2012

Could Space Mirrors Stop Global Warming?
By Rachel Kaufman August 08, 2012

Space Mirrors Climate Change
A constellation of billions of mirrors free-floating at the Earth-Sun Lagrange point blocks solar radiation and cools earthly global warming. (Image credit: Dan Roam)
The record-breaking temperatures of the past few years are getting more people thinking about bigger solutions to climate change. Ideas once thought of as wacky are now receiving careful consideration, including an idea that sounds straight out of science fiction: cooling the earth by launching reflective mirrors into space.

Lowell Wood of Lawrence Livermore National Laboratory proposed a giant space mirror in the early 2000s, though he cautioned that the mirror should be considered only as a measure of last resort. Why? Because the mirror would have to have an area of 600,000 square miles – a slightly smaller area than Greenland – and launching something that big would be prohibitively expensive. Another option: billions of smaller mirrors. Roger Angel, researcher and optics expert at the University of Arizona, proposed that idea in 2006.

In either case, the mirror or mirrors would orbit at Lagrange point L1, a gravitationally stable point between the Earth and the sun that's about four times the distance from the Earth to the moon. The mirrors would barely be visible from Earth and would block just 1 percent to 2 percent of the sun's light, but that would be enough, advocates of the schemes say, to cool the planet. Even with Angel's plan, the current cost of launching a trillion mirrors would be $10,000 per pound, or, in total, 26 times more than the current U.S. national debt.

A nightmare to maintain

Another option was proposed in 2002 by space consulting firm Star Technology and Research. Star's experts calculated that a network of steerable space mirrors orbiting Earth's equator, like one of the rings of Saturn, could lower the average air temperature by up to 3 degrees Celsius (5.4 degrees Fahrenheit) while simultaneously generating power from onboard solar panels and beaming it to Earth.

But such an approach could generate problems. Report author and Star Technology president Jerome Pearson calculated it would take 5 million spacecraft to achieve the desired result, and even if each individual craft could last 100 years, that means 137 ships would have to be replaced or repaired per day. And the craft would produce "stars" that would be visible from the ground. (Pearson's other hypothetical proposal, a ring of reflective rocks in the same position, would light the night sky with the equivalent of 12 full moons.)

Even if a space mirror scheme was technically and financially feasible, it could result in unintended consequences, like drought. A recent study from the Max Planck Institute for Meteorology added the effect of space mirrors to four climate models. In each model, the space shades lowered the average global temperature to preindustrial levels, but unevenly. The poles warmed while the tropics cooled, fewer clouds formed, and the planet received less rainfall, especially in the Americas and northern Eurasia.

A short-term solution?


Such a plan also would do nothing about ocean acidification and little about sea-level rise, since sea levels respond slowly to changes in Earth's temperature. A space mirror could offset air temperature warming until at least 2070, according to a 2010 Proceedings of the National Academy of Sciences paper, but the oceans would still rise by two feet in that time.

Aggressive carbon reductions coupled with a space reflector could limit sea level rise to one foot by 2100, and stopping it completely would require a mirror that was constantly getting larger and more efficient at blocking sunlight.

While a giant space mirror isn't currently possible with today's tools, technology is in fact catching up to science fiction, and the idea seems less outlandish than it did a decade ago. Will an enormous mirror someday be the world's last hope to stop global warming? Only time will tell.

Editor's Note: The original version of this story incorrectly stated that a relative change of 3 degrees C is equal to 37 degrees F. The correct conversion is 5.4 degrees F.

This story was provided by InnovationNewsDaily, a sister site to LiveScience. Follow InnovationNewsDaily on Twitter @News_Innovation, or on Facebook.

https://www.livescience.com/22202-space-mirrors-global-warming.html

Renewable and Sustainable Energy Reviews
Volume 31, March 2014, Pages 792-834
Renewable and Sustainable Energy Reviews
Fighting global warming by climate engineering: Is the Earth radiation management and the solar radiation management any option for fighting climate change?
Author links open overlay panelTingzhenMingaRenaudde_RichterbWeiLiuaSylvainCaillolb
https://doi.org/10.1016/j.rser.2013.12.032Get rights and content
Under a Creative Commons licenseopen access
Abstract
The best way to reduce global warming is, without any doubt, cutting down our anthropogenic emissions of greenhouse gases. But the world economy is addict to energy, which is mainly produced by fossil carbon fuels. As economic growth and increasing world population require more and more energy, we cannot stop using fossil fuels quickly, nor in a short term.

On the one hand, replacing this addiction with carbon dioxide-free renewable energies, and energy efficiency will be long, expensive and difficult. On the other hand, meanwhile effective solutions are developed (i.e. fusion energy), global warming can be alleviated by other methods.

Some geoengineering schemes propose solar radiation management technologies that modify terrestrial albedo or reflect incoming shortwave solar radiation back to space.

In this paper we analyze the physical and technical potential of several disrupting technologies that could combat climate change by enhancing outgoing longwave radiation and cooling down the Earth. The technologies proposed are power-generating systems that are able to transfer heat from the Earth surface to the upper layers of the troposphere and then to the space. The economical potential of some of these technologies is analyzed as they can at the same time produce renewable energy, thus reduce and prevent future greenhouse gases emissions, and also present a better societal acceptance comparatively to geoengineering.

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Abbreviations
AVEatmospheric vortex engineBCblack carbonCCScarbon capture and sequestrationCDRcarbon dioxide removalCEclimate engineeringCSPconcentrated solar powerDETdowndraft energy towersERMearth radiation managementGEgeoengineeringGHgreenhouseGHGgreenhouse gasesGWglobal warmingHMPTHoos mega power towerIPCCIntergovernmental Panel on Climate ChangeMRmeteorological reactorsOTECocean thermal energy conversionPCMphase change materialsSCPPsolar chimney power plantSRMsolar radiation managementSRMsunlight reflection methodsUREunusual renewable energiesUVultraviolet
Keywords
Earth radiation managementGeoengineeringThermal shortcutsSolar updraft chimneyDowndraft evaporative towerHeat pipeClear-sky radiative cooling
1. Introduction
The most serious and important problem humankind has ever had to face might be global warming with disastrous consequences and costly adverse effects [1]. Adaptation and mitigation strategies might not be sufficient. In May 2013 the CO2 concentration in the Earth's atmosphere officially exceeded 400 ppm, according to the Mauna Loa Observatory in Hawaii, which has been monitoring atmospheric CO2 since 1958 when that figure was around 320 ppm. At the time the Intergovernmental Panel on Climate Change (IPCC) issued its 2007 assessment [2], it recommended to keep atmospheric greenhouse gases below 450 ppm in order to keep the temperature rise under a 2 °C target [3].

Many scenarios have been considered in order to slowly decrease our greenhouse gases (GHG) emissions to try to keep the average temperature heat rise under +2 °C. But without an international agreement signed by the biggest polluters, this <2 °C figure will remain only empty words and will not be followed by actions and effects.

Human GHG emissions have already been so important and some of these GHG have such extraordinarily long lifetimes that even if by a magic wand we could stop all emissions overnight, the average temperature of Earth would continue to rise or stay at current levels for several hundred years [4].

Global warming results from the imbalance between the heat received by the Earth and, the heat reradiated back to space. This paper proposes methods to increase the IR radiation to space. The surface outgoing longwave radiation is defined as the terrestrial longwave radiative flux emitted by the Earth's surface beyond the 3–100 µm wavelength range. The shortwave incoming solar radiation also called global irradiance or solar surface irradiance [5] is the radiation flux density reaching a horizontal unit of Earth surface in the 0.2–3 µm wavelength range. Both are expressed in W m−2.

The GHGs trap some heat and, by greenhouse effect, warm the Earth surface. Incoming and reflected shortwave sunlight patterns are represented on the right side of Fig. 1 from NOAA [6] (inspired by Kiehl [7] and Trenberth [8]); outgoing infrared or longwave radiation modes are symbolized on the left side. The Earth's energy budget expressed in W m−2 is summarized in this figure. The principal atmospheric gases ranked by their direct contribution to the greenhouse effect are [7] water vapor and clouds (36–72%), carbon dioxide (9–26%), methane (4–9%) and ozone (3–7%).


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Fig. 1. “Earth's Annual Global Mean Energy Budget” (from NOAA) [6].

Tackling climate change will require significant reductions in the carbon intensity of the world economy. Developing new low-carbon technologies and adopting them globally is therefore a priority. But even moving relatively quickly toward a carbon-neutral economy will still result in a net increase in CO2 in the atmosphere for the foreseeable future. It seems that we are nowhere close to moving quickly in this direction: gas and fossil fuel reserves have effectively increased, due to improved technologies for extraction. Huge underwater oceanic reserves of methane hydrates or clathrates [9], [10] will possibly become extractible in the near future. The recent shale gas boom in USA and the methane reserves do not militate in favor of a reduction of the energy consumption, nor in a reduction of CO2 and CH4 emissions. With gas prices hitting rock bottom, the cost competitiveness of renewable energies in the short- to mid-term will be harder to meet than ever before. This has brought further uncertainty about the future of solar projects and offshore wind technologies, particularly solar ones. The innovation challenge spans the development of new unusual renewable energies based on low-carbon technologies as well as – and possibly even more pressing – improving the performance, the efficiency, and particularly lowering the costs of the existing ones.

This review intends to be an element that provides an update on proposed solutions to the control and the management of the climate, and to propose a tool of choice among new and innovative ones.

Geoengineering aims at stabilizing the global climate, reducing global warming and fighting anthropogenic climate change owing to two strategies: shortwave (0.3–3 μm) sunlight reflection methods and carbon dioxide removal technologies. After a short overview of a set of geoengineering strategies, this paper then proposes innovative methods for increasing outgoing terrestrial (4–100 μm, and most often 4–25 μm) radiant energy fluxes by thermal longwave radiation methods. One of the main ideas developed in this review is that GHGs are good insulators that prevent normal interactions with the Earth atmosphere with the space, and keep the Earth too hot, so “atmospheric thermal bridges” have to be created. By analogy to the expression of “thermal bride” used in civil engineering where heat is transferred by conduction from one part of a building to another, with the result of a cooling of the hotter part, we define an atmospheric thermal bride has a way to transfer longwave radiation from one part of the atmosphere (generally the Earth surface) to another (generally in the higher troposphere, the stratosphere, or to the open space). One natural phenomenon illustrating this concept is the atmospheric window, by which IR radiation in the range 8–13 µm can escape directly to space.

After an overview of the principal geoengineering techniques of solar radiation management (SRM or sunlight reflection methods), we present in this review technological breakthrough alternatives, many of them are little known, misunderstood or ignored, that can decrease or decelerate global warming (GW), and also might help to cool the Earth surface.

A 30 years power-purchase agreement of the Southern Californian Public Power Authority [11] for the construction of the first solar updraft chimney in La Paz County, Arizona, USA was announced in February 2011. Another company [12] published plans to combine downdraft evaporative cooling towers with wind towers to produce electricity. The opportunity to take stock of similar disrupting technologies and their benefits is examined in this paper.

These recent announcements for the construction of industrial scale power plants of solar updraft chimneys and downdraft energy towers have been made. These unusual renewable energy power plants belong to the family of large scale power stations called by us “meteorological reactors” which can convert heat into artificial wind inside a duct and produce electricity by driving turbines. Despite many interesting advantages such as the low cost of the kWh produced, a long lifespan, clean energy production and environmentally friendly operations with almost no maintenance, their current commercial applications are limited because of their large initial investment cost and low conversion yield.

Several energy-neutral ideas and techniques will be described, followed by a description of a number of innovative and unusual renewable energies (UREs), from the family of the meteorological reactors (MR), which can at the same time help cooling the planet by Earth radiation management (ERM), produce CO2-free electricity and prevent further CO2 emissions.

This review focuses on using several MR, night sky radiation and giant heat pipes as active heat transfer tools to cool down the Earth by artificial vertical wind generation and, at the same time, production of sustainable CO2-free renewable energy without the drawbacks of current climate engineering strategies. This review sheds light on innovative activity and innovation dynamics in heat-transfer technologies and CO2-free renewable energy production.

2. Overview of the major SRM geoengineering proposals
Proposals for GE projects can mainly be divided into two categories: SRM and carbon dioxide removal (CDR) [13]. CDR techniques (that curiously are considered as CE, but probably might not) are out of the scope of this paper and thus will not be depicted. The IPCC Fourth Assessment Report [14a] defines geoengineering (GE) as “technological efforts to stabilize the climate system by direct intervention in the energy balance of the Earth for reducing global warming”. Geoengineering [15] or climate engineering (CE) consists in a large set of technologies that deliberately reduce solar insolation or increase carbon removal directly from the atmosphere, on a large scale, with the aim of minimizing, counteracting, mitigating, limiting, counterbalancing or reversing anthropogenic climate change in order to reduce GW or its consequences. The raise of geoengineering on the scientific and policy agenda is no doubt at the international level, as it has been assessed by the 5th IPCC working groups (WP) 1 and 3. The 5th IPCC report of WP1 issued in September 2013 [14b] cites geoengineering 50 times only in its chapter 7 and 16 publications on geoengineering are cited in Chapter 6.

In a Royal Society [16] report, geoengineering is defined as the “deliberate large-scale manipulation of the planetary environment to counteract anthropogenic climate change”. This Royal Society report reviews a range of proposals aimed to reflect the Sun's rays back to space, and, among means to remove CO2 from the air for instance oceanic carbon sequestration, by injecting iron into the world's seas to rapidly increase the amount of phytoplankton that feeds itself from CO2.

An almost exhaustive list of proposed GE projects has been established [17], a very large review of CE proposals has been given by Vaughan [13], and numerous other strategies have been listed [18], [19]. Literature is now abundant about geoengineering proposals, describing them in detail and discussing their advantages, effectiveness, potential side effects and drawbacks [16], [20], [21], [22], but also governance, legitimacy and ethical aspects [23], [24], [25]. As a matter of fact, criticism about CE research focuses on international consequences of possible unilateral use of GE techniques [26], [27].

SRM proposals aim to reduce GW by reducing the amount of light received on the Earth and by its atmosphere [28]. It includes (Fig. 2) several techniques like space solar reflectors; stratospheric injection of aerosols; seeding tropospheric clouds by salt aerosols or ice nucleation to make them whiter and also surface albedo change (urban, rural, or atmospheric approaches). Numerous other strategies have been proposed [29], [30], but the aim of this review is not to be exhaustive. GE has been quite studied since 2008 and is envisioned as a plan B in case the governments do not succeed to reduce CO2 emissions. At the international level of climate change politics, the positioning of CE as an option between mitigation and adaptation is taking concrete form. The elaboration of an alternative plan C developing the concept of Earth radiation management (ERM) is at least appealing and entailing and is the goal of this review which has in mind the need for innovative breakthroughs. Those new strategies have the potential to address 2.2 times more energy flux (69%) than SRM (31%).


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Fig. 2. Overview of the principal SRM geoengineering techniques that attempt to increase the reflection back to space of the incoming solar radiation. These techniques are often referred as acting by a “parasol or umbrella effect”.

2.1. Space mirrors [31,32] and science fiction-like proposals
The idea of this GE scheme is to send into orbit giant mirrors (55,000 orbiting mirrors each of 100 km2) made of wire mesh; or to send trillions of light and small mirrors (the size of a DVD), in order to deflect sunlight back to space. In other words numerous artificial mini-eclipses that will obscure the sun. This option is widely considered unrealistic, as the expense is prohibitive, the potential of unintended consequences is huge and a rapid reversibility is not granted.

Similarly, the reduction of incoming solar radiation was considered by placing a deflector of 1400-km diameter at the first Lagrange Point, manufactured and launched from the Moon [33]. The idea to mine the moon [28] to create a shielding cloud of dust is in the same league.

Several other proposals have been studied and discussed by some scientists but, at our knowledge, not by the space industry which probably fears that the thousands of orbiting debris could damage the satellites in orbit.

2.2. Sulfate aerosols [34,35]
This scheme is inspired by studies of the Mount Pinatubo volcano eruption in the Philippines in 1991 and by the cooling effect of its sunlight blocking sulfur plume. This “artificial-volcano” idea is one of the least costly, and very small sulfate particles in the stratosphere could last for a couple of years. The two main problems are acid rain creation and probable damage of the ozone layer.

But, currently burning fossil fuels and coal in particular and other anthropogenic emissions, already introduce every year nearly 110 million tons of SO2 in the lowest levels of the atmosphere [36]. With other reflective tropospheric aerosols this has a direct cooling effect evaluated by Hansen [37] to 1 W m−2, plus an indirect cooling effect of 0.8 W m−2. Not all these aerosols are anthropogenic and volcanic aerosols also tamped down Earth warming: recent work from Neely [38] revealed that moderate volcanic eruptions, rather than Asian anthropogenic influences, are the primary source of the observed 2000–2010 increases in stratospheric aerosol. Sulfates in the troposphere have a much shorter resilience time than those in the stratosphere, that is why 1–5 million tons of small size particles of SO2 in the stratosphere every year [39] would have a more efficient cooling effect than current emissions in the troposphere.

Reducing the sulfates emissions from power plant, as is already done in the US, Europe and Japan, is helpful for reducing acid rain, but it removes the umbrella of sulfates protection that reflects solar radiation back to space and shields the Earth from the warming effect of GHGs and thus has a net warming effect. The problem is complex but if by magic tomorrow it was possible to stop completely burning coal, the result would be an immediate major global warming effect.

This paradoxical existing incentive in favor of non-reduction of pollution could be a possible rationale for promoting SRM in spite of the moral dispute over GE. But to become morally acceptable, SRM should be limited to the idea of compensating for the warming effect of local air cleaning. SRM should not be aimed to substitute to the needed efforts of GHG emissions down-curving, as CO2 levels will continue to rise in the atmosphere soon breaking the 450 ppmv level limit climatologists recommend, to eventually reach 800 ppmv or even more.

Among several other critics [40] to the use of sulfates in the stratosphere, there is the need to deliver every year at least one million ton of SO2 using thousands of balloons, planes or rockets, costing between $25 and $50 billion annually and having to be maintained continuously. Also a change in overall rain patterns and a non-uniform cooling effect obtained over the entire Earth with winners and losers is among the drawbacks, together with the non-resolution of the problem of ocean acidification.

The addictive character of this techno-fix will not encourage decreasing our CO2 emissions and if stopping this geoengineering scheme was mandatory for whatever reason (unexpected effects, financial crisis…), the stratospheric sulfate sunshade would rapidly lift and several decades' worth of warming would hit the Earth and all living organisms with no left time for adaptation.

One of the greatest fears for the opponents to CE comes from the fact that due to the relatively low cost of SRM (compared to CDR) and potential to act quickly (e.g. like after the Pinatubo eruption), SRM may be adopted by some governments without consulting other countries, as in this particular case national policies have international effects. GE attempts made by some countries may conduct their neighbors to perceive them as rogue states. The fair amount of the current research on governance of GE might unintentionally convince that if appropriate governance frameworks, principles and codes are in place thus developing GE options can be a responsible option. The debate about governance, legitimacy and ethics cited early [23], [27] is still mostly centered on the sulfate aerosol option.

2.3. Cloud whitening [41,42]
The idea is that sea water can be pumped up and sprayed into the air to increase the number of droplets, and produce fine sea salt crystals increasing the reflectivity of low altitude clouds. Together, many droplets and salt aerosols are expected to make whiter clouds and reflect more intensity of sunlight. It seems harmless and not too expensive, but needs to be done on a huge scale to have any global effect. This proposal (and several others) is backed financially by former directors from Microsoft. According to Latham [43], in the first decades of operation, the amount of disseminated salt over land would be several orders of magnitude less than naturally produced. This mechanism is based on the Twomey and Albrecht effects: increasing number or surface area of droplets increases the scattering of light, thus increasing albedo. As reducing droplet size lowers their sedimentation velocity, precipitation could be delayed or inhibited, increasing cloud lifetime, so there will be an increased cloudiness.

A “Flettner” rotary ship using the “Magnus effect” is studied by Salter and Latham [41], [43] for spaying sea water aerosols.

If a meaningful amount of tankers and fleet of commercial vessels was equipped to vaporize small droplets of salted water for cloud whitening as experimented by Salter, the SRM effect could become regular, global and at low cost. For Latham [43] 1500 spray vessels can produce a negative forcing of −3.7 W m−2. If any unforeseen adverse effect occurs, the reversibility is rapid, as the system can be switched off instantaneously and in a few days the clouds properties will return to normal. This technology produces local cooling and can also reduce the intensity and severity of hurricanes.

A similar technique has been proposed by Seitz [44] using the vessels of the commercial fleet to inject micron-size bubbles in the oceanic waters in order to increase albedo and cool the water.

2.4. Other albedo changes [45,46]
The proportion of light reflected from the Earth's surface back to space is called albedo after the Latin word albus for white. In the Earth radiation budget it is identical to the outgoing shortwave radiation, with spectral properties in the range of those of the incoming light from the sun.

Road asphalt is hotter during the summer, meanwhile white roofs stay cooler [47], allowing saving some electricity used for air conditioning and thus avoiding CO2 emissions. According to Akbari [45], replacing 1000 ft2 (93 m2) of a dark roof by a white roof, might offset the emission of 10 t of CO2 (air conditioning savings and albedo effect). Several companies have developed asphalt road coatings and asphalt roofs coatings that reduce surface heat by up to 15–20 °C and thus the urban heat island effect.

Painting roofs and roads in white, covering glaciers and deserts with reflective plastic sheeting, putting white or pale-colored plastic floating panels over oceans or lakes, and planting genetically engineered paler crops have all been proposed to reflect sunlight back into space (Fig. 2). Gaskill [48] gave an extensive overview of rationale, pros and cons of global albedo projects. Replacing tropical forests by high albedo deserts is not an option, but the development and advance of the forests to the North could have a positive retroaction on global warming: in this case, the refusal of a GE scheme (like planting whiter trees) for moral reasons does not seem justified as it fights against a GW positive feedback and also decreases CO2 atmospheric levels by wood production.

Boyd [49] and then the UK Royal Society [16] evaluated recently and ranked the main SRM and CDR techniques for their safety, effectiveness, affordability and cooling potential (calculations details inside the report). Table 1 and Fig. 3 from the Royal Society report summarize their findings.

Table 1. Estimates of the cooling potential of several geoengineering techniques by Lenton and Vaughan [13] and the Royal Society report [16] (includes CDR techniques not discussed in this paper).

Geoengineering technique Cooling potential
Stratospheric aerosols 3.71
Albedo increase of clouds, mechanical 3.71
Albedo increase of deserts 1.74
Air capture and storage 1.43
Ocean phosphorus addition 0.83
Albedo increase of grassland 0.64
Bio-char production 0.52
Carbonate addition to oceans 0.46
Albedo increase of croplands 0.44
Ocean nitrogen fertilization 0.38
Iron fertilization 0.29
Afforestation 0.27
Albedo increase by human settlement 0.19
Enhance upwelling 0.028
Albedo increase of clouds by biological mean 0.016
Enhance downwelling 0.016
Albedo increase in urban areas 0.01

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Fig. 3. Geoengineering proposals classified in the Royal Society report [16] for their safety, effectiveness and affordability.

Air capture consists to capture diluted CO2 in air with alkaline polymers and BECS consists to produce bio-energy followed by carbon storage.

The 2005 IPCC special report on carbon capture and sequestration (CCS) [50] provides a full description of these technologies. CDR and CE techniques have been reviewed elsewhere [13], [16], [51] and will not be mentioned here.

2.5. Some examples of small scale SRM experiments already performed
It is worth pointing out that several small scale field SRM studies, or experiments have already been carried out, or are planned, but with no global GE aim. Some are isolated or individual initiatives, with different levels of maturity and sophistication, many without scientific research purposes, but they have received some public attention, for instance:
•
In 2005 a small pilot project on the Gurschen glacier of the Swiss Alps was conducted to try to stop the ice melting of glaciers [52a] with a “ice protector” textile made of a lightweight dual-layer composite with polyester in the top side to reflect light, and polypropylene on the bottom to block heat and slow ice melting during the summer. It proved successful as the blanketed area had 80% less melt than surrounding ice. Covering an area of 30,000 m2 was projected on the Vorab glacier.

•
In the Peru Andean region, a local team that painted rocks in white won a $200,000 prize from the World Bank as part of its “100 Ideas to Save the Planet” competition [52b–c]. Meanwhile the “Fund for Innovative Climate and Energy Research” is financed by Gates [52d] and is more devoted to SRM projects, the “Virgin Earth Challenge” financed by Brandson [52e] is more concentrated on CDR and offers a $25 million prize for a commercially viable invention able to permanently remove significant volumes of GHGs out of the Earth's atmosphere, so as to contribute materially to avoid global warming.

•
In cold countries, with winter freezing rivers [53] and lakes it can be drilled bore holes into the ice that has started to form. The water will be discharged across the surface, where it will freeze and add layers of ice rinks. The ice cap itself being a good insulator, if no holes were drilled in it, much less water would freeze. This process could be repeated at regular intervals throughout the winter with the aim to produce a big block of ice several meters thick as refrigeration storage, to cool and water the cities as it melts during summer. The insulation capacity of the ice is broken by one of the “thermal bridge” strategies that will be developed later.

•
Over the summer months, up to 40–50% of the water stored in small farm dams may be lost to evaporation, but using white reflective covers to reduce this loss increases agricultural water use efficiency and participates to global cooling by modifying albedo [54].

•
Rising salty groundwater currently threatens many agricultural lands, but a salinity mitigation strategy[55] is already applied in Australia. The aim is to prevent the clear ground water to mix with the salty one, and consisted during the dry season in pumping salty groundwater into shallow evaporation basins to form a salt pan with higher reflectance than the surrounding farmland which resulted in an immediate mitigation of local warming both by evaporation and by albedo modification. The main goal is achieved too: preventing salty ground water to mix with the clear one.

•
The SPICE project [56]a–b (stratospheric particle injection for climate engineering) consisted in using a small hose-augmented balloon up just over one km high, pumping water into the air. The aim was to test the feasibility of later piping sulfates at 25 km high (see Fig. 4). Although only water was to be sprayed, GE opponents succeeded to stop this experiment. Partanen [57] showed that multiplying the mass flux by 5 or reducing the injected particle size from 250 nm to 100 nm could have comparable effects on the GE radiative efficiency.


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Fig. 4. The SPICE experiment [58]a-b


In 2002, an artificial cloud making method was patented in China [59]. It replicates Earth's Hydrologic Cycle, using a pipeline facility constantly conveying air, from a lower altitude to a higher altitude, with water vapor which condenses to form a pervasive artificial cloud. More recently several US patents [59] from former Microsoft scientists described a very similar concept, with a 15–50 km high altitude duct “conduit” like in Fig. 5 for the aerosol injection in the stratosphere. That sounds quite high, but several articles from NASA describe the feasibility of multi-kilometer height tall towers [61], [62], [63], [64], [65]. Later in this review, some ERM strategies propose to make use of quite high meteorological reactors, but civil engineers and architects are confident on their feasibility, as already almost kilometric high buildings have been successfully built and numerous projects all over the world target taller ones.


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Fig. 5. Kilometric high conduit for aerosol spraying in the stratosphere [60].

2.6. Discussion about SRM
SRM methods may be able to reduce temperatures quickly and some of them like stratospheric aerosols at comparatively low cost. However, even if they could reduce some of the most significant effects of global warming and lessen some of its harmful impacts, these technologies could also have significant unanticipated harmful side effects. Moreover, they would not eliminate the cause of climate change, the emissions of GHGs and the associated threat of ocean acidification. For many experts the whole idea of pursuing these “technical fixes” is controversial since SRM can probably restore on average the Earth's global radiative balance, but regional climate discrepancies will remain [66].

Also, if CO2 levels continue to rise during SRM, that means it must be maintained indefinitely to avoid abrupt and catastrophic warming and there must happen no technological, economical or political failure.

In a position where avoidance of one danger exposes one to another danger, CE has been widely shunned by those committed to reducing emissions and by the public which feels that SRM and GE (often only associated to sulfate aerosols) is far too risky to attempt, since tampering with Earth's and climate systems could lead to new climatic and ecological problems.

The principal GE schemes are represented in Fig. 6 reproduced from Matthews [67]. In a paper whose title is “Can we test geoengineering?”, MacMynowski [68] noted that SRM tests could require several decades or longer to obtain accurate response estimates, as the hydrological and temperature responses will differ from a short-duration test and also from what has been observed after large volcanic eruptions. Robock [40] found “20 reasons why geoengineering may be a bad idea”.


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Fig. 6. Several geoengineering schemes as represented by Matthews [67].

By pumping massive amounts of CO2 and other GHGs into the atmosphere and by building mega-cities and thousands of kilometers of black paved highways, humans have already engaged in a dangerous geophysical experiment. The only difference with CE is that it was unintentional. The best and safest strategy for reversing climate change is to halt this buildup of atmospheric GHGs and stop CO2 emissions, but this solution will take time, and it involves a myriad of practical and political difficulties. Meanwhile, the dangers are mounting and even with a serious effort to control GHGs emissions, meaningfully reducing them in the very near term is an unattainable goal.

As Myhrvold and Caldeira [69] showed, the rapid deployment of low-emission energy systems can do little to diminish the climate impacts in the first half of this century: conservation, wind, solar, nuclear power, and possibly CCS appear to be able to achieve substantial climate benefits only in the second half of this century.

So maybe GE will be needed, although serious research on CE is still in its infancy, and till recently has received little financial funding for scientific evaluation of benefits and risks.

But even if the ethics of geoengineering as well as political aspects has been widely discussed [20], [21], [22], [23], [24], [25], [26], [27], neither international nor public [70], [71] consensus has been yet obtained even for research on this subject: stopping the “spice experiment” previously cited is an illustration.

It is worth noting that since 1977 there is an Environmental Modification Convention, which has so far been ratified by 76 countries [72]. It prohibits the hostile use of techniques that modify the dynamics, composition, or structure of the Earth (including the atmosphere) or of outer space. One of the main questions of the debate is: in a fragile and globalize world, who would govern geoengineering actions that can severely affect climate and, for this reason, might be potentially used as weapons?

Also, till date the most successful international agreement is the Montreal Protocol on Substances that Deplete the Ozone Layer [73] that was agreed in 1987. It included trade sanctions to achieve the stated goals of the treaty and offers major incentives for non-signatory nations to sign the agreement. As the depletion of the ozone layer is an environmental problem most effectively addressed on the global level the treaty include possible trade sanctions, because without them there would be economic incentives for non-signatories to increase production of cheap depleting substances, damaging the competitiveness of the signatory nations industries as well as decreasing the search for less damaging alternatives. All UN recognized nations have ratified the treaty and continue to phase out the production of chemicals that deplete the ozone layer while searching for ozone-friendly alternatives. In the presence of halogenated compounds, the sulfate aerosols in the stratosphere might damage the ozone layer [39] thus this SRM might be a violation of the Montreal Protocol spirit and goal.

The intergenerational transfer of atmospheric carbon and GHGs stocks and pollution is also part of the discussions [74], [75] as this is equivalent to delay current generation's abatement efforts. Future generations will have to limit the damages of the atmospheric carbon stock that they will inherit from current society. Together with radioactive nuclear wastes, this implies future costs and a poisoned chalice to leave to our heirs and successors.

In the absence of adequate reductions in anthropogenic CO2 emissions, GE has been put forward as the only remaining option that might fix our rapidly changing climate, even if scientists are reluctant to encourage governments to deploy CE rather than invest in cutting emissions and making efforts to control them.

CDR and CCS techniques address the root cause [16] of climate change by removing the most abundant GHGs from the atmosphere, but will require decades to have significant effects. SRM techniques are much faster (months) and attempt to offset the effects of increased GHGs concentrations by reducing the absorption of solar radiation by the Earth. Both methods have the same ultimate aim of reducing global temperatures.

3. Earth radiation management (ERM)
Proposed SRM GE schemes act by the parasol effect: reducing solar incoming radiation. However CO2 traps heat both day and night over the entire world whereas diminished solar radiation would be experienced exclusively in daytime and on average most strongly at the equator.

The technologies described in this paper, although seasonal, are expected to be less intermittent and cover more than the diurnal cycle and are well distributed from equator to pole as they are complementary. Fig. 7 shows on which radiation fluxes SRM geoengineering schemes might be useful acting on shortwave radiation (0.2–3 µm), which represents less than 1/3 of the total incoming radiation. ERM proposed in the next part of this review focuses on more than 2/3 of the global radiative budget and is possible night and day all over the Earth. The goal of this paper is to demonstrate that several other ways of action are possible acting on the longwave radiation (4–25 µm) flux.


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Fig. 7. Principal energy fluxes in yellow the ones corresponding SRM geoengineering schemes, and in red the others concerned by the sustainable ERM proposed new ways of action (i.e. increase sensible and latent heat transfer to the outer space).

3.1. Targeting high and cold cirrus clouds: not a SRM strategy but a ERM one
Mitchell [76] proposed to cool the Earth surface by increasing outgoing longwave radiation by reducing the coverage of high cirrus clouds.

Cirrus clouds tend to trap more outgoing thermal radiation than they reflect incoming solar radiation and have an overall warming effect. As they have a greater impact on the outgoing thermal radiation, it makes sense to target the colder cirrus clouds. This proposal consists in increasing outgoing longwave radiation by dispersing clouds over the polar ice caps. Thus by changing ice crystal size in the coldest cirrus, outgoing longwave radiation might be modified. According to Mitchell, the coldest cirrus have the highest ice super-saturation due to the dominance of homogeneous freezing nucleation, so seeding cold cirrus (high altitude) with efficient heterogeneous ice nuclei (like bismuth tri-iodide BiI3) should produce larger ice crystals due to vapor competition effects, thus increasing outgoing longwave radiation and surface cooling. BiI3 is non-toxic and Bi is one order of magnitude cheaper than Ag (sometimes used to increase rainfall).

Preliminary estimates by Mitchell [76a,b] and by Storelvmo [76c] show that global net cloud forcing could neutralize the radiative forcing due to a CO2 doubling. Airline industry is the potential delivery mechanism for the seeding material and reversibility should be rapid after stopping seeding the clouds. This approach would not stop ocean acidification, buts seems to have less drawbacks that stratospheric injection of sulfates.

3.2. Preventing a possible weakening of the downwelling ocean currents: also an ERM strategy?
The formation of North Atlantic Deep Water releases heat to the atmosphere, which is a contributor to a mild climate in Europe. Without the warm North Atlantic Drift, the UK and other places in Europe would be as cold as Canada, at the same latitude. But the increase of CO2 in the atmosphere might produce a weakening of the North Atlantic Deep Water by modification of downwelling ocean currents. The slowdown of new sea ice formation might lead to the abatement of the thermohaline circulation. With the aim to prevent it, Zhou and Flynn [77] assessed the costs of several methods for enhancing downwelling ocean currents, including the use of existing industrial techniques for exchange of heat between water and air. They proposed the use of snow-cannons powered by wind turbines on floating barges during the winter to help the formation of thicker sea ice by pumping ocean water onto the surface of ice sheets. Sea ice that forms naturally in the ocean does so at the bottom of an ice sheet and is not very salty (ice rejects the salt as it freezes). As sea ice formation increases the salinity (salt content) of the surrounding water, this cold and salty water is very dense, and sinks creating the “global conveyor belt”. Zhou and Flynn make the assumption that if seawater freezes on top of an ice sheet, salt would mainly be trapped on the surface or within the ice, both as brine cells and solid salts, especially if the thickness of ice built up to several meters thick. In this case, then incremental downwelling current would occur when the sea ice melted in the spring, since the melting ice would lower the temperature of seawater and the surrounding ocean salinity would be unchanged. On the contrary, if brine was able to flow from the top of the ice sheet back into the ocean in the winter as incremental sea ice will be formed, then incremental downwelling current would occur in the winter driven by salinity. In both cases the goal of enhancing downwelling ocean currents is reached.

Of course on the one hand the Zhou and Flynn proposal can be classified in the albedo modifications schemes of the SRM strategies (Fig. 8), as it participates in maintaining the polar ice caps which help to regulate global temperature by reflecting sunlight.


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Fig. 8. Representation of two EMR strategies. The technology proposed by Zhou and Flynn [77] to re-ice the Arctic during the winter uses ice cannons powered by wind turbines floating on barges, and the technology proposed by Bonnelle [78] consists in transporting sea water till the top of northern mountains where the air is very cold and just before freezing carrying it back downhill to the ocean where floating ice and saltier water are released.

But on the other hand their strategy can also be evaluated differently. As the first layers of floating ice are good thermal insulators, natural heat transfer from the winter cold air to the liquid water under the ice is not very efficient, so the growth of the ice caps is slow and the increase of the thickness limited. The Zhou and Flynn strategy overpasses this problem, and they obtain a thicker ice cap that can last longer in spring and thus reflect more sunlight back to space. But also, during the winter manufacture of the ice by sending a seawater spay in the cold air, the latent heat of solidification (freezing) will be released in the atmosphere: cold ice is created on the ocean surface meanwhile the hot air generated, will by natural buoyancy go upper in the troposphere. So a heat transfer from the surface to a higher elevation has occurred. A similar strategy was previously described for rivers [53] or lakes in cold countries. It is as if a thermal bridge was created by the ice canons between warmer water and cold air to bypass the insulation caused by the first thin ice cap.

Adapting some of the previous processes to slowdown glaciers melting during summer seems possible. Quite often lakes form below melting glaciers. Those lakes hidden under the snow make the risk of giving jog suddenly, releasing large quantities of water and mud. To prevent avalanches and floods in the summer, some cities upstream of these under-glacier lakes install pumping systems for emptying them as they are formed, and water is discharged into rivers. Instead, at night the water could be pumped up above the level where temperatures remain negative and with snow-cannons used to produce new fresh and clean snow with high albedo. This technique also transfers heat from the water to the air. At lower altitudes thermosyphon heat pipes, as well as other mechanisms to facilitate the sublimation of water, can also help to contain the summer glacier melting.

3.3. Alternatives to SRM do exist
These last examples show that complementary strategies or approaches to SRM are possible, in particular those targeting infrared radiation out to space. Several ideas concentrating in the Earth radiation management, including latent heat and sensible heat riddance methods and anthropogenic waste heat energy removal means, will be presented in the following paragraph.

In the next chapters, unusual renewable energies (UREs) are described, which can at the same time produce electricity, avoiding the CO2 emissions that would otherwise have been made by conventional fossil power plants and also help cooling down the Earth. Last but not least, these UREs can also help to avoid the waste heat energy associated with nuclear power plants as well as with almost all other thermal power plants.

4. Why looking for energy removal methods?
4.1. Waste heat and thermal emissions also warm Gaia
Human activities are not only releasing GHGs into the atmosphere, but also waste heat at the Earth surface and into the oceans. Fossil fuel powered plants emit most of the GHGs, but also add significant amount of their intake energy as waste heat. Generation of 1 kWh of electricity by a “typical” coal-fired power plant emits 1 kg of CO2, but also releases about 1.8–2 kWh of low grade heat into the surrounding environment [79] which, although a minor one, is another form of forcing on the climate system. Of course this is on average, as CO2 emissions depend on technology used (combined cycle, integrated gasification combined cycle, conventional pulverized coal, oxygen combustion…), and also on the type of fuel [95b]. For instance the CO2 emissions in gCO2/kWh of electricity produced are 920 for anthracite, 990 for lignite, 630 for crude oil, 400 for natural gas, etc. In 2010, 43% of CO2 emissions from fuel combustion were produced from coal, 36% from oil and 20% from gas.

The electricity generated both by conventional power plants and by renewable ones is also largely dissipated as waste heat. These anthropogenic heat sources have generally been considered quite small compared with radiative forcing due to GHGs [80]. The Earth thermal energy fluxes from the sun's energy received in connection with GHG and aerosols emissions are represented in Fig. 1, Fig. 7, but power plants in converting energy from thermal to electrical energy also generate waste heat, mostly released at the Earth surface.

For more than a century, scientists had suspected that cities impact rain patterns. Nowadays there is increasing observational evidence [81] that urban land cover can have a significant effect on precipitation variability. The urban heat island, the city structures and the pollution all interact to alter rain storms around cities [82]. Increased temperature may provide a source of buoyant unstable air that rises and the city's buildings provide a source of lift to push warm, moist surface air into the cooler air above it. Thanks to urban aerosols that act as cloud condensation nuclei, this hot humid air can develop into rain clouds that soak the area downwind up to 50–100 km. Large urban areas and urban environment alter regional hydro-climate, particularly precipitation and related convection processes which are key components of the global water cycle and a proxy for changing climate.

Even if the total human-produced waste heat is only about 0.3% of the heat transported across higher latitudes by atmospheric and oceanic circulations, recently the research conducted by Zhang [83] showed that, although the net effect on global mean temperatures is nearly negligible (an average increase worldwide of just 0.01 °C), the waste heat generated by metropolitan areas can influence major atmospheric systems, raising and lowering temperatures over hundreds of kilometers. However, the noticeable impact on regional temperatures may explain why some regions are experiencing more winter warming than projected by climate computer models.

In this paper, Zhang based his calculations on the 2006 world's total energy consumption that was equivalent to 16 TWh (20.4 TWh in 2011 according to the IEA [95c]), of which an average of 6.7 TWh was consumed in 86 metropolitan areas in the Northern Hemisphere, where energy is consumed and dissipated into the atmosphere as heat. The results of the Zhang computer model show that the inclusion of the energy use at these 86 model grid points exceeds 0.4 W m−2 that can lead to remote surface temperature changes by as much as 1 K in mid- and high latitudes in winter and autumn over North America and Eurasia.

The effect of waste heat is distinct from the so-called urban heat island effect. Such heat islands are mainly a function of the heat collected and re-radiated by pavement, buildings, and other urban features, whereas the Zhang study examines the heat produced directly through transportation, heating and cooling units, and other activities. The long lifetime of CO2 and GHGs in the atmosphere and their cumulative radiative forcing are higher than waste heat warming. However the latter may be important for the short-term effects, and the next decades as the growth of total energy production will not stop [84].

According to Nordell [85a,b] heat dissipation from the global use of non-renewable energy sources has resulted in additional net heating. His 2003 paper “Thermal Pollution Causes Global Warming” was quite commented [86a–d] but since then, modeling performed by Flanner [87] suggested that waste heat would cause large industrialized regions to warm by between 0.4 °C and 0.9 °C by 2100, in agreement with Chaisson's estimates [88], thus showing that anthropogenic heat could be a minor but substantial contributor to regional climate change, and have local climate effects [89], [90], [91].

Besides the GH effect, for later generations the anthropogenic heat release can become dangerous. The UREs presented in this paper can contribute to the growth of global energy production without GHGs emissions, and cooling the Earth instead of warming it.

The global average primary energy consumption (0.03 W m−2) is relatively small compared with other anthropogenic radiative forcing effects, as summarized in the 2007 IPCC report [80]. Nevertheless, despite its relatively small magnitude, power plants waste heat may have a considerable impact on local surface temperature measurements and important potential impact in future climate. Even if our current global primary energy consumption which amounts only to 16 TW and is nothing compared with the 120,000 TW of solar power absorbed by the Earth, what matters is the balance between how much heat arrives or leaves the Earth. The UREs presented in this paper might cool the Earth at the ground level and not warm it, thus help to maintain the Earth's energy budget.

4.2. Renewable energies have some dark side
Even renewable energies produce local heat, although they provide a greater thermal reduction benefit by avoiding CO2 emissions.

Photovoltaic [92] solar panels are mainly black or dark with very low albedo and high emissivity, typically absorbing about 85% of the incoming light, 15% of this is converted into electricity, the remainder 70% of the energy is turned into heat. Millstein [93] found that the large-scale adoption of desert PV, with only 16% albedo reduction, lead to significant local temperature increases (+0.4 °C) and regional changes in wind patterns. Of course several studies have proven the utility of roof PV panels urban cites [94] and the overall balance is positive.

According to IEA [95] the total (dark) collector area of unglazed water collectors for swimming pool represents 18 km2 in the USA and 4.7 km2 in Australia. Inside urban environments PV and solar thermal panels for warming domestic water might increase the local heat island effect, because they modify the albedo of the place where they are installed. However, the benefits of PV systems are bigger, as the direct effect of providing local power and the indirect ones as avoiding the use of fossil-fuel power plants (reduced emissions of GHGs and other pollutants, such as ozone precursors and regional improved air quality).

Concentrated solar power (CSP) [96] is a technology where a fluid is warmed by concentrated sunlight and this heat is used to produce vapor and rotate turbines. Depending on the CSP type, the Carnot efficiency is around 15–20%, the remaining energy is released as waste heat.

Some hydroelectric dams might also present some drawbacks in our warming world: in equatorial and tropical regions the anaerobic organic matter decomposition in the reservoirs depths releases methane in the atmosphere [97], [98], [99], and methane is a GHG with a global warming potential 25 times higher than for CO2. Dams also release N2O which is an ozone depleting gas and also a potent GHG nearly 300 times more harmful than CO2.

Large dams might also be related to earthquakes [100], [101], as well as deep geothermal energy which might be associated with induced seismicity. For instance deep geothermal research led to the cancellation of a project in Basel, Switzerland, after the high-pressure fracturing of rock around the well caused hundreds of seismic events some of them large enough (magnitude 3.4, 2.6 and 2.7) to damage property [102], [103].

cOcean thermal energy conversion (OTEC) consists to produce electricity by driving turbines with a hot source and a cold sink, thus pumping warm surface seawater and cold deep seawater through heat exchangers. It works best when the temperature difference between the warmer, top layer of the ocean and the colder, deep ocean water is about 20 °C, or more when possible. Deep injection of heat poses problem, as the heat life expectancy in depth and at the surface is quite different. Depth heat will stay there for years as the thermal resistance between the bottom of the oceans and the biosphere is large, while the surface heat will quickly be ended by exchange with the atmosphere, and with the cold source which is the space by clear sky. As a result, the deep layers of the ocean are warmed and by thermal expansion can add up to the current sea level rise problem due to global warming, which causes floods concerns for coastal cities and low altitude islands. Also, as the amount of energy transferred to the cold source is more than 20 times the work removed from the system, it could be better to develop solar ponds than OTEC: a greater temperature difference can be obtained, with a better Carnot yield, and neither the sea level rise, nor the biodiversity and biotopes modification problems.

If in the near future wind energy manages to represent a significant part of the energy production, large scale wind farms might affect local climate. Keith [104] found that very large amounts of wind power can produce non-negligible climatic change at local and continental scales and Keith also observed some large-scale effects. Wang [105] found that if wind turbines can meet 10% or more of global energy demand in 2100, they could cause surface warming exceeding 1 °C over land installations, but in contrast, surface cooling exceeding 1 °C is computed over ocean installations. Thus, if horizontal man-made surface wind modifications can impact the local climate, why not vertical updrafts? This idea will be developed in the next chapters of this review.

4.3. Can we enhance heat transfer?
In order to cool down the Earth at a global scale several techniques will be proposed in the next chapter as able to enhance heat transfer from the Earth surface to the middle or the top of the troposphere. The rationale can be explained with the help of Fig. 1, Fig. 7. The energy from the sun that reaches the Earth is primarily in the form of visible and near infrared light (although some other wavelengths of the electromagnetic spectrum are also present, as infrared energy (heat) and ultraviolet energy). About 31% of the sunlight (the albedo) is reflected back to space as it reaches the Earth system, by clouds, dust particles, aerosols in the atmosphere, and also by the Earth surface, particularly from snow- and ice-covered regions. About 69% of the sunlight is absorbed by the Earth system (atmosphere and surface) and heats it up; the amount transferred in each direction depends on the thermal and density structure of the atmosphere. Then the heated Earth (land, ocean and atmosphere) will radiate back this heat as longwave radiation, in some cases after having handled it by several processes: dry convection (sensible heat), evaporation (latent heat) and some conduction. But because the Earth system constantly tends toward equilibrium between the solar energy that reaches the Earth and the energy that is emitted to space, one net effect of all the infrared emission is that an amount of heat energy equivalent to ~69% of the incoming sunlight leaves the Earth system and goes back into space in the form of IR radiation (this process is referred as Earth's radiation budget).

4.4. Earlier computer study
While studying the effect of adding “ghost forcings” (heat source terms), Hansen and Sato [106] noted that the feedback factor for the ghost forcing they applied to the model varies with the altitude of the forcing by about a factor of two. Their study showed that adding the ghost forcings at high altitudes increases the efficiency at which longwave radiation escapes to space. Of course, the analysis of these results will depend on the cloud cover and of the altitude, but their results can be understood qualitatively as follows. Considering ∇T at the surface in the case of fixed clouds, as the forcing is added to successively higher layers, there are two principal competing effects. First, as the heating moves higher, a larger fraction of the energy is radiated directly to space without warming the surface, causing ∇T at the surface to tend to decline as the altitude of the forcing increases. Second, warming of a given level allows more water vapor to exist there, and at the higher levels water vapor is a particularly effective GHG. Nevertheless, the net result is that ∇T at the surface tends to decline with the altitude of the forcing.

Considering that clouds are free to change, the surface temperature change depends even more on the altitude of the forcing as shown by Fig. 9 from Hansen and Sato [106]. The principal mechanism is that heating of a given layer tends to decrease large-scale cloud cover within that layer. The dominant effect of decreased low level clouds is a reduced planetary albedo, thus a warning; while the dominant effect of decreased high cloud is a reduced greenhouse effect, thus a cooling. However, the cloud cover, the cloud cover changes, and the surface temperature sensitivity to changes may depend on characteristics of the forcing other than altitude (e.g., latitude), so the evaluation requires detailed examination of the cloud changes and was further studied in Hansen's paper.


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Fig. 9. (a and b) Surface temperature change as a result of a forcing (reproduced from Hansen [106]).

In Fig. 9a Hansen has represented the surface air temperature sensitivity to a globally uniform ghost forcing of 4 W m−2 as a function of the altitude of the forcing. ∇T0 is the surface temperature response without any climate feedbacks allowed to operate. In Fig. 9b the feedback factor at which the ghost forcing is inserted is represented as a function of the altitude.

4.5. Cooling by irrigation
It is well known that increased evaporation has a cooling influence locally and a warming influence wherever water condenses. It could be anticipated that if water condenses at high altitude, the drier hot air will rise and release part of its energy out to space.

The extent of global warming might have been masked to some extent by increased irrigation in arid regions using ground water and demonstrated by Boucher [107]. For instance Lobell [108] found that, by introducing large amounts of water to the land surface via irrigation, there is a substantial decrease in daytime surface air temperatures during the dry season, with simulated local cooling up to 8 °C and global land surface cooling of 1.3 °C. Each year, irrigation delivers an amount of about 2% of annual precipitation over land, or 2600 km3 of water to the land surface. Sacks [109] has confirmed local alteration of climate by irrigation, but concluded to an average negligible effect on global near-surface temperatures.

The semi-arid pasture land in Almeria, south-eastern Spain, has been progressively replaced by plastic and glass GHs for horticulture and intensive culture. Today, Almeria has the largest expanse of GHs in the world – around 26,000 ha. Campra [110] studied temperature trends in several regions and found that in the Almeria region, the GHs have cooled air temperature by an average of –0.3 °C per decade since 1983, meanwhile in the rest of Spain temperature has risen by around +0.5 °C.

The net influence of evaporation in global mean climate has been assessed by Ban-Weiss [111] and coworkers, who perform a highly idealized set of climate model simulations and showed that altering the partitioning of surface latent and sensible heat by adding a 1 W m−2 source of surface latent heat flux and a 1 W m−2 sink of sensible heat (i.e. decreasing the Bowen ratio) leads to statistically significant changes in global mean climate. This study suggests that for every 1 W m−2 that is transferred from sensible to latent heating, on average, as part of the fast response involving low cloud cover, there is approximately a 0.5 W m−2 change in the top-of-atmosphere energy balance (positive upward), driving a decrease in global mean surface air temperature of 0.54 K. This occurs largely as a consequence of planetary albedo increases associated with an increase in low elevation cloudiness caused by increased evaporation. Thus, their model results indicate that, on average, when latent heating replaces sensible heating, global, and not merely local, surface temperatures decrease. Ban-Weiss's “latent heat source simulation” consisting to increase the upward latent heat flux from the land surface to the atmosphere by 1 W m−2 resulted at the top of atmosphere in an increase in net shortwave radiation of 0.2+0.1 W m−2 (upward positive), and an increase in upward longwave radiation of 0.80+0.06 W m−2. Ban-Weiss concluded that his study points to the need for improved understanding between changes at the Earth's surface, and how they interact with fluxes at the top of the atmosphere to drive regional and global climate change.

5. ERM to produce thermal bridging
The GHG effect occurs in the longwave range and is mainly caused by the increase of CO2 concentrations. CCS and CDR address the cause of the GHG effect and if the CO2 concentration decreases, then the outgoing longwave radiation to the space increases.

SRM reduces the amount of energy reaching the Earth surface, addressing the incoming shortwave radiation by strategies and techniques producing a “parasol effect”. But compensating longwave radiation problems by shortwave radiation management, even if able to compensate on average for the same amount of temperature increase, is not equivalent and might for instance in more rain in some parts of the Earth, and droughts [112] in others.

We propose ERM that addresses the longwave radiation portion of the spectra represented in Fig. 10. The goal is to increase by different ERM techniques, both sensible and latent heat transfer out to space, and also radiation through the atmospheric window, the thermals, and all global surface longwave radiation.


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Fig. 10. Outgoing longwave radiation [7], [8] types targeted by ERM.

As GHGs are good insulating “materials”, we propose to create thermal bridging in the GHG envelope surrounding the Earth with gaps or breaks in this GHG insulating envelope in order to create pathways for heat loss that bypass the thermal insulation that causes global warming. These atmospheric thermal bridges are thermals and warm updrafts or cold downdrafts. Several devices called meteorological reactors are proposed, that can provide an uninterrupted “short circuit” between the surface level and the top of the troposphere, and then the outer space.

The atmospheric thermal bridges provided by these devices will result in a bypass with an accelerated heat loss from surface to the space through the thermal insulation caused by the GHGs. The MR proposed are power-generating systems that are able to transfer heat [113] from the Earth surface to the upper layers of the troposphere.

For instance, it is known that thunderstorms influence the climate system by the redistribution of heat, moisture and momentum in the atmosphere. The effects of convective updrafts from various types of clouds have been explored by Masunaga [114] and Folkins [115]. On short timescales, the effect of deep convection on the tropical atmosphere is to heat the upper troposphere and to cool the lower troposphere by moisture transport from the atmospheric boundary layer to the free troposphere. Cold rain and an atmospheric boundary layer cooling is linked with the atmospheric response comprising a lower-tropospheric cooling and upper-tropospheric warming, leading to a momentary decrease in temperature lapse rate.

Jenkins [116] as shown that especially in the case of mineral dust, the aerosols can also act as effective ice nuclei, enhancing the freezing of cloud droplets and thus increasing cloud updrafts and cold-rain precipitation.

From previously described computer simulations made by Hansen [106] and Ban-Weiss [111] and from real life global scale observations, it can be anticipated that a method or a strategy that will allow a power plant or an industry to transfer a considerable amount of their waste heat (dry or humid) at high altitude instead of rejecting it at the surface will somehow participate to cool the hearth. If numerous wind turbines can do it by acting on horizontal winds [105], probably that vertical drafts also. Unfortunately current dry or wet cooling towers used by the power industry do not fulfill these criteria. If a method or strategy that rejects heat in altitude is CO2-free, cheap and at the same time allows production of renewable energy the benefit could be important, not only for the climate but also for human and other living beings. Fig. 10 represents the different longwave radiation origins that are the target of ERM and Fig. 11 the principal unusual renewable energies that are proposed to reach this goal.


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Fig. 11. Principal longwave radiation targets of meteorological reactors.

6. Transferring surface hot air several kilometers higher in the troposphere
6.1. Solar updraft Chimneys: power plants that run on artificial hot air
A solar tower [117], [78], also called a solar aero-electric power plant, is like an inverted vertical funnel. The air, collected at the bottom of the tower, is warmed up by the sun, rises up and drives a turbine which produces electricity [118] (Fig. 12). Indeed, the thermal radiation from sunlight heats the air beneath a glass or plastic cover, the hot air rises up a tall chimney which causes a decrease in pressure. Thus, cold air is sucked by the rising hot air within the chimney, which creates surface wind inside the GH. At the bottom of the chimney there are several turbines [119] that catch the artificial wind coming into the chimney. The turbines generate electricity. Thermal energy storage [120] under the collector allows peak load and night production. The promoters of this technology expect it to be cost-competitive with electricity from the grid, meeting the demand profile and thus being the first non-intermittent renewable energy source to reach a primary provider status. Of course several solutions exist or have been developed for energy storage of other intermittent renewable energies, as thermal storage for CSP (high temperature melted salts in tanks, for 2 or 3 h), chemical batteries or hydrogen production (from water electrolysis) for wind turbines and PV. All these storage systems have for the moment low storage capacity and require high investment costs.


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Fig. 12. Working principle of a solar updraft chimney [121] the air buoyancy is due to the temperature differential, which implies density differences under the greenhouse cover, thus a pressure difference and an updraft.

Pumped-storage hydroelectricity is the most established technology for utility-scale electricity storage and has been commercially deployed for decades. The world pumped storage generating capacity is currently about 130 GW. This energy storage method is in the form of potential energy of water. The facilities generally use the height difference between two natural or artificial water reservoirs and just shift the water between reservoirs. Low-cost off-peak electric power from nuclear power plants or excess electricity generation capacity from wind turbines is used to run the pumps and transfer water to the higher reservoir. During peak load or for load balancing water is released back into a lower reservoir through a turbine generating electricity. Reversible turbine/generator assemblies act as pump and turbine.

Compressed air energy storage in underground caverns or in old salt mines is also an energy storage possibility, but few locations exist and storage capacity is lower than for pumped storage hydroelectricity. Also as the compression of air generates heat and the air expansion requires heat (the air is colder after expansion if no extra heat is added) the system is more efficient if the heat generated during compression can be stored and used during expansion, but this increases the investment costs and the complexity of the system.

For the SCPP, which is a low temperature difference thermal power generation system, gravel, water in plastic bags or tubes, and even the soil can be natural energy storage materials. Adding the storage capacity is relatively cheap. Considering the large area of the collector, the SCPP can generate output power continuously and steadily day and night. The use of low temperature solid/liquid phase change materials (PCM) will considerably increase the initial investment of building a commercial scale SCPP.

Quite numerous prototypes have been built in different countries, but only a unique large SCPP prototype was built in the 1980s in Manzanares, Spain by Schlaich [122], [123], [124] and produced 50 kW. According to an announcement from the private company EnviroMission at the end of December 2011, the 200 MW La Paz Solar Tower Project in Arizona, USA, should be on line during the first quarter of 2015 [125]. However, the 200 MW figure seemed over estimated, as the same company announced for 2006 a similar power plant in Buronga, Australia, which targets the same power output with a 1.3 times taller chimney and an almost 2 times larger GH, but nevertheless a 30 year power purchase agreement was signed with the Arizona power authority [125].

Another private new competitor appeared [126] which intends to develop 200 MW projects and announced having already purchased a 127,000 ha site surrounding the township of Tuckanarra, in the Mid-West region of Western Australia.

Two years earlier, in December 2009, it was announced that a much smaller 200 kW SCPP demonstration pilot was completed in Jinshawan, Wuhai, Inner Mongolia, China, and that a 25.1 MW SCPP was scheduled for December 2013, the construction being expected to account for 2.510.000 m2 of desert area and 1.26 billion RMB investment [127] ($200 million).

The effects of water vapor and possible condensation in a large SCPP are an important issue and were investigated by several researchers, particularly by Kröger [128]. Of course, water should not be evaporated under the GH as it will reduce the power output because of the latent heat of vaporization needed, and as a result the air temperature differential will decrease; but if moist air enters inside the GH, it improves the plant driving potential and condensation may occur inside the chimney of the plant under certain conditions, releasing inside it the latent heat of condensation. Pretorius [129] described a plant model that takes into account the effect of water vapor in the air inside and outside the plant, and considers the possible condensation of the air inside the chimney of the plant.

Ninic [130] studied the impact of air humidity on the height potential (the height at which disappears the buoyancy force of the collector air ascending with no solid chimney) and on the increase of the operating potential and efficiency of the whole plant. The height potential could be considerably increased if the air entering the collector is already moistened.

The cloud formation in the plumes of SCPPs was studied by VanReken [131] and the results indicate that for very high water vapor concentrations, cloud would probably form directly inside the chimney; with possible precipitation in some cases. For more moderate water vapor enhancements, the potential for cloud formation varied seasonally and was sensitive to the assumed entrainment rate. In several cases there was cloud formation in the plume after it exited the chimney. The power plant performance can probably slightly be reduced by these clouds, but these low altitude clouds could also have a beneficial effect on GW by albedo modification.

Zhou [132] studied the special climate around a SCPP and then, using a three-dimensional numerical simulation model, investigated the plume of a SCPP in an atmospheric cross flow [133], with several wind speeds and initial humidity hypothesis. It was found by Zhou that relative humidity of the plume is greatly increased, due to the plume jet into the colder surroundings. In addition, a great amount of tiny granules in the plume, originating from the ground or contained in the air sucked, act as effective condensation nuclei for moisture, and condensation would occur. A cloud system and precipitation would be formed around the plume when vapor is supersaturated, with maybe some beneficial effects in the deserts where SCPPs are intended to be built.

Furthermore, the latent heat released from the condensation of supersaturated vapor can help the plume to keep on rising at higher altitudes. Even if it depends on wind conditions [134], the plume often reaches more than 3 km up to 4 km which was the upper limit of the Zhou simulation model (Fig. 13). The numerical model can probably be improved, especially with larger spatial dimensions, but nevertheless this first work on the subject is instructive. Several chimney shapes have been studied [135] but little research as yet been done on implementing a convergent nozzle throat at the top of the tower, increasing output air speed and condensation nuclei concentration by reducing the flux area.


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Fig. 13. From Zhou [133] streamlines for atmospheric cross flow with 40% (a) and 80% (b) relative humidity.

In order to release the air at higher altitudes, the SCPPs can be built in higher locations on Earth, where insolation and average yearly temperature are optimal. In Mexico, the city of Nogales is located at an altitude of nearly 1200 m, and has almost the same temperature and insolation [136] characteristics than La Paz, Arizona, USA, located only at roughly 130 m altitude. So the air of a similar SCPP will already get out 1 km higher. Similar high insolation locations at high altitude can be found elsewhere, for instance in the Atacama Desert.

Zhou [137] described a SCPP with a floating chimney stiffened onto a mountainside and analyzed the power generation potential in China's Deserts. This type of SCPP is expected to be less expensive taking profit of local mountains, and is suitable for the special topography in China with vast desert belt surrounded by high mountain chains up to thousands of meters. His results show that the possible power obtained from the proposed floating SCPP in the Taklamakan or in the Badain Jaran Deserts can satisfy the total electricity consumption in China, and that the total expected power in the 12 Chinese deserts and sands, reaching more than 25,000 TWh per year, can even supply the electric power needs of the entire world.

Over the course of the 21st century we will probably progressively shift to an electricity-based economy [138], as all the renewable energies (wind, hydro, concentrated solar power, photovoltaic, geothermal, tidal, wave and biomass) and nuclear energy essentially produce electricity. The global electricity output is currently estimated at ~5000 GW. Some scientist like Cherry [139] and Doty [140] proposed that after peak load, the unused capacity of power plants could be diverted to recycle CO2 and produce high energy content and easy to carry domestic or vehicle synthetic fuels. Thus maybe more than 10,000 GW will be needed for the entire world needs.

As the GH collector of a SCPP is the most expensive constituent (50–70% of total cost), Bonnelle [141], [142] imagined several concepts of solar chimneys without collector canopy, for instance tropical ones floating over the hot ocean, and whose working mechanism is based in latent heat of condensation. Hot air saturated by moisture enters the bottom of the chimney, and the driving force that lifts up all the air column is the water condensation several hundred meters before the tower exit: the latent heat released quite high in the tube warms the air inside and the buoyancy produced pumps up the entire air column. Distillated water is a sub-product than can be valued; Authors imagined that lots of these SCPPs could cool the ocean surface, and might prevent or reduce intensity of hurricanes. Hagg [143] developed similar concepts called “hurricane killers”.

6.2. Discussion about the cooling effects of kilometric high chimneys and towers
It should be noted that the SCPPs are generally intended to be built in deserts, where albedo is generally high and air is quite dry, whereas the concepts proposed by Bonnelle or Hagg are applied at oceanic locations, where albedo is lower and air is quite humid. The purpose of the SCPP is to produce renewable electricity, but the yield is relatively low, 3% in theory, but only 1–1.5% in practice for a 1 km high tower after subtracting pressure drop and other loses [142]. So at the exit of the chimney, the hot air has still some kinetic and thermal energy. This thermal energy could be radiated back to space and thus help cooling the Earth by outgoing longwave radiation, increasing mostly sensible heat flux, as targeted by SCPPs in deserts, or latent heat flux for Bonnelle's tropical towers.

At this point the question is: will the heat released by the SCPP at the top of the tower be trapped in the troposphere, or a meaningful amount of it will escape out to space as longwave radiation? The “ghost forcing” simulations made by Hansen [106] give confidence on a positive answer, but at what extent? At ground level the greenhouse of the conventional SCPPs trap almost all the solar radiation that otherwise would have been reflected: thus will the radiation back to space be larger on average?

This evaluation is out of the scope of this review and needs further studies, but maybe a very simplified calculation can be intended here. According to the NASA Earth observatory [144] at an altitude of roughly 5–6 km the concentration of GHGs in the overlying atmosphere is so small that heat can almost radiate freely to space. A simplified calculation can be made at the altitude of 5500 m, which is roughly the point in the atmosphere where half the amount of air is below and half is above [145]. Thus, it can be assumed that if heat is transferred at this altitude, as the infrared radiation will be emitted in all directions, the re-absorption will at least be cut in half (some downward radiation will be reflected). The Sun's energy electromagnetic radiation output is composed of approximately 9% ultraviolet (UV) rays, 41% visible light, and about 50% IR. At the Earth's surface the composition of the electromagnetic radiation is on average 3% UV, 52% visible light and 45% IR. Of course the longwave radiation of the hot air gases that are rejected by the SCPPs have not the same spectral composition than the Sun's radiation. The 50% IR downward radiation of our hypothesis will mainly be in the 7–15 µm region as water absorbs IR in the 7 µm and in the 15 µm region, but reemits at 7, 10 and 15 µm meanwhile CO2 absorbs in the 15 µm region and reemits also at 7, 10 and 15 µm. Both H2O and CO2 reemit in the atmospheric window around 10 µm. Depending on the Earth location, there will be more or less aerosols, dust, humidity, clouds and scattering particles, but on average instead of normal distribution we will have y ~69% of energy gone to space and only ~31% reabsorbed into the atmosphere. In other words, the air which will be either warmed up at 5500 m or transferred already hot at this altitude will lose at least 30% more heat than “normal” air warmed at ground level and submitted to the GH process.

As the polar tropopause is reached at an altitude of nearly 9 km, and the tropical tropopause at 17 km, the altitude of 5500 m might be too conservative as the upper layer of many clouds and dust particles might reflect backwards some radiation going down. As a matter of fact
•
man-made aerosols and cloud formation nuclei are mainly located at a lower altitude;

•
cirrus clouds (found in 43% of some satellite observations [146]) are semi-transparent in the infrared and their mean effective emissivity is between 0.5 and 0.6;

•
in coastal environments, coarse particles are found to account for roughly half of the total scattering and 70% of the backscattering for altitudes up to 1000 m [147].


SCPPs concentrate the heat [148] of a very large area (38 km2 for a 200 MW model power plant), trapping it under a canopy of glass, instead of letting that heat dissipate into the surrounding countryside or rise to the atmosphere just above. The SCPPs release this heat at a higher altitude (1 km for the Australian project, 1.5 km for the Namibian project) through a chimney of smaller cross-sectional area (13,300 m2, 130 m diameter,). Thus at the output the thermal column escaping the tower is concentrated more than 50 times (as well as the moisture condensing nuclei naturally present in the air).

The idea of transporting heat upwards through the atmosphere and contributing to lower surface temperature by increasing the flow of upward energy via convection, and then dispersing that heat with the aim to radiate it to space has been proposed by Wylie-Sears [149] and by Pesochinsky [150], but not at the same height. The idea of dissipating energy by high altitude thermal radiation was also suggested by Mochizuki [151a,b]. Both ideas will be exposed later.

Artificial thermals are created by the hot air exiting from the chimneys of the SCPPs. The warmer air expands, becoming less dense than the surrounding air mass. The mass of lighter air rises and, as it does, it cools due to its expansion at high-altitude lower pressures. Colder air is displaced at the top of the thermal, causing a downward or lateral moving exterior flow surrounding the thermal column. The rising parcel, if having enough momentum [152], will continue to rise to the maximum parcel level until it has cooled (by longwave radiation in all directions) to the same temperature as the surrounding air, or until negative buoyancy decelerates the parcel to a stop.

About 89% of the outgoing infrared radiation is affected by the GH effect. The GHGs cause both the absorption and the emission and as the heat must be radiated away, IR fluxes have to be considered. As all gases radiate both up and down, some of the lifted energy by the chimney will be radiated down, with maybe on average little cooling effect, taking into consideration the albedo change made by the solar collector of the SCPP at ground level.

6.3. The two hypotheses for the air released in altitude by SCPPs
First case: if the extra heating is released by the chimney at an insufficient altitude (2–3 km high) it might suppress natural convection to the same extent as the injected extra heat, tending to keep the troposphere with a constant lapse rate and having caused no significant net effect as the atmosphere is often stratified at some ranges of altitudes. If the environmental lapse rate is less than the moist adiabatic lapse rate, the air is stable – rising air will cool faster than the surrounding air losing its buoyancy. Also, a part of the extra heat released can be compensated by the drag produced by the updraft which creates a similar and opposite force to counter that from the buoyancy, thus leading to a temperature increase in the Polar Regions.

Second case: if the extra heating is released by the chimney at a higher altitude (3–5 km), as the lapse rate cannot get any greater than the dry adiabatic rate, local convection will increase the heat. The heat has still to be radiated after that; but the warmer the air or the clouds, the more heat will be radiated from them. Of course radiation will occur in all directions, and thus some of the heat will be radiated down, but the net outgoing longwave radiation to space will still be increased, even if the effect will be partially offset by decreased convection elsewhere after the plume dissipates.

Indeed, the average global cloud height is linked to the average global temperature. Generally, the higher the average cloud height, the higher the average surface temperature, and vice versa [153]. The IR emission by clouds to space represents 26% of the incoming solar radiation, almost the same amount that all the reflected short-wave solar radiation (31%) by clouds, aerosols, dust and the surface. And the lower the average clouds height is, the hotter the clouds are, and thus the more radiation they lose to space, which means the surface stays colder. So SCPPs releasing quite humid hot air can probably be shorter than those releasing relatively low moist hot air (the height potential previously described by Ninic [130]).

It should be noted that SCPPs work on a 24 h/day basis, thanks to thermal storage, there is no intermittency, so the air flux never stops and is quite important: for a 200 MW SCPP with a chimney diameter of 130 m and an air speed of 11.3 m s−1 at the turbines, the amount of air that comes out at the top of the tower is of 12.6 km3 day−1. Although the current objective of SCPPs is to produce electricity and not to accelerate air in order to send it higher in the troposphere, kinetic energy can be given to the air by reducing the tower outlet diameter, thus increasing the air speed and the initial diameter of the thermal column.

To answer the initial question, more numerical simulations are needed using, for instance Kelvin–Helmholtz instability in a spatial way and at a larger scale than used by Zhou [133] to take into account all the parameters, including the altitude of the location where the SCPP will be built, night and day temperatures during the different seasons and different registered wind speeds and humidity levels. Even if in some cases SCPPs do not cool directly the Earth by longwave radiation back to space, at least they provide indirect benefits like avoiding nearly 900,000 t of CO2 emissions every year for each 200 MW SCPP and this should also be taken into consideration.

Transporting heat upward through the atmosphere and contributing to lower surface temperature was the goal of this chapter.

Further to the previous subsection about waste heat and thermal power plant emissions, it could seem obvious that if all nuclear, thermal and fossil carbon power plants increased significantly the height of their exhaust chimneys (currently only 100–200 m high) in order to their exhaust gases to pass the boundary layer, local cooling could occur not only by heat transfer but also by plume cloud formation, even if at the global scale no net cooling would result. This idea will be developed later in a further subchapter giving examples of the endless possibilities of use of high towers for global warming reduction. The idea is that for polluting coal power plants, higher chimneys mean sulfate pollution emissions at higher altitude with better dilution and transport and longer tropospheric duration.

6.4. Super chimney
The super-chimney imagined by Pesochinsky [150], [154] consists in a huge vertical open duct at both ends, which works as a giant vacuum cleaner, transferring hot air from the sea level to the atmosphere 5 km higher, where temperature is −30 °C. The principle consists in the chimney effect based on the fact that hot air rises by buoyancy above cold air, because hot air is less dense and therefore lighter than cold air. But the process can be made more intense preventing the mixing of warm and cool air, so a chimney prevents inside air from mixing with the outside air until the air exits. The chimney stack effect needs a differential of temperature between the air inside and outside to run correctly. Moreover, the higher the chimney is, the more efficient it is.

It is a similar concept to previously described SCPP [148], except that there is no solar collector at the bottom of the tower, which usually couples the GH effect to the sucking effect of the chimney. According to Pesochinsky the temperature difference between the bottom and the top of the tower is sufficient. Another difference with conventional SCPPs concerns the size, 5–10 times bigger: up to 10 km high with a diameter of up to 1 km. Even if these heights have never been reached by human buildings, some GE / CE projects reported in the initial part of this review envisioned similar heights [59], [60], [61], [62], [63], [64], [65].

Furthermore, some authors reported, with such a large duct and in certain atmospheric conditions, that a cold air inflow could occur at the top and as a result a layer of cold air could get out at the bottom of the chimney, the hotter air surface being just pushed up, with the creation of a thermal inversion. In terms of heat transfer the result is nearly the same: cold air down and hot air up.

Indeed, on some Pesochinsky's designs, the tower is alongside a mountain slope or drilled inside a mountain (which seems too expensive) and numerous air pipes are connected on the sides. Di Bella [155] suggested a similar concept by using giant open pit-mines and also recycling waste-heat from power plants. This heat input could be useful to prevent cold inflow entering these large diameter chimneys. To illustrate the potential of these devices, according to Pesochinsky's calculations [156] only 10 super chimneys 5 km high can offset the heat surplus in the Earth atmosphere, which causes current global warming. This would mean that all the atmospheric circulation would be completely reorganized from only 10 points on the Earth’s surface: the climate induced perturbations could be much worse than what we want to avoid. Hopefully with smaller, cheaper and more numerous super chimneys, better distributed on the surface of the planet, this deleterious effect can be avoided. The calculations done are rather simple, and were confirmed by Mudde [157] from Delft University of Technology. They are based on a difference of temperature of 50 °C and as the super-chimney will facilitate air convection by bringing masses of warm air up to 5 km, then when the heat from the air radiates out, as it will be already at high altitude, less energy will be reabsorbed by the atmosphere, due to a thinner layer of atmosphere to go through. Therefore, more heat will be leaving the atmosphere, thus reducing the global atmospheric temperature. The authors believe that more scientific studies are needed to prove the concept, and that the technology still fairly mature to build 5 km high chimneys.

Constructional generalities are given by Pesochinsky with no real details: tall skyscrapers already exist; unlike chimneys, buildings entail much heavier construction because there are floors, ceilings, several fluids and lifts going up and down, and all other elements within buildings which are necessary to make it useful for humans. A chimney is just a cylinder, thus is a much lighter structure and can be build a lot taller than any building with new “super-strong” materials, not even described by Pesochinsky.

6.5. The hot air balloon engine to release air in altitude
Edmonds [158a,b] developed the idea of producing electricity by using hot air balloons directly filled by air heated by the sun, by means of a glazed collector (Fig. 14) like in solar chimneys. This system can be summarized as a SCPP where the tall chimney is replaced by a balloon, filled by the hot air produced under the greenhouse. For Edmonds, the lift force of a tethered solar balloon can be used to produce energy by activating a generator during the ascending motion of the balloon. The hot air is then discharged when the balloon reaches a predefined maximum height. Edmonds predicted the performance of engines in the 10 kW–1 MW range. The engine can operate over 5–10 km altitude with thermal efficiencies higher than 5% comparatively to 1–3% for SCPPs.


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Fig. 14. Elements of the hot air balloon engine as proposed by Edmonds [158] with balloons operating till 10 km in altitude.

The engine thermal efficiency compares favorably with the efficiency of other engines, which also utilize the atmospheric temperature gradient but are limited by the much lower altitude than can reach the concrete chimney. The increased efficiency allows the use of smaller areas of glazed collectors than for SCPPs and the preliminary cost estimates suggest lower prices of the kWh produced, as there is a lower building cost.

Then Grena [159] proposed some variants, for instance the use of two balloons bound together (Fig. 15a), the use of warm saturated air from a source such as the cooling tower of a power station or the use of transparent balloons containing inside a black absorber that is directly warmed by the sunlight (Fig. 15b). Grena suggested that the upward drift due to solar energy and the lateral drift due to wind can both be used to generate energy.


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Fig. 15. (a) Double balloon variant of solar balloons proposed by Grena [159]: one balloon ascends meanwhile the second descends and recharges, slowing down the ascent of the first. [158]. (b) Another variant of the solar balloons proposed by Grena: the sun radiation is absorbed by a black collector inside the transparent balloon and heats the air, generating a lift that actions a turbine. (c) Another variant of the solar balloons proposed by Grena: a support balloon filled with Helium is associated with a drive balloon filled with waste heat from power plants, for instance warm and saturated air from a cooling tower. The latent heat of condensation of the humidity allows the balloon to go much higher, up to 10 km. For the descent, the drive balloon is emptied of the hot air at its maximum altitude. The support balloon slows down the descent. The water condensate can be recycled [158]b.

In a variant, a couple of balloons are used: a big drive balloon filled with hot air and a smaller support balloon filed with helium (Fig. 15c), both connected to an electric generator by a rope. While ascending several kilometers the balloons perform work on the electric generator. At some maximum height of the order of 10 km the larger drive balloon discharges all its hot air into the cold upper atmosphere (thus transferring heat from the Earth surface to the upper layers of the troposphere). Then meanwhile the two balloons are hauled back to ground, the smaller balloon provides support for the empty envelope of the larger balloon. At some height, the latent heat of condensation of water vapor inside the drive balloon maintains the internal air temperature above ambient temperature and provides an increasing lift force with height, plus water. This balloons technology seems quite promising both to produce renewable energy with smaller investment costs than SCPPs, but also to cool the Earth as higher altitudes can be reached by the hot air.

The GE community might also be interested by this hot air balloon concept, as filling similar balloons by the hot flue gases coming out from the exhaust chimneys of polluting coal power plants, can be a very inexpensive way to send sulfates at high altitude, and at the same time produce electricity to compensate for the cost of the installation. The goal can be to install filters on the exhaust of the power plants to remove 95% of the sulfates released in the local environment. Only 5% of the flue gases coming out of the chimney will be used for filling the balloons that will climb 10 km high (or till the stratosphere). In this CE scheme it can be argued that the sulfates sent higher would have been released in the lower troposphere anyway and in an amount 20 times more important. Removing 100% of the SOx is not desirable as an immediate warming will result by the elimination of the low altitude reflecting aerosols.

7. Transferring cold air to the Earth surface
7.1. Downdraft evaporative cooling tower for arid regions
The hydro-aero power generation plants, also called downdraft energy towers (DET) [160], were first developed by Carlson [161] from Lockheed Aircrafts, and then by Zaslavsky and Guetta [162], [163], [164]. This concept was extensively studied by researchers, was the subject of numerous PhD theses [165], [166], and has even been associated to pumped storage and desalinization plants [167].

The DET is a power plant that uses seawater and solar energy accumulated in hot dry desert air to produce electricity. It includes [168] a tall downdraft evaporation tower, water reservoirs, pipes, pumps and turbines (Fig. 16). The DET has to be built inland, in the driest possible location, as the yield is reduced by moisture; but DET should not be too far from the sea, as sea water is needed and pumped by ducts till the DET. Seawater is then pumped till the top of the tower where it is sprayed with numerous nebulizers. The water droplets fall down and evaporate, creating a downdraft cold air flow which is denser than ambient air. The tower is quite large and high (typically 400 m in diameter and 1.4 km high) in order to reach humidity saturation. At the bottom of the tower the heavier artificial wind drives turbines. Only a nearly 1/3 portion of the electricity produced is needed to pump the water to the top of the tower (and from the sea).


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Fig. 16. Schematic illustration of the DET operation reproduced from Czisch [169] and Technion – Israel Institute of Technology.

Excess water is used and is not evaporated in order to collect the salt byproduct, also using an electro-coalescence device. In a hot and arid desert, the tower releases at its bottom huge amounts of cold and very humid air that might help to green the desert if some condensation occurs outside during colder nights. If there is a mountainous landscape around, a DET might produce inversion layers. Small altitude inland clouds might form, as sand dust could be good condensing nuclei (Fig. 17), thus increasing in-land albedo.

So this technology might well be the inland counter part of the ocean cloud whitening SRM geoengineering proposal by Salter and Latham [41], [43] using Flettner ships. Brackish water is returned to the sea, but geoengineers [170], [55] can imagine albedo strategies using this salty water to whiteness controlled and limited areas of desert where there is no groundwater tabs under it.

According to Zaslavsky [171] DET might help cooling the Earth, and actually reverse global warming, as by cooling air in desert regions, the DET could expand the effects of a global natural cooling process called “Hadley Cell Circulation” whereby the Earth cools itself, but occurring mostly only near the equator.

An US private company is developing a similar concept [172] making also profit of the lateral wind (Fig. 18a) to increase downward flow similar to a Japanese wind tower [173] project, and to existing cooling towers that also harness the wind, but at lower altitudes (Fig. 18b).


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Fig. 18. (a) First energy tower designs from wind clean energy tower [172]. (b) Example of existing fan less, cross-flow induced draft cooling tower. (c) Current energy tower designs from wind clean energy tower [172] very similar to the ones developed by Zaslavsky [162], [164] at the Technion institute (see Fig. 14). (d) Textile cooling tower in Bouchain [176], France 1980–1991

Multiple air inlets at several heights have also been experimented by Erell and Pearlmutter [174] in downdraft cooling towers. The initial structure (Fig. 18a) proposed by the Arizona “wind clean energy tower” can probably be cheaper to build, as there is less wind pressure outside. This company announced recently [175] having selected a site located in San Luis, Arizona, to pursue the construction of their DET facility, but they also come back to a conventional DET type structure (Fig. 18c) similar to the one developed at the Technion institute.

The cooling tower of a 250 MW power plant [176] has been made of a steel skeleton and textile cover made of polyester and PVC coating and with 10 t per meter of tensile strength. This was done in France by the national electricity company EDF after the original cooling tower made of concrete collapsed. After only 6 months of development and construction this refrigerant entered in service in 1980 at the Bouchain site, where it was kept in use for 10 years (Fig. 18d). When deconstructed the plastic sheet was still in perfect condition. The advantages noted by the company were its low cost, the short time of construction and the savings by the faster recovery of the production. If the DET planned in Arizona is made the same way instead of concrete or iron, it can probably be less expensive to build.

In 2003 Bonnelle [142] proposed a DET with a lighter structure (Fig. 19) from the fact that in DETs the pressure inside the duct is higher than ambient pressure, so the walls made of concrete or iron could be replaced by textile ones. Sorensen made a similar proposal with a SCPP, but needed to put the turbines at the top of the chimney as the air pressure inside the tower is lower than outside (Fig. 19).


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Fig. 19. Light structure proposed for DETs by Bonnelle [142] with the chimney made of textile as pressure inside is higher than outside

A comparison has been made of pros and cons of conventional DETs versus SCPPs by Weinrebe [178]: SCPPs appear more profitable, but the potential profits of the lateral winds have not been evaluated.

If the investment costs are much lower, for instance using ETFE foils, and if wind power can also be harnessed at the same time as proposed in the initial design by the Arizona company [172], [175], those mixed wind and DETs plants can probably be built closer of the coast, even if the humidity levels are slightly higher. Of course the moister the outside atmosphere, the lower will be the water evaporation so less cold air flow is produced and the power output will be smaller by the evaporative part of the plant. But saving pumping energy (estimated to consume almost 1/3 of total energy produced) and harnessing land breeze and sea breeze can compensate somehow thanks to the wind part of the plant. Thus the brine could be sent directly to the sea, saving initial investment in the tubes and in the electro-coalescence devices. With an investment in a directional output of the cold air towards the sea, these DETs would also be able to produce cloud whitening over the sea (CE SRM technology), with no need of the Flettner boats proposed by Latham and Salter [41], [43].

SCPPs and DETs present numerous advantages comparatively to the current wind turbines: their maintenance is easier as the turbines are at ground level; they use artificial hot or cold wind with no intermittency, 24 h/7 days production and, to increase the power, bigger ones can be built with local materials and adding more turbines of the same size, with no need to change the road infrastructure. As a matter of fact, for a single current giant wind turbine of 5-6 MW reached such a big size that transportation is becoming problematic from the manufacture site till their final installation working site.

Wind turbines operate with horizontal winds meanwhile the SCPPs and DETs exploit vertical air currents. Tidal turbines are the underwater equivalent of wind turbines and operate horizontal ocean currents. The perspective is that maybe the equivalent of underwater SCPPs or DETs will be developed to exploit the vertical currents or temperature or salinity differences among the great ocean conveyor belt [179] without disturbing it, and on the contrary with the aim to stabilize the thermohaline circulation [77].

Recently Bauer [180] has developed a one-dimensional low Mach number model applicable to both DETs and SCPPs.

8. Transferring latent (or sensible) heat to the top of the troposphere
8.1. Creating artificial tornadoes: the atmospheric vortex engine
The basic source of energy for tropical cyclones is heat transfer from the ocean. According to Renno [181], atmospheric convection is a natural “heat engine”. During one cycle of the convective heat engine, heat is taken from the surface layer (the hot source) and a portion of it is rejected to the free troposphere (the cold sink) from where it is radiated to space. The balance is transformed into mechanical work. Since the heat source is located at higher pressure than the heat sink, the system is capable of doing mechanical work. The mechanical work of tropical cyclones is expended in the maintenance of the convective motions against mechanical dissipation. Ultimately, the energy dissipated by mechanical friction is transformed into heat. Then, a fraction of the dissipated energy is radiated to space while the remaining portion is recycled by the convecting air parcels.

The energy cycle of the mature hurricane has been idealized in 1986 by Emanuel [182] as a Carnot engine that converts heat energy extracted from the ocean to mechanical energy. He derived the Carnot's theorem from Bernoulli's equation and the first law of thermodynamics. In the steady state, this mechanical-energy generation balances frictional dissipation, most of which occurs at the air-sea interface. The idealized Carnot cycle as illustrated by Emanuel is in Fig. 20. In the third leg of the Carnot cycle, air descends slowly in the lower stratosphere, retaining a nearly constant temperature while losing heat by electromagnetic radiation to space.


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Fig. 20. The idealized Carnot cycle as illustrated by Emanuel [182].

As represented by Emanuel in Fig. 20, in the hurricane Carnot cycle the air begins spiraling in toward the storm center at point acquiring entropy from the ocean surface at fixed temperature Ts. Then it ascends adiabatically from point c, flowing out near the storm top to some large radius denoted symbolically by point o. According to Emanuel the excess entropy is lost by export or by electromagnetic radiation to space between o and o' at a much lower temperature To. The cycle is closed by integrating along an absolute vortex line between o' and a.

Michaud [183] proposed several models for heat to work conversion during upward heat convection and completed a model [184] for calculating hurricane intensity. It is clear that real hurricanes are open systems that continually exchange mass with their environment, nonetheless, the Carnot cycle was considered as good enough until Emanuel's hurricane model has been improved, for instance by Smith [185]. In 2008 Renno [186] proposed a more general theory that includes irreversible processes. A heat engine cannot operate with heat flowing from a single reservoir; the second law of thermodynamics states that it is impossible to achieve 100% efficiency in the conversion of heat into work. Any real heat engine must absorb heat from a warmer reservoir and reject a fraction of it to a colder reservoir while doing work. Renno [186] published a thermodynamically general theory for various convective vortices that are common features of atmospheres: they absorb lower-entropy-energy at higher temperatures and they reject higher-entropy-energy to space, ranging from small to large-scale and playing an important role in the vertical transport of heat and momentum.

Emanuel [187] made estimates of the kinetic energy dissipation of real storms and showed that an average tropical cyclone dissipates approximately 3×1012 W. This is equal to the rate of US electrical power consumption of the year 2000; but an exceptionally large and intense storm can dissipate an order of magnitude more power. The thermodynamic disequilibrium that normally exists between the tropical ocean and atmosphere allows convective heat transfer to occur. For Emanuel [188] the increasing GHGs alter the energy balance at the surface of tropical oceans in such a way as to require a greater turbulent enthalpy flux out of the ocean, thereby requiring a greater degree of thermodynamic disequilibrium between the tropical oceans and the atmosphere. In 2001 Emanuel [189] made the supposition that much of the thermohaline circulation is actually driven by global tropical cyclone activity. In 2007 Sriver [190] computations showed that mechanical stirring of the upper layers of the ocean by cyclones may be responsible for an important part of the thermohaline circulation and provided some evidence that cyclone-induced mixing of the upper ocean is a fundamental physical mechanism that may act to stabilize tropical temperatures and cause polar amplification of climate change. If this proves to be the case, then the tropical cyclones are integral to the earth's climate system. D’Asaro [191] found that for hurricane Frances the net upwelling was about 15 m. The heat capacity of the ocean is much higher than that of the atmosphere. The heat provided by cooling a layer of water 1 m thick by 1 °C is sufficient to increase the temperature of the bottom kilometer of the atmosphere by 4 °C which would be a large increase in the heat content of the atmospheric boundary layer. Thus, according to Michaud [192], hurricane sea-cooling is primarily due to cooling from above and not to mixing of cold water from below as stated by Sriver [190] and D’Asaro [191] for whom sea surface cooling is due to ocean vertical mixing and not to air-sea heat fluxes.

Warm seawater is the energy source for hurricanes. Emanuel [193] argued that sea spray could not affect enthalpy transfer because droplets that completely evaporate absorb as much sensible heat as they give off in latent heat. As a matter of fact, without spray the interfacial sea-to-air heat transfer ranges from 100 W m−2 in light wind to 1000 W m−2 in hurricane force wind. Spray can increase sea-to-air heat transfer by two orders of magnitude and result in heat transfers of up to 100,000 W m−2, similar to the heat transfer per unit area obtained in wet cooling towers [192] (with a thermal capacity of 1000 MW, a diameter of 100 m and a the heat transfer area of 5000 m2). In hurricanes, drops of spray falling back in the sea can be 2–4 °C colder than the drops leaving the sea, thus transferring a large quantity of heat from sea to air. Michaud's calculations show that if the heat of evaporation is taken from the sensible heat of the remainder of the drop; evaporating approximately 0.3% of a drop is sufficient to reduce its temperature to the wet bulb temperature of the air. The heat required to evaporate hurricane precipitation is roughly equal to the heat removed from the sea indicating that sea cooling is due to heat removal from above and not to the mixing of cold water from below.

Trenberth [194] found that a large hurricane can produce 10 mm h−1 of rain over a 300 km diameter area. That gives a mass of rain of nearly 200×106 kg s−1; multiplying by the latent heat of vaporization, the heat required to vaporize the water amounts to almost 500 TW, an enormous amount of energy as the world's average electrical energy production is of 2 TW. According to Michaud [192], assuming that the intense heat flux takes place under the 5000 km2 area of the eyewall that gives an eyewall heat flux of 100,000 W m−2.

Based on the huge amount of mechanical and thermal energies of cyclones, Michaud [195], [196] proposed a very original and unusual device for capturing mechanical energy during upward heat-convection in the atmosphere.

Other scientists like Nazare [197], Mamulashvili [198], Coustou [199] or Nizetic [200] also proposed devices for producing an artificial vortex by capturing the energy produced when heat is carried upward by convection in the atmosphere like in hurricanes, tornadoes or dust devils. A man-made vortex reaching miles into the sky would act much like as a very tall chimney, where air density and temperature effects can be harnessed to produce electricity from low-energy content gases, such as those rejected from a cooling tower.

The heat source can be solar energy, warm sea water, warm humid air, or even waste heat rejected in a cooling tower. The atmospheric vortex engine (AVE) developed by Michaud consists of a cylindrical wall, open at the top and with tangential air entries around the base. Heating the air within the wall using a temporary heat source such as steam starts the vortex. Once the vortex is established, it could be maintained by the natural heat content of warm humid air or by the heat provided by cooling towers. Of course, there is reluctance to attempt to reproduce such destructive phenomenon as a tornado, but according to Michaud, controlled tornadoes, rather than create hazards, could reduce them by relieving instability. Indeed, a small tornado firmly anchored over a strongly built station would not be a hazard and the AVE could increase the power output of a thermal power plant by 30% by converting 20% of its waste heat into work.

Cooling towers are commonly used to transfer waste heat to the lower atmosphere. Michaud's AVE is supposed to increase the efficiency of a thermal power plant by reducing the temperature of the heat sink from +30 °C at the bottom of the atmosphere to −70 °C at the bottom of the stratosphere. The AVE process can provide large quantities of renewable energy, alleviate global warming, providing precipitation as well as energy. Recently Ninic and Nizetic [201a–c] as well as Natarajan [202] studied vortex engines and the technical utilization of convective vortices for carbon-free electricity production.

The Michaud's AVEs have the same thermodynamic basis as the solar chimneys. The physical tube of the solar chimney is replaced by centrifugal force in the vortex and the atmospheric boundary layer acts as the solar collector. The AVE needs neither the collector nor the high chimney. The efficiency of the solar chimney is proportional to its height which is limited by practical considerations, but a vortex can extend much higher than a physical chimney. The cylindrical wall could have a diameter of 200 m and a height of 100 m; the vortex could be 50 m in diameter at its base (Fig. 21) and extend up to the tropopause. According to Michaud, in a vortex, the centripetal force in the rotating column of air replaces the physical chimney and prevents cooler ambient air from entering the rising warm air stream. The rising air in the vortex chimney is continuously replaced by moist or warm air at its bottom. The chimney and the rising air column are essentially the same.

Each AVE is expected to generate 50–500 MW of electrical power. The energy will be produced in turbo-generators located around the periphery of the station (Fig. 21).


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Fig. 21. Atmospheric vortex engine concept from Michaud [195] (illustration by Charles Floyd).

The AVEs have the capacity at the same time to transfer heat from the surface till the tropopause (thus cool the Earth) and to produce large quantities of carbon-free energy because the atmosphere is heated from the bottom by solar radiation at the Earth surface and cooled from the top by infrared radiation back to space and this will be the driving force of AVE power plants. According to Michaud, the AVEs could be controlled and even turned off at will. This means that while the vortex may possess great power, it cannot become destructive and therefore is far safer than some CE proposals. The AVE concept has already been tested in small-scale models. The larger of the models was 4 m in diameter. A 34 m high vortex is exhibited at a museum in Germany. The main criticism against the AVE comes from the fact that it still sounds very theoretical and no extraction of energy from the vortex has yet been realized. Also the AVE would only work in very specific conditions designed to prevent the air vortex to leave the AVE as soon as power is drawn off to generate electricity and a pilot plant producing more energy than consuming is still expected. Nevertheless, the feasibility of the concept has been demonstrated theoretically and with small scale models, but not yet in an installation large enough to power turbines. Building a prototype of 8 m is underway and a 16 m is planned [203]. To fully demonstrate the AVE concept, a test a prototype might be built at an existing thermal power plant where a controlled heat source of relatively high temperature will be available. With 20–30% of the capacity of the existing cooling tower, the prototype would be able to accept a fraction of the waste heat from the plant and as a minimum will add valuable cooling capacity and reduce cooled water temperature for the plant without risk to the existing plant operation. Then, once the vortex control will be demonstrated under low-heat and low-airflow conditions, turbines could be added to the air ducts and a complete operational AVE system could be tested.

Recently a system similar to the AVE but using Papageorgiou floating type SCPPs has been proposed [204] to be installed on tropical oceanic barges.

It is worth noted that at a SolarPaces congress a SCPP has been proposed without turbines [205], to be used as dry cooling tower for large scale CSP field. In the context of this review where cooling the Earth surface is the goal, a similar approach can be discussed for AVEs. Even if AVEs where only used to replace cooling towers without any production of electricity, after the initial investment done to build them, a real benefit for the local climate can be expected. The saving on water and pumping can compensate for its cost and the AVE will dissipate at high altitude huge amounts of waste heat for almost free. The cost of an AVE with no turbines can be anticipated as quite small compared to the full cost of a large scale SCPP, which costs of construction and land acquisition have been a stumbling block for groups trying to replicate the Manzanares prototype design on a commercial level.

So even if to date the scientific and technological stage of development of AVE is less advanced than the SCPP and DET, it would take little investment for this technology to be in place quickly. Very rapid progress could be made especially since unlike SCPPs and DETs, AVE prototypes of intermediate size have their interest and can be profitable, whereas gigantism is required for the others. For this interesting tool to start it would suffice of the simple but real commitment of a single industrial of the conventional thermal energy sector, even without the purpose of producing energy.

9. Transferring surface sensible heat to the troposphere
9.1. Heat pipes and thermo-siphons
Heat pipes and thermo-siphons consists generally of a sealed metal shell, usually cylindrical [206a–c], and can transport large quantities of heat even with a very small difference in temperature between the hotter and colder interfaces. The devices are filled with a two-phase fluid, and the heat is removed thanks to evaporating and condensing processes. Inside them, at the hot interface, a fluid turns to vapor. Because of its higher pressure, the vapor generated, moves inside the pipe to the colder end zone, where condensation takes place at the cold interface. In a heat-pipe the liquid is then subjected to a capillary-driven flow, generating passive recirculation back to the hot interface to evaporate again and repeat the cycle. In a thermo-siphon the liquid falls down by gravity [207] back to the hot interface to evaporate again and repeat the cycle. Heat pipes and thermo-siphons differ by size (respectively small and big) and by the way the liquid comes back to the heat source (respectively by capillary action or by gravity). The main advantages of the heat pipes and thermo-siphons are their simplicity, the lack of moving parts, no electric power required, absence of noise and compactness and typically require no maintenance.

Heat pipes can work in all positions, included horizontally, thermo-siphons need to be vertical or inclined with a convenient slope and evaporator is bellow the condenser. For heat pipes, a wick structure exerts a capillary force on the liquid phase of the working fluid. The wick is generally located on the internal side of the tube's side-walls and is typically a sintered metal powder or a series of grooves parallel to the tube axis, but it may in principle be any material capable of soaking up the coolant [208]. Quite often both denominations are used indistinctly for any one of the two devices.

Typical heat pipes and thermo-siphons [209] (Fig. 22) consist of sealed hollow tubes made of a thermo-conductive metal such as copper or aluminum and containing a “working fluid” or coolant (such as water, ammonia, alcohol or mercury) with the remainder of the pipe being filled with vapor phase of the working fluid, all other gases being excluded. The materials and coolant chosen depends on the temperature conditions in which the device must operate, with coolants ranging from liquid helium for extremely low temperature applications to mercury for high temperature conditions.


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Fig. 22. Working principle of a heat pipe: a thermosyphon [209] (or heat pipe) is a metallic cylinder filled with a refrigerant (liquid+gas). When heat is absorbed at the bottom in the evaporation section A, the filling fluid boils and the vapor raises B. As the vapor reaches the condensing area C at the top of the cylinder, the heat is transferred to the outside environment and the vapor condenses inside. The liquid returns to A by gravity (or in heat pipes by capillarity through a wick D). The cycle can then start again.

The advantage of heat pipes is their great effectiveness in transferring heat. They are far more effective for heat conduction than an equivalent cross-section of solid copper. Heat flows of more than 230 MW m−2 have been recorded at Los Alamos Laboratories [210] for satellite and space flight applications (nearly 4 times the heat flux at the surface of the sun) with lithium inside a molybdenum pipe, which can operate at temperatures approaching 1250 °C. NASA is working with Los Alamos Laboratories to develop heat pipes for use in nuclear reactors to produce propulsion and generate electricity for spacecraft journeying to the solar system's outer limits.

The use of heat pipes has become extensive over past years for space satellites as they work well in zero gravity environments and also in many electronic devices, such as notebooks and microelectronics. In fact, for computers a remote heat exchanger is often used in order to allow a more compact design.

Other applications of heat pipes are “endless” [211] and include waste heat recovery in industrial boilers, gas–gas exchangers, steam generators, liquid metal heat pipes, high-temperature heat pipe hot air furnaces [212], etc. Heat pipes are also applied to solar heat collection for snow road melting, cooling for CSP power plants [151], cold energy storage for cooling data centers or hospitals, extraction of geothermal heat. Several studies have been conducted to use heat pipes in the nuclear [213], [214] industry, for instance to capture nuclear process heat, and transport it to a distant industrial facility producing hydrogen requires a high temperature system of heat exchangers, pumps and/or compressors. The heat transfer system envisioned by Sabharwall [215] is particularly challenging because of very elevated temperatures up to 1300 K, an industrial scale power transport (≥50 MW), but also due to a large distance horizontal separation of more than 100 m between the nuclear and industrial plants dictated by safety reasons. As will been seen later, vertical thermosyphons of this size [216] are already operating.

As seen in Fig. 23, thermo-siphons are also used to keep the permafrost frozen preventing the hot oil of the Trans-Alaska pipeline [217] to warm the soil; and also for permafrost preservation under roads and railways like in the Qinghai-Tibetan railway [218]; to prevent seepage in Earth dams by freezing soils in the structure foundations.


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Fig. 23. (a and b) Thermo-siphons on the Trans-Alaska Pipeline [217].

It has been suggested to use heat pipes to prevent icebergs and glaciers melting in Arctic ocean [219]. This latter application can counteract this effect of global warming, which acts as a vicious circle as the more polar ice melts, the lower is the albedo of the free water and the more heat is trapped instead of being reflected. A massive deployment of this already existing heat pipes technology can be considered as geoengineering if done in order to help counteract the sword of Damocles hanging over, with the possible melting of permafrost and methane hydrates and the release in the atmosphere of CH4, a GHG 25 times more potent than CO2.

9.2. Super power station or mega thermo-siphon
In 1996, the Dutch energy and environment agency Novem examined together with the industry group Hoogovens, a concept invented by the ocean engineer Frank Hoos [220], [221], [222], [223]. This project was beyond anything of what had been considered previously in terms of renewable energy power plant. The project called Hoos “Mega Power Tower” (HMPT) was developed to harness the difference in temperature between the warm ocean current of the gulf stream and the icy sub-zero (freezing) temperatures of the atmospheric upper air layers.

In 1992, a thermosyphon with a 37 m long evaporator has been studied [224], but the technological leap was huge.

The smallest version of the huge Hoos tower (Fig. 24) would have been 5 km high, with a diameter of 50 m. The highest and more efficient was set at 7.5 km high. It has to be installed floating on a pontoon at about 30 km from the coast where continuous water currents exist. At the time of the project, a mixture of butane gas and ammonia gas was chosen to circulate inside. This liquid mixture evaporates at the bottom of the giant thermosyphon, thanks to the ocean thermal energy (Gulf Stream), with gas velocities up to 180 km h−1 according to Hoos (but in between 20 and 60 km h−1 for operation). The top of the tube is frozen between −10 °C and −35 °C, thus liquefying the inner medium. By a central down-comer the condensed liquid falls down back to the heat source at the bottom, evaporates again and falls down again and again. That is at the same time the working principle of the weather machine (evaporating–condensing–raining), but with another fluid and the working principle of thermo-siphons (which generally have no moving parts, on the contrary of the HMPT).


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Fig. 24. Representation of the Hoos mega power tower HMPT project [225].

To make profit of the gravity, Hoss planned to install hydroelectric type turbines at the bottom of the central duct in order to generate electricity. The turbines of such a system were supposed to achieve performance up to 7 GW. The structure total weight was estimated to be 400,000 t, and in order to offset its own weight, four ellipsoidal balloons filled with lighter than air gas and with diameter of 360–900 m were proposed to be attached to the tower and sustain it.

Some more recent studies for other technologies can be adapted to this old project. As seen in Fig. 25 for an updraft floating SCPP developed by Papageorgiou [226], [227], more balloon compartments support the own weight of the structure and offer less resistance to lateral winds.


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Fig. 25. Papageorgiou's floating solar chimney concept (image from Bonnelle [228]a).

A higher two-tower version has been studied by Hoos who proposed a height of 7.5 km and at the tower top temperatures of −45 °C prevail. In the top of the 2d tower part, hydrogen would have been the circulating fluid, which in turn generates enough lift to be able to support its own weight without the support pillow on the tower shaft. In the lower segment, which would ground with a diameter of 2.5 km, a mixture of ammonia and butane was planned to be used as working medium. The finned heat exchanger at the top of the Hoos mega power tower would have a diameter of 1.2 km. The estimated cost was 30 billion dollars.

A feasibility study was conducted during more than 1 year on several technical aspects. It seems that under wind load conditions only small displacements can be traced, due to the enormous weight of the condenser, which functions as a stabilizer for the pipe below, and the floating base on the ocean. At that time therefore, the mechanical structure appeared technically possible, and the project credible for both the company who developed it and for the Netherlands. This 5 km or 7.5 km high HMPTs could seem unrealistic, but geoengineering projects envisions for instance a 15–25 km high hose to spray sulfates, and NASA conducted feasibility studies for “space elevators”. The authors believe that more scientific studies are needed to prove the concept, and that it is worth being reevaluated in light of technology evolutions made since the initial proposal by Hoss. Of course public acceptance of such high structures will probably be poor and building technology is not yet mature to build 5–7.5 km high pipes.

Also, due to its big size and power, the system makes it somewhat vulnerable. For example, a fault in the gas flow and half of a European country runs out of power. Better locations with lower wind speed patterns can probably be found.

Since April 2012, a 95 m high thermosyphon filled with a Freon gas is in operation to cool at −40 °C the inner detector of the ATLAS experiment at LHC [216] (CERN – Geneva in Switzerland). Its dimensions are quite modest and small compared to the ones of the Hoos project, but it is worth knowing it. Of course this system has no moving parts and is not intended to produce electricity.

A major problem in 1996 for Hoos was that the functionality of such a system can be hardly reduced to a scale test. But nowadays pilot tests might be possible: progress has been made in scientific knowledge on heat pipe and thermo-siphons [229], materials and technologies as well as in all the other engineering aspects of this ambitious project. For instance in membranes, shells [230] and fiber textiles for the lifting balloons, light steel tower, wind load, reduced structural risk. Also heat transfer efficiency of gas mixtures and new gases. Back in 1930, Einstein and Szilard were granted a patent [231] for a refrigerator with no moving parts (no compressor), like a double thermo-siphon, using ammonia–water and ammonia–butane mixtures together.

9.3. Mega thermo-siphon or ultra large scale heat-pipe
Mochizuki and his coworkers developed the concept [151] of giant thermo-siphons (Fig. 26) to help cooling-down the Earth.


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Fig. 26. (a and b) Concept of ultra large scale 10 km high vertical thermo siphon for cooling the Earth by Mochizuki [211]. The upper figure represents the secondary heat pipes placed horizontally at top.

The giant heat pipe proposed by Mochizuki [229] is not intended to produce electricity but has an improved design by an additional set of a different kinds of heat pipes at the top of the device that might improve the heat transfer. Preventing ice formation on the outside part of the heat exchanger can be a technical issue.

To evaluate the benefit of using such engines to transfer heat to the upper atmosphere to help offset radiative forcing due to GHGs and global warming can be done in the following way [232]: according to the IPCC and Hansen [233] the radiative forcing due to anthropogenic CO2 is about 1.7 W m−2. Over the Earth surface of 5.1014 m2 this amounts to 850,000 GW. The maximum of Carnot efficiency of a HMPT engine would be of about 20%. If the efficiency of the device is one half the maximum Carnot efficiency (i.e. about 10%) then in generating all the global electrical output of 5000 GW, the mega power tower engines would transfer about 50,000 GW to the upper atmosphere at an altitude of 7.5 km. Assuming the estimate made by Pesochinsky [150], [154] that infrared re-absorption would be cut in between half and 70%, this heat transfer would correspond to a decrease in radiative forcing of 35,000 GW or about 0.07 W m−2. That would offset only 4% the radiative forcing due to anthropogenic CO2 present in the atmosphere. But at the same time it will end all CO2 emissions from fossil fuel electricity production, and their wasted heat released at the surface, as the hypothesis was that these devices provided 100% of our current electricity consumption. So, less than 750 mega power towers can in theory solve the anthropogenic global warming problem. Of course the energy mix of tomorrow will be and has to be as large as possible, with a wide a range of technologies and solutions. In the same manner than for the HMPT concept, the authors believe that more scientific studies are needed to prove the concept, and that building technology for such high structures is not yet mature.

As a conclusion, heat pipe is a known and reliable passive technology that for the moment has been extensively used for heat transfer. For instance, 124,000 heat pipes are used to dissipate heat at the Trans-Alaska Pipeline, mounted on top of the pipeline's vertical supports and keep the permafrost frozen and intact by conducting heat from the supports to the ambient air. Without such pipes, heat picked up by the oil from its underground sources and through friction and turbulence (as the oil moves through the pipeline) would go down the pipelines supports anchored to the ground and would likely melt the permafrost: these 124,000 heat pipes prevent the pipeline to sink.

10. Other energy transfers to the troposphere to cool the earth surface
10.1. Polar chimney
Two similar concepts to the HMPT heat pipe engine have been proposed in 2008 and 2010 by Bonnelle [78], [142], [228]b as seen in Fig. 27 for the latter and in Fig. 8 for the former. In Polar Regions like in northern Norway or Alaska, where high mountains are close to the sea, this thermal machine has a gas mixture evaporated at the sea level in a first heat exchanger, and conveyed by a large diameter duct leaning against the relief, to the top of a mountain. A high tower sucks polar air by chimney effect and also captures the winds. A second heat exchanger at the bottom of the tower helps the gas mixture to condensate and to return downhill by gravity through a second duct (smaller in diameter), meanwhile warming up the air, that rises inside the chimney. A set of turbines collect the energy from this buoyant air, and other turbines make profit of the falling liquid. Not only the device can produce renewable electricity, but also helps sea ice formation and cools down the sea which reinforces denser water sink in deep currents.


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Fig. 27. Bonnelle's 2nd polar chimney concept [228]b.

In Bonnelle's previous concept [142] (Fig. 8), the difference was in the working fluid (water), transported in an open conveyor, cooled at the top of the mountain under a similar chimney, and carried back downhill just before freezing and being released in the open ocean [234]. With this previous configuration, at the tower output moist air is released, which can favor snow falls, and thus increase the polar albedo replacing old ice on glaciers, probably polluted with soot and black carbon by whiter and fresher snow with high albedo. The authors believe that this technology deserves more scientific studies to prove the concept, which is worth being evaluated in light of its capacity to re-ice the Arctic and to prevent methane hydrates destablization.

10.2. Taking advantage of energy potential of the undersea level depressions to install other pipelines and ducts useful to produce electricity and increase local albedo
The concept of helio-hydroelectric power was proposed in 1970 in a progress report on the feasibility of such a plant on the Eastern shore of Saudi Arabia, published by the King Fahd University of Petroleum and Minerals from Saudi Arabia. When topographical and hydrological conditions are favorable to build a dam from the sea or the ocean (an infinite reservoir at a constant level, the source), to a depression well below sea level (the closed reservoir or sink), the evaporation at the “closed reservoir” will tend to decrease its level inducing a flow to move from the infinite reservoir. Therefore the flow of water evaporated by the sun is transformed into a discharge from the “open sea” to the “closed reservoir”. Solar energy of evaporation has thus been transformed into hydraulic energy.

In 1972 Bassler [235] proposed the Qattara Depression near El Alamein, only 80 km away from the Mediterranean. The depression is 300 km long and 150 km wide and 135 m deep below sea level at its lowest point. Also in 1972–1973 Kettani and Peixoto [236], [237] suggested that the Dawhat Salwah of the Arabian Gulf (Persian Gulf) can be transformed into a large water reservoir, by building a dam from Saudi Arabia to Bahrain, and another from Bahrain to Qatar. Cathcart, Badescu and Schuiling also developed the concept [238], [239] and made several other science-fiction like proposals of helio-hydroelectric power plant locations.

In 1980, Assaf proposed a similar concept to this one and to the Zaslavsky's DET, covering a natural canyon [240]. According to Bassler [235], combining with pumped storage, the attainable capacity can reach about 4 GW peak load energy, as in the Qattara Depression region at a level of −60 m the surface area is of 12,000 km2 and the annual evaporation volume can be of more than 20,000 million m3 with current evaporation levels of 1800 mm per year. As salt deposits have a higher albedo than surroundings, a global cooling effect can be expected (and no risk for groundwater at proximity).

Hafiez [241] showed that the transformation of Qattara Depression into an isolated anthropogenic inland sea could provide some ocean level adjustment, as well as generate energy, induce rainfall over some of the adjacent desert, reduce hottest desert daytime and coldest night time air temperatures, and permit new local-use fisheries (aquaculture) as well as international tourism resorts. The concept of ocean level adjustment is worth being evaluated in the context of sea level rise by thermal dilatation and melting of continental glaciers.

Thus, these helio-hydroelectric power plants are able at the same time to produce renewable energy, prevent future CO2 emissions, change local albedo by salt crystallization [54] thus increase global cooling of the Earth, increase latent heat transfer from the ground to the atmosphere and energy transfer back to space, increase evaporation that might help green the deserts and stabilize sand dunes [242], provide useful raw materials for multi-industry use. Last but not least, it can prevent sea level rise, which is one of the principal global warming concerns, thus reducing the 200 million climate refugees expected by 2050 [243], [1] to be displaced by climatically induced environmental disasters.

10.3. Examples of the endless possibilities of high towers use for global warming reduction
Although CO2 is generally considered as well-mixed in the atmosphere, data indicate that its mixing ratios are higher in urban than in background air, resulting in urban CO2 domes: for example Idso [244] reported measurements showing that in the Phoenix city center, peak CO2 was 75% higher than in surrounding rural areas and averaging 43% on weekdays and 38% on weekends.

In 2009 Jacobson [245] reported that local CO2 emissions can increase local O3 and particulate matter (PM) due to feedbacks to temperatures, atmospheric stability, water vapor, humidity, winds, and precipitation. According to Jacobson, although the pollution health impacts are uncertain, results suggest that reducing local CO2 may reduce 300–1000 premature air pollution mortalities per year in the U.S. even if CO2 in adjacent regions is not controlled. Jacobson proposed CO2 emission controls and regulations on the same grounds that for NOx, HC, CO, and PM.

London was famous in the 19th century for its smog mainly due to air pollution, and as explained by Asimov [246] the air pollution declined by the construction of higher chimneys that disposed pollution in height in a way that made it fall back to Earth several hundred kilometers away. Of course, the initial problem of poor air quality in London, transformed in a problem of acid rain and sulfur deposits in Scandinavia, may be seen as if the situation had not improved, passing from one problem to another. As with most technological arrangements, the problem has been moved without being resolved. But as noted by Lomborg [247], this argument does not raise the issue of assessing the severity of problems. Highly polluted air in big cities and very dense urban areas kills every year thousands of people, making sick many more and reduces life expectancy. Diluting the pollution and exporting it is not the best solution, but saves lives, reduces illness severity and citizens live longer. When trying to establish the more effective priorities, the self-restoration and remediation ability of our planet has also to be taken into consideration when analyzing the relative importance of problems.

Several engineers like Moreno [248] and Bosschaert [249] have proposed using SCPPs as giant vacuum cleaners for urban atmosphere of highly polluted cities (Fig. 28), thus not necessarily primarily conceived for its energy generating capacity. A tall urban tower could be fitted with particulate and carbon air filters so that the air rushing through the chimney would be cleaned, resulting in urban air quality improvement. The constant air pull of the SUP will partially combat the heat island effect. In hot climates, a shadowing layer with a semi-transparent membrane could be installed to increase albedo, partially blocking out the sun, causing the temperature gradient to drop. A light pressurized inflatable rising conduit as proposed by Sorensen [177] might be easy to install in height between tall skyscrapers (and easy to remove in winter) and will not be too expensive, as the “vacuum cleaner” function does not require turbines and the structure is much lighter than of a conventional SCPP.


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Fig. 28. Using solar updraft chimneys to reduce urban heat island and particulate matter over big cities

PM, black carbon (BC) and soot also are a big health problem, and together with tropospheric ozone contribute to both degraded air quality and global warming. According to Shindell [250] dramatically cutting them with existing technology would save between 700,000 and 4.7 million lives each year. Shindell identified 14 measures targeting CH4 and BC emissions that reduce projected global mean warming of ~0.5 °C by 2050 and avoid 0.7–4.7 million annual premature deaths from outdoor air pollution. He calculated that by 2030, his pollution reduction methods would bring about $6.5 trillion in annual benefits from fewer people dying from air pollution, less global warming and increased annual crop yields production by from 30 to 135 million tons due to ozone reductions in 2030 and beyond. According to Shindell, since soot causes rainfall patterns to shift, reducing it would cut down on droughts in southern Europe and parts of Africa and ease monsoon problems in Asia. Shindell calculated that only in the U.S. his measures could prevent by year 2030 about 14,000 air pollution deaths by year in people older than 30.

As seen in the previous example, tall chimneys can be used as giant vacuum cleaners for dense megalopolis. Taking as a model size the 200 MW SCPP project from EnviroMission, as the air speed is estimated to be 11.3 m s−1, with a diameter of the tower of 130 m, we can calculate that the amount of air pumped by only one SCPP will be 4600 km3 every year.

If similar devices were associated (for a short period of time) with the most polluting fossil power plants, using waste heat as driving force and equipped with filters for particulates, the air quality will improve considerably. The SCPP efficiency will probably be poor because of pressure drop by dust filters, and less electricity will be produced, but the investment cost will be reduced as no huge greenhouse collector has to be built. Filtration of the exhaust of power plants, cement factories and other dust polluting industries is a well established technology. The pressure drop for particulates will anyway be smaller than for coal and other fossil power plants equipped with CCS, as in this particular case we do not focus on acid gases removal or neutralization, only on solid matter elimination.

As an example, using a general circulation model to investigate the regional climate response to removal of aerosols over the United States, Mickley and Leibensperger [251] found that reducing U.S. aerosol sources to achieve air quality objectives could thus have significant unintended regional warming consequences. They calculated an annual mean surface temperature increase by 0.4–0.6 K in the eastern US, but the temperature rise can be as much as 1–2 K during summer heat waves in the Northeast due to aerosol removal, meanwhile nearly negligible warming occurs outside the US.

Black carbon emissions have steadily risen this last two decades, largely because of increasing emissions from Asia. Soot and BC particles produced by industrial processes and the combustion of diesel and biofuels absorb incoming solar radiation and have a strong warming influence on the atmosphere [252]. On the one side, increasing the amounts of BC and decreasing the amounts of sulfates both encourage warming and temperature increases. On the other side, as several European and North American countries have passed a series of laws that have reduced sulfate emissions by more than 50% over the past three decades, although improving air quality and public health, the result has been less atmospheric cooling from sulfates.

Removing the dust and BC emissions by low cost particulate filters with small pressure drop of the principal Asian coal-fired power plants which account for the higher soot and PM emissions, can help separating the gas (SO2) from the particles (soot and PM). If at the same time the height of the exhaust chimneys of a part of the main Asian coal-fired power plants which account for the higher sulfates emissions in order to these flue gases (still containing SOx but no more BC and soot) to pass the boundary layer, not only the pollution will be diluted, but probably it will slightly increase the effective area and atmospheric life-time of the reflecting aerosols, that are normally flushed out of the atmosphere by precipitation. Although atmospheric pollution and aerosols are not well distributed and vary in space and in time [253], the IPCC global estimates of aerosols' direct cooling effect is −0.5 W m−2 and for their indirect cooling effect (by increasing the reflectivity of clouds) is −0.7 W m−2, with an uncertainty range of −1.8 to −0.3 W m−2.

As a matter of fact, a high SCPP-type chimney associated with a coal power plant and equipped with low pressure drop and low cost filters for solids will not only improve the air quality and aid public health, reducing global warming by soot and BC removal, but might be able to preserve the atmospheric cooling from sulfates. The fact is that sulfates in the troposphere have a much shorter resilience time than those in the stratosphere. But taller chimneys can send sulfur gases at a much higher altitude than conventional ones, for a longer period if they pass the boundary layer. Until Asian countries apply similar clean-air regulations than the U.S. and European countries, a progressive transition path can be proposed, for instance one out of five polluting coal power plants is equipped with a higher chimney and the 4 others are equipped with systems to wash out and neutralize the sulfates and NOx of their exhaust. Whatever the localization of the coal power plants with higher chimneys, the height needed for the exhaust can be calculated in order to obtain a five times longer residence time of the sulfates in the troposphere and an increase of the cooling effects of the corresponding aerosols, even if not as efficient as if they were in the stratosphere as proposed by CE SRM. The major difference is that this proposal uses already ongoing tropospheric pollution and reduces it progressively; meanwhile geoengineering has to inject sulfates intentionally in the stratosphere. Geoengineering proponents can study the effects of these actions, without performing themselves experiments on the stratosphere. One thousand SCPPs could pump at ground level 4 million km3 of air every year and send it in the troposphere.

The current cost estimates made by EnviroMission are of nearly $0.5 billion each for a 200 MW SCPPs with GH for solar collection (at least for the first prototypes, one could imagine costs going down and overall performances going up). In order to compare, the construction of a coal plant often costs more than 1 billion (3 billion for the last AMP-Ohio coal plant), with an operational life expectancy of 30–40 years, compared to more than 100 years for a solar tower.

Crutzen estimated that the costs to send SO2 in the stratosphere will be in between $25 and $50 billion every year. With $25 billion, at least 50 conventional SCPPs of 200 MW can be built every year (and four times more if built to use waste heat from power plants, as they do not need a solar collector). Each one of the conventional solar towers will annually prevent over 900,000 t of GHGs from entering the environment: this represents for the 50 SCPPs all together 45 million tons of saved CO2 emissions per year with only the cost of 1 year of stratospheric sulfate sunshade.

Of course the new 50 solar towers built every year will together generate 50×650 GWh per annum (i.e. 32.5 TWh). This is enough to provide electricity to power around 10 new million households every year. The life expectancy of SCPPs is anticipated to be of roughly 100–120 years. For the same $25 billion needed each year for SRM by sulfates, nearly 200 SCPPs associated to conventional power plants can be built, filtering the soot and BC off the air, with an immediate cooling effect (in particular in the Arctic region) and saving thousands of lives.

Synergies between direct CO2 capture from the air and SCPPs [254] were evaluated and at least a 25% cost reduction of the CDR process arises, with also a simplified scheme for carbon sequestration.

11. Clear sky radiative cooling or targeting the atmospheric window
Matter continuously exchanges energy with its surroundings. Heat transfer can occur by conduction, convection, radiation and also by evaporation combined with convection and condensation at altitude. After sunset when a surface on the earth faces the sky, it loses heat by radiation, but might gain heat from the surrounding air by convection. If the surface is a good emitter of radiation, at night it radiates more heat to the sky than it gains from the air and the net result is that the surface temperature drops to below that of the air. Surfaces can only experience subambient cooling if the thermal radiation given off is larger than that coming in from surrounding surfaces and from the atmosphere. This phenomenon is called night sky radiation cooling. When protected from wind, by clear sky and dry weather, heat transfer from ground surface by IR radiation is much faster than air convection, so a net cooling of the ground can occur resulting in well above air temperatures [255].

In SRM strategies, high-albedo surfaces are proposed to reduce solar heat gains by reflecting an increased amount of solar energy and increasing the albedo. In ERM strategies sky cooling surfaces can pump heat away by radiative cooling to the atmosphere and get rid of the heat directly into outer space. The longwave energy is removed directly by transmission through the atmospheric window. So SRM and ERM are complementary as they can make profit of two distinct types of coolness, the first connected to the whiteness (high albedo) of the surface, which prevents excessive temperatures through reflection of incoming solar radiation, and the second with the coolness that can be captured under a clear sky making use of the atmospheric window, which allows to lose longwave radiation of energy directly into outer space.

The radiational cooling of selective surfaces has been studied by many authors [256], [257], [258], [259] since the 1970s, in order to match the atmospheric window (8–13 µm) for more effective cooling by exposition to the clear sky. As seen in Fig. 7, the outgoing longwave radiation through the atmospheric window represents 12% of the total outgoing radiation (17% of the longwave radiation). Space cooling (or nocturnal radiation cooling to the night sky) is based on the principle of night heat loss by long-wave radiation in the atmospheric window (8–13 µm). This occurs from a warm surface (the ground or the roof of a building) to another body at a lower temperature (the sky). By clear sky, ground can act as “nocturnal sky radiator” and its cooling by night sky radiation can often reach temperatures 5–10° below ambient (and even much more), so the correlation with the air temperatures measured under a shelter 2 m above the ground are often different. For instance, recent Moderate Resolution Imaging Spectro-radiometer confirmed [260] that at night-time by dry night, the air temperature is often consistently higher than the satellite-measured land surface temperature.

In the 1980s Martin and Berdahl [261] developed an algorithm for calculating the thermal radiant temperature of the sky, based on an empirical and theoretical model of clouds, together with a correlation between clear sky emissivity and the surface dew-point temperature. Hourly sky temperatures have been calculated based on typical meteorological year weather data sets. A typical sky temperature map for the US in July was published by ASHRAE Handbook 2011 based on this work (Fig. 29).


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Fig. 29. Average Monthly Sky Temperature Depression (Tair–Tsky in °C) for July. (Adapted from Ref. [261].)

Berger [262] developed an inexpensive apparatus to measure sky temperature. A procedure to calculate the radiative heat exchange between two bodies to be used in the determination of sky temperature, clear sky index or plate emissivity was published by Armenta-Déu [263]. Argiriou [264] showed that more than 90% of the total sky radiation is emitted by the lowest 5 km of the atmosphere, to which water vapor contributes over 95%. He published the frequency distribution of the sky temperature depression for a list of locations over a given period of time.

Over the years, radiative cooling of buildings has attracted considerable research, mainly focused on evaluating the magnitude of the resource and the variations in cooling potential among different locations. Granqvist [265] was also interested in the design of radiative materials for heating and cooling purposes, in particular surfaces capable of reaching below ambient temperatures by benefiting from the spectral emittance of the clear night sky.

Underlying mechanisms have been described by Martin [266]. Granqvist [267] discovered selectively emitting SiO films and Lushiku and Granqvist [268] studied several selectively infrared emitting gases like ammonia, ethylene, and ethylene oxide. Meanwhile Etzion and Erell [269] studied several low-cost long-wave radiators for passive cooling of buildings.

Tsilingiris [270] tested several polymer layers, poly vinyl fluoride being especially good and Berdahl [271] studied MgO and LiF layers. Practical experiments have been conducted by Eriksson and Granqvist [272], [273] on thin films of materials such as silicon oxynitride, alumina, and by Granqvist [274] and Tazawa [275] on silicon monoxide or silicon nitride [276]. Other alternatives include nanoparticles of SiC and SiO2, that turn out to be of special interest as proved by Gentle and Smith [277].

Currently, the research areas for space sky cooling focus on alternative cooling systems [278], [279], [280] for instance for hot regions where evaporative cooling cannot be used. The aim of these scientists is energy savings compared to mechanical vapor compression systems, by collecting at night cold water [281] in storage tanks to be used in a cooling coil [282] unit during the day, creating a cold storage for the following day. Phase change materials [283] can replace the cold water storage.

One can image improving the daily water production of cheap semi-closed solar stills, as with stored night coolness the condensation yield can be improved. A reverse greenhouse effect was described by Grenier [284] in 1979. Using two water tanks, one for hot storage during the day and one for cold storage during the night, can reduce the temperature differences between day and night in greenhouses for agriculture purposes in hot arid and dry regions of numerous countries. Providing some shadow and with the water recycled inside the greenhouse by night sky condensation might help for a better and more efficient irrigation use, and for the development of a sustainable and self-sufficient food production.

As at night, water freezing can damage PV panels and thermal collectors, research also focused on preventing frost formation and maintaining transparency of a window exposed to the clear sky, for instance using the low-emittance coating SnO2 on covered glass [285]. Combining heating and cooling in a single surface or single stacked system having suitable spectral properties can be done sequentially with daytime heating and night-time cooling with surfaces designed for sky cooling. The cold sky radiation constitutes a heat sink mainly used for passive cooling systems. Under tropical climates like in Thailand, cooling by night radiation is feasible mainly during the tropical winter season [286] where experimental results showed four different surface temperatures nearly 4 °C below ambient temperature under clear sky (Fig. 30).


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Fig. 30. Hourly variations from 18 h to 06 h of ambient air and different temperatures of 4 different outer surfaces of radiators tested by Khedari [286] (24 January 1998, cloud cover 5%).

Erell [287] reviewed this research work. A cooling effect can also be obtained during the day [257]. Combining in one surface high solar reflection and efficient sky cooling can lead to daytime cooling. As demonstrated by Nilsson [288] and Addeo [289] meanwhile solar reflection keeps the building cool, sky cooling contributes to make its radiative output surpass the solar heat gain so that subambient cooling starts earlier in the afternoon than would be the case without sky cooling. So through some special arrangements it is also possible to achieve useful sky cooling in the daytime and high levels of cooling can be achieved with surfaces of this type as long as there is no incident solar energy and the air convection exchanges are poor.

Among convection covers for radiative cooling radiators, there is polyethylene and zinc sulfide [290] which is mechanically stronger and more resistant to solar UV. They are used as window material associated with selective radiator materials.

Since 2005 the U.S. Department of Energy as conducted extensive research on theoretical [291a] and experimental [291b] evaluation of the “NightCool”, nocturnal radiation cooling concept and performed performance assessment in scale tests buildings.

Recently Smith [292] succeeded in amplifying radiative cooling by combinations of aperture geometry and spectral emittance profiles and Gentle [293] applied to cool roofs and sky cooling a polymeric mesh which is a durable infra-red transparent convection shield.

A very complete and extensively review of the night sky research and potentials in many areas has been published in 2010 by Grandqvist and Smith [294]. They also described many possible applications of sky cooling to save energy, increase efficiency and prevent new CO2 emissions.

Together with reverse osmosis, a commonly used method for desalination of sea water is multi stage flash distillation, but both processes are energy intensive methods. Water condensation, occurs when surface temperature falls below the dew point. Several authors [295] have studied dew water recovery using radiative cooling to condense atmospheric vapor [296] on surfaces which can pump heat at subambient temperatures. The technique is referred to as “dew-rain” and typically uses pigmented foils like a unit depicted in France [297] which was able to produce significant amounts of water. A polyethylene foil containing a ZnS pigment helped to collect dew [298] at night in Tanzania and in India [299a] dew collection is being implemented for drinking water. In proper climatic conditions even simple galvanized iron roofs are capable of collecting some dew [299b]. The emitter surface being the coldest, condensation happens first on it and may sometimes occur as dew on the cover as well. But as water has high thermal emittance and is hence strongly IR absorbing, it is essential to remove it from the cover. Of course the low amounts of dew water collected with current clear sky cooling systems cannot compete with the worldwide desalination capacity of 78 million m3 per day (consuming more than 80 TWh of energy per year) [300]. More than 1 billion people lack access to clean water supplies and an extensive use of desalination will be required to meet the needs of the growing world population. Energy costs are the principal barrier and as for instance by 2030 the total electricity demand for desalination in the MENA region is expected to rise [301] to some 122 TWh. Synergies with sky cooling for increasing process efficiency improved with overnight-generated coolness and complementarities for water collection in remote areas far from the sea or in altitude are worth envisioned.

In order to trap additional power from waste heat from conventional power stations and industries, Grandqvist and Smith [294] suggest that overnight-generated coolness can add significantly to the power output of turbines working at low temperatures. Any low temperature thermal power system can benefit significantly in efficiency by having the cold sink temperature fall by 10–15°. Collecting coolness via sky cooling for an engine condensation cycle with sufficiently cheap and simple materials can also boost up the efficiency of the output from renewable power thermal systems. Grandqvist and Smith also propose that, as large-scale photovoltaic generation systems are commonly located in near-perfect locations for night sky cooling under clear skies and in dry air, they can benefit during the day from night collected cooling in fluids, which may be able to decrease the daily temperatures of the solar cells by 5 °C and thus increase the photovoltaic efficiency [302] as shown in Fig. 31.


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Fig. 31. decrease of electrical efficiency as a function of PV temperature increase reproduced from Tonui [302].

One of the major problems of concentrated solar power and related concentrated thermal electricity technologies installed in hot deserts is their need of water as a cold sink, as in arid deserts the water resource is scarce and the use of non-renewable groundwater goes against sustainable development. Although the area needed for this purpose is important, the possibility of storing night sky coolness in phase change products can considerably reduce the daily water needs for cooling. A similar approach was proposed by Bonnelle [205] with SCPPs for CSP dry cooling. Other water use of the CSP or PV industry in deserts comes from the need to remove sand dust from the mirrors or panels: maybe night sky dew condensation can wash out these particles by natural gravity in the early morning.

Electrically powered compressors for cooling systems dump outside the buildings into the close local environment an amount of heat that is larger than the heat removed from the inside rooms. Often, for absorption cycle cooling the coefficient of performance has a value of one and up to three for high performance chillers. That means that between twice and four times the heat that is pumped away is released outside, contributing to the urban heat island of large agglomerations.

In 2003, 395 TWh yr−1 were consumed for air conditioning in the world, and by 2030 this consumption is projected to rise by more than three times. Thus around 45 GW of additional external heat load is globally due to air conditioning and, 180 GW is continuously heating up outside urban air. As discussed earlier in Section 3, given typical efficiencies of thermal power plants, their total atmospheric heat load is now probably around 35,000 TWh each year, or at any one time around 4 TW of heat. That is one of the reason why Grandqvist and Smith [294] suggest that it is very important to make more use of solar reflectance and sky cooling, as the more heat derived from cooling will be pumped into the outer space the better. They encourage greater use of night cooling with conventional compressors plus storage as able to send much of the exhaust heat into the outer space instead of into the nearby air.

They also note that when the cooling for buildings is obtained by water evaporation, there is a higher demand on water resources and an elevation of local humidity, both of which are undesirable. In contrast, sky cooling avoids this, and has no adverse impacts on the local environment. On the contrary, sky cooling actually helps ameliorate the urban heat island effect, whereas electrically powered cooling systems and other options will exacerbate it. Sky cooling devices may also be applicable in homes for collecting and storing cold fluid overnight to supply part of the cooling needs of the next day. As shown by Akbari [45], reducing the heat island effect by high albedo roofs can not only reduce the need for air conditioning and lead to energy savings, but also improve air quality and thus have health benefits. Reducing urban smog and ozone [303] will also contribute to healthier cities. By reducing air conditioning needs, the cool-roofs and sky cooling strategies can reduce leakage of greenhouse refrigerant gases that are often worse GHGs than CO2.

The cool roofs ideas [45], [94] have conducted to the development of high reflectivity plus high emissivity tiles or coatings. As most of the NIR solar energy lies at 0.7<λ<1.2 μm, it is important that the reflectance is high in this range. The SRM strategy targeting surface albedo [48] can be completed by high thermal emittance materials to also benefit of sky cooling when possible, with adequate covers to prevent thermal conductance. But whitish-looking surfaces are not necessarily very good solar reflectors and may absorb as much as half of the incident solar energy getting warm and leading to a significant internal heat transfer by conduction. It is easily realized that in hot climates roofs should have high solar reflectance combined with high thermal emittance. As recalled by Grandqvist and Smith, the high emittance not only helps to keep down daytime temperatures on roofs and walls but also at night it allows the roof and often the interior and building mass, to cool to a temperature a few degrees below that of the ambient. Among others, they suggest the use of aluminum flakes that have been precoated with nano-thin SiO2 layers via sol-gel coating before an iron oxide layer is applied. The clear top overcoat imparts a high emittance to this two-layer coating that might be affordable as it is produced by making use of two of the earth's most abundant oxides. Sheet glass in which the iron content is almost zero is also suggested as possible material that has very small solar absorption and very large radiation output. They also recommend to have a convection-suppressing shield that reflects or back scatters solar radiation while it transmits in the thermal infrared, for instance with microparticles of ZnS, another option being nanosized TiO2 incorporated in polyethylene [304].

In the purpose of increasing natural convection at a much larger scale, whenever it can be advantageous, for instance to increase valley breeze, mountain breeze, sea breeze or land breeze, the concepts previously described in this section can probably be extended. A strategy could consist in trying to favor, during the summer nights, the amount of cold air coming down from the mountains along the slopes to the valleys, in order to keep cities colder during the day and thus reduce the use of air conditioning and decrease the CO2 emissions.

Wind is simply air in motion, caused by the uneven heating of the earth by the sun. Solar heating varies with time and with the reflectance and the emittance of the surface. Differences in temperature create differences in pressure. When two surfaces are heated unequally, they heat the overlying air unevenly. The warmer air expands and becomes lighter or less dense than the cool air. The denser, cool air is drawn to the ground by its greater gravitational force lifting, thus forcing the warm air upward. The rising air spreads and cools, eventually descending to complete the convective circulation (Fig. 32a and b). As long as the uneven heating persists, convection maintains a continuous convective current.


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Fig. 32. (a) The convection process. (b) The generation of artificial vertical currents by albedo and by radiative forcing modification.

Based on the heat island effect on rain described early in this paper [81], [82], an international team built up the idea of working with a black material (low reflectivity, low emittance) absorbs energy from the sun and then radiates it back into the atmosphere at night. The air above the black surface could be raised by 40–50 °C above the surrounding temperature, creating a “chimney” of rising air currents.

According to Bering [305], covering nearly 10 km2 with an appropriate material can make it rain downwind. As it is done near a humid sea coast, clouds will form in the afternoon along a strip as wide as the black surface, and then go up several kilometers. The artificial thermal will boost water vapor to around 3 km where it can condense into water droplets that create clouds, and rain will fall in upwind regions as far as 50 km. The technique is to be applied to any subtropical dry region within 150 km of an ocean. The physical feasibility of the technique was ascertained by computer simulations at a Spanish location in the Mediterranean coast, but the results were not as good as expected. The forest “biotic pump” hypothesis [306] might let think that the association of a first “black” similar area, with a second area covered with a “white” high reflectivity and high emittance material close to a very humid coast like in the Red sea might give better results in case of the presence of cloud condensation nuclei, which is the case in dusty deserts.

Using man made tornados (rotating in the opposite direction) has been proposed [307] to divert or stop natural ones. Maybe large convection surfaces as studied by Bering, associating sky cooling and also numerous small AVEs, can reduce the destructiveness and the number of fatalities of inland tornadoes by reducing the differentials of temperatures and pressures between the upper layers and the surface, by constantly removing energy to the convective system.

As a conclusion, Grandqvist and Smith make the observation that sky cooling to subambient temperatures has only received spasmodic attention over the years. For them, to date the great potentials of sky cooling have not yet been successfully exploited, despite our ready access to it, probably because the field is not widely understood or appreciated. Also few scientists are active in it, and also as, apart some arrangements for water collection, little effort has been made to develop products based on sky cooling. They think that the quite diverse technological scope beyond these applications seems not yet well understood, and this “knowledge gap” has yet to be bridged. For instance practical cooling at a low cost down to 15 °C below the coldest ambient temperature of the night has been demonstrated. Clearly, the field of sky cooling has so far fallen short of its potential by a wide margin, but it has too much to offer to be neglected. The number of possibilities allowed by sky cooling is huge: of course the goal of this paper is to focus on the possibility to increase the outgoing IR radiation to space by the atmospheric window. But sky cooling presents many other possibilities, like improving renewables electricity production in particular of PV; increasing the efficiency of fossil fueled power plants and of any thermal one; reducing water needs and consumption of the power industry; reducing waste heat release in the close environment at the Earth surface; improving the efficiency of air conditioning systems; reducing the heat island effect; reducing the electricity consumption and the CO2 emissions by reducing building cooling needs (in synergy with cool roofs); improving human health in urban areas by reducing aerosols and smog; allowing water collection from the atmosphere, and an aid to water condensation in distillation, improving drinkable water access in dry or poor countries; and many other potentials.

The numerous synergies of sky cooling with other energy related technologies have not yet fully been explored. For instance, the thermosyphons used to prevent permafrost melt under pipelines or railways in Northern countries work mainly when the ambient air temperatures are lower than underground temperatures, transferring heat from the ground to the air and keeping the permafrost cool. During the summer, as the air temperature is higher than the condensation temperature of the gas inside the thermosyphon, no heat transfer occurs. It might be possible to improve the upper part of the heat pipe, to make if benefit of the clear sky radiative cooling. High reflectance and high emittance coatings on the top of the thermosyphon, some shadow to protect the condenser part from direct sunlight during the day, a shelter to reduce air convection and a larger surface area exposed to the clear sky at night, all might be possible in order to obtain more effective heat pipes throughout the whole year.

12. Overview of the principal ERM techniques proposed
In Table 2 the principal characteristics of the meteorological reactors described in this review are summarized, with their main heat removal targets and advantages and both physical and technical potentials description. The possible carbon credits have not been taken into consideration.

Table 2. Overview of principal ERM strategies and their characteristics.

Type of MR SCPP in deserts DET AVE Thermo-syphon Night sky radiation Tropical SCPP Polar SCPP
Outgoing radiation target Sensible heat Latent heat Latent heat+sensible heat Surface radiation Thermals+sensible heat Latent heat evaporation Latent heat crystallization
Possible additional climate benefitsa
−
Rain in deserts

−
Heat island effect reduction in urban areas

−
Low altitude clouds (albedo)

−
Green the deserts

−
Increase planetary albedo

−
Maintain thermo-haline circulation

−
Re-ice the artic

−
Reduce hurricanes intensity

−
Dew water collection

−
Heat island effect reduction

−
Cloud cover increase

−
Rain in deserts

−
High albedo fresh snow at poles

−
Sea ice cover increase

Research results available +++ ++ + +++ +++ +- +−
Small prototypes built +++ ++ + +++ + no no
Renewable energy production Yes Yes Possible Possible No but can improve existing power systems Possible Possible
Useful without turbines Yes dry cooling csp Yes cooling GH for agriculture in hot deserts Yes replace cooling towers Yes many industrial uses Not applicable Yes water production Yes to re-ice the arctic+increase polar albedo with fresh snow
Possible synergies for cost reduction b
−
CO2 capture

−
GHG removal

−
GH agriculture

CO2 capture Use waste heat of thermal power plants Dry cooling
−
Cooling PV panels

−
Cool paints and coatings

−
With heat pipes

unknown unknown
Estimated cost for full operational scale $300–400 million (750 m high) $100–150 million (750 m high) $50–100 million or $10–20 million without turbines [203b] $100–150 million without turbines (750 m high) small covers, or coatings in numerous locations floating $200–400 million $200–300 million on mountain side
Is a rapid implement-tation possible? (a couple of years) Yes Yes
−
Yes without turbines

−
No with turbines

−
Yes without turbines

−
No with turbines

−
Not at high altitude

Yes No No
Possible variants, (other than mountain side)
−
Many: floating

−
urban ventilation

−
etc.

−
With wind towers

−
With ETFE textile shell

Variants [197], [199] Multiple industrial uses: ex. H2 production by nuclear Several to increase breezes from sea or land, from valley or, mountain Variant [143] Similar to thermo-syphon
a
Additional to: avoided CO2 emissions; heat transfer out to space; and renewable energy production.

b
Except for tropical SCPPs and thermosyphons, it may be advantageous to use the relief to support the chimney by the mountain side, which reduces the cost for building it; part of the duct structure can be in steel covered with textile sheet instead of concrete.

Thinking to the possible climatic benefits (i.e. avoided hurricane costs, avoided CO2 emissions and improved human health), the economical potential is expected to be wide, but the costs estimates are yet approximate as only little literature and data is available for the moment.

For SCPPs, the principal costs estimates given in Table 2 are extrapolations from the evaluation made in 1995 by Schlaich [117], then in 2007 by Pretorius [308a], in 2009 by Fluri [308b], and finally from the 2013 actualization made by Krätzig [309]. As Krätzig also gives costs estimates for a 750 m high chimney with no collector, these figures were extrapolated for 750 m high DETs and thermosyphons. For Bonnelle's equatorial SCPP and polar SCPP an extrapolation of both sources was made, together with figures given by Papageorgiou [310] for a floating solar chimney. No figures are available for clear sky night cooling.

For AVEs, Michaud [203b] evaluates to $80 million the total cost of a real scale AVE prototype 45 m height and 60 m base diameter. The vortex obtained will probably reach an height of 9000 m with a base diameter of 6 m. The heat source can be waste heat from a 200 MW thermal power plant. Michaud's major objectives are to increase the power output of the existing thermal power plant by 10–20%, reduce the GHG emissions by 10–20% and also to eliminate the need for the cooling tower.

In Table 3 several parameters of comparison are given between SRM and ERM techniques, in terms and potential benefits for the climate, but also for the humanity.

Table 3. Comparison of the principal SRMa and ERM techniques (CDR not included)

Parameters SRM ERM Comments
Type of strategy Parasol effect Thermal bridge Global warming is caused by GHGs which keeps the infrared radiation in the lower layers of the atmosphere
Targeted radiation Shortwave /visible 31% Longwave/IR 69% It is difficult to compensate a longwave positive forcing by a shortwave negative forcing [311]=> in the case of SRM by sulfates, the rain and wind patterns are modified [112], that means more rains or drought in some places with winners and losers. ERM compensates longwave positive forcings by longwave negative forcings
Global climate benefit of cooling 2 °C Yes Yes Both strategies can be constructed to target compensation of 2 °C global warming on average
Indirect profitability Yes Yes Both techniques if they reach their goal of cooling the Earth will prevent some of the consequences of global warming [1]
Direct profitabilityb No Yes Almost all meteorological reactors (MR) can produce electricity at a competitive cost. The SRM techniques have a cost but do not produce something that can immediately be soldb
Proportionality of costs and expenses No Yes SRM global cooling needs a large scale implementation and to permanently maintain it during decades. ERM can have immediate local effects and be progressively implemented. Once built, a SCPP can last 100 years (50 years for other MR) with almost no consumables needs
Prevent ocean rise Long Rapid Some ERM techniques can provide more rain over the continents [312], more fresh snow in the Arctic, more sea ice
Improve crops yield No Yes See Shindell [250]
Prevent CO2 rise, avoid future CO2 emissions No Yes Opponents to geoengineering fear that it will not encourage governments to reduce CO2 emissions [40]. The MR proposed in this review produce CO2-free renewable energy and can replace fossil power plants
Prevent GHG rising No Yes As clean electricity can be produced by MR, they will favor less coal mining and less shale gas production, thus lower methane and soot emissions. The electric cars will replace internal combustion engines thus less NOx and PM pollution
Avoid other pollutions No Yes SRM might release sulfates in the stratosphere or salts in the oceanic clouds. An ERM strategy described at the end of paragraph X of this paper might allow to progressively reduce currently existing tropospheric pollution (soot and BC+4/5 of sulfates) and still keep the current cooling level of these aerosols
Prevent ocean acidification rising No Yes ERM can avoid future CO2 emissions and SCPPs can drastically reduce de cost of direct air capture [254]
Hurricane reduction Not directly Directly and indirectly Global cooling can indirectly reduce hurricane intensity. But MR like AVE and tropical SCPP [142] can directly cool the oceanic waters. Maybe sky cooling large convection surfaces (end of chapter X1) associated with numerous small AVEs can prevent in land tornadoes
Improve human health No Yes See Shindell [250]
Improve development No Yes Poor, hot countries can build SCPPs or DETs in desert locations with local labor and local raw materials
Public acceptance Poor Anticipated to be good Producing clean renewable electricity and avoiding future CO2 emissions can be better accepted than the “business as usual scenario”
Possible Military use of the technology. Destructive capacity or possibility of misuse High risk Low risk The relatively low financial cost of SMR using sulfates, their high efficiency, with the possibility of a rapid and massive deployment can be feared [40]. ERM is not a rapid action: MR construction is slow and quite expensive; individual MRs cannot be big enough to make significant damages and it seems difficult to build several MR in order to harm. Even the AVE are safe in theory because in case a tornado leaves the generator, it loses immediately the energy that gives him birth and thus dies as soon [203]. Supposing that cyclones could be created with AVE, it is impossible to guide or orient them
a
As in the most abundant scientific and ethical literature on CE talking of SRM deals with the sulfates in the stratosphere strategy (and although SRM includes a wide range of other different strategies), in the following table the comparisons are made to this particular SRM technique.

b
The possible carbon credits have not been taken into account, but clearly ERM with MR deserves them, on the contrary of SRM.

SRM do not address the problem of ocean acidification and of atmospheric GHGs (CDR and CCS address only one of the GHGs). Sunlight reflecting methods could cause significant environmental harm, like changing weather patterns and reducing rainfall, damaging the ozone layer, reducing the effectiveness of solar renewable energies, as well as causing sudden and dramatic climatic changes if deployment is stopped, either intentionally or unintentionally. Technical, political and ethical uncertainties are numerous.

ERM strategies seem to have fewer drawbacks. ERM is a set of power-generating systems producing renewable energy and able to transfer heat from the Earth surface to upper layers of the atmosphere allowing heat loss into space. But as this review is the first to propose the concept of longwave energy removal methods, a careful evaluation and examination by other scientists is necessary, recommended and highly desirable. This review suggests several enhanced raise manners for latent and sensible heat energy riddance methods to lose longwave radiation directly into outer space, and a peer evaluation of their potential is required before a correct comparison with SMR strategies can be performed.

13. Discussion
Climate change and global warming is an increasing problem and there is serious concern about the international organizations and governments capacity to take good decisions and fight them effectively. Alternative solutions like CDR and SRM are proposed by the geoengineering community.
On page 20 of the 5th IPCC's summary for policymakers (released September 27, 2013), it is mentioned that “Methods that aim to deliberately alter the climate system to counter climate change, termed geoengineering, have been proposed. Limited evidence precludes a comprehensive quantitative assessment of both Solar Radiation Management (SRM) and Carbon Dioxide Removal (CDR) and their impact on the climate system.

CDR methods have biogeochemical and technological limitations to their potential on a global scale. There is insufficient knowledge to quantify how much CO2 emissions could be partially offset by CDR on a century timescale.

Modelling indicates that SRM methods, if realizable, have the potential to substantially offset a global temperature rise, but they would also modify the global water cycle, and would not reduce ocean acidification. If SRM were terminated for any reason, there is high confidence that global surface temperatures would rise very rapidly to values consistent with the greenhouse gas forcing.

CDR and SRM methods carry side effects and long-term consequences on a global scale”.


Carbon dioxide removal would need centuries before acting, but addresses the real problem as well as ocean acidification and other CO2 induced problems. SRM could provide rapid cooling, in few months, but would require to be maintained at least for decades and presents serious side-backs. Some SRM strategies are also cheap and could be so efficient that they can become addictive and may result in forgetting the progress needed in reducing our CO2 and GHGs emissions.

Public acceptance of geoengineering is poor and CE is often presented as Ulysses choices between Scylla and Charybdis, because the most discussed CE option is the stratospheric sulfates one. But soft-GE options with low risk and with good cooling potential, effectiveness and affordability might exist; some CDR strategies seem to answer these criteria and somehow should not be considered as CE.

The goal of this paper is to demonstrate that other ways exist, like Earth thermal radiation management by several complementary techniques that allow more heat to escape to space.

There is an increasing interest in the development of hybrid renewable energy devices for simultaneously harvesting various unusual forms of energy to produce electricity, thus preventing further CO2 and other GHGs emissions, and also at the same time allowing cooling the Earth by ERM.

No single source, type or form of energy will answer the enormous energy needs of humankind. Fell [313] described numerous strategies for global cooling and Jacobson [314] reviewed the “solutions to global warming, air pollution, and energy security”. We believe that some of the technologies presented in this review paper are complementary and deserve being included in the energy portfolio mix of the future.

Stabilizing climate will require within the coming decades the development of primary carbon emission-free energy sources and efforts to reduce end-use energy demand. Of course, no single adaptation or mitigation method, no single geoengineering scheme or idea, and no single MR or URE will solve alone by miracle the global warming and climate change problems.

In a 2002 paper in Science entitled “Advanced Technology Paths to Global Climate Stability: Energy for a Greenhouse Planet”, Hoffert [315] and 17 other scientists, after noting that non-primary power technologies that could contribute to climate stabilization have severe deficiencies that limit their ability to stabilize global climate, concluded that a broad range of intensive research and development is urgently needed to produce technological options that can allow both climate stabilization and economic development. ERM with several of the UREs presented in this paper have this potential as they de-carbonize electricity generation, are not intermittent and reduce the mismatch between supply and demand.

Meteorological reactors as large as those described in the paper do not yet exist, and as such, we view this work as a theoretical problem, but with real potential for real world applications in the coming decades. The SRM techniques also imply a long way of research before safe and large global implementation can be envisioned, if ever.

As a brief summary of what authors think that even if the most advanced MR technique is currently those of SCPPs to be built in hot deserts, SCPPs with large collectors will probably be more useful to produce renewable energy than to cool the Earth (but they can also be used for GHGs removal which will be the subject of a different paper, compared to CDR).

The authors believe that although only theoretical work has been done on other technologies, they deserve further development:
•
The polar SCPP variant proposed by Bonnelle (Chapter 10, Fig. 27) has a great potential, as not only it can produce a significant amount of the electricity needs of northern Europe, but it can also re-ice the Arctic, and remove the sword of Damocles of a large destablization of methane hydrates with a possible tipping point.

•
The night sky cooling materials to send back to the outer space some IR radiation by the atmospheric window are of great interest, as at small scale the technology readiness level is satisfactory and the scalability also seems good.

•
The Grena variant with two balloons of the hot air balloon engine proposed by Edmonds (Chapter 6, Figs. 14 and 15a) to release air in altitude, reduces the investment cost of SCPPs because it suppresses the chimney. As a much higher altitude can be reached by the balloons than by a concrete chimney, the heat transfer to the space can be more efficient, if the amounts of hot air transported per day and per device are similar (several km3). Small scale prototypes can be built and studies can be performed to use waste heat from existing power plants instead of solar energy. Filling the balloons with humid hot air (from tropical seas), can help reaching higher altitudes when water will condense inside, as latent heat will be released and will warm the inner air.

•
To produce renewable energy the Michaud's AVEs will probably still need a long development time. But independently of electricity production, if the AVE can be proven safe and that there is no risk of producing free tornados or free hurricanes, they can probably be rapidly used only for heat transfer from the Earth surface to the top of the atmosphere, the cooling benefits can maybe easily proven. If an AVE system uses the waste heat from power plants as driving force, water will be saved in cooling towers and waste heat will no longer be released in the rivers or in the oceans. In very humid and hot climates, if proven safe the AVEs can help drying the local atmosphere, and transfer huge amounts of energy to the stratosphere, thus cooling the earth.

The authors' opinion is that among MR the most effective and practical technologies to fight against climate change can be AVEs (if proven safe) and night sky cooling. In a configuration where they are not designed to produce renewable energy the R&D required to reach an industrial scale as well as the investment costs will be reduced.


Maybe some of the concepts presented in this review, either for climate engineering SRM as for ERM will never be of practical use; some of them can probably be categorized as pure science fiction; some others could seem “naiveties”, but nonetheless the fight of global warming and its tremendous disastrous potential consequences deserves this review. The real problem is the climate change and the global warming being caused by ongoing GHG emissions, not to try to solve it by CE, SRM, CDR, or by the ERM proposals made is this paper. The roadmap for scaling up carbon sequestration from megatons to gigatons [316], or stratospheric sulfates seeding from tons to megatons is not an easy task, and neither is easy the task of convincing investors to build the very first kilometric high DETS and SCPPs that will allow ERM.

The development of renewable energies which are environment-friendly alternatives to fossil fuels use might be completed by new ones which have no anticipated adverse effect on the environment. One of the aims of this review is to give an overview of the state in the art of the numerous ideas and theoretical progress that have been accomplished to date in the development of different MR for energy harvesting. These URE devices can harvest solar, wind and temperature difference energies and allow energy storage or peak production 24 h/7 days. The status and outlook of commercial perspectives is given, in particular for SCPPs and DETs. But, even though significant progress has been made in the research on several of these MR, quite a few have reached the prototype level and SCPPs are the only ones to have been very recently launched at a nearly industrial scale. The industrial potential of these UREs seems important as many of them have the economic potential of provide work to local labor, using local raw materials, and development benefits (electricity and water production in deserts areas). The public and societal acceptance might also be higher than for CE methods.

Coupled with a desire for a miracle that will mitigate energy and environmental concerns, these clean URE are an area ripe for hype. Perhaps several of the technologies described here exhibit good feasibility, yet will never be practical or will not deliver all the promises made, even if they offer high theoretical potential to address the energy challenge. In the future, scientists and engineers will show us among these MR what is possible and also practical. The main idea developed in this review is that GHGs are good insulators of the Earth that prevent normal interactions with the atmosphere (Fig. 33) and keep the earth too hot, so atmospheric thermal bridges have to be created and these UREs can do it.


Download : Download high-res image (757KB)Download : Download full-size image
Fig. 33. Creating thermal bridges between the surface and the higher troposphere can help cool down the Earth by ERM by increasing the amount of outgoing IR radiation, meanwhile SRM aims to reduce the incoming shortwave sunlight.

Permafrost melting is considered has an important issue by the scientists who fear leakage or massive emissions of methane, a GHG with a GWP100 25 times higher than for CO2. Already an enormous amount of thermosyphons are currently used to prevent permafrost melting along pipelines, roads and train-rails over Alaska or Siberia. Large scale use of numerous, more efficient and cheap heat-pipes can help relive the side effects of this global warming induced-problem, as well as for glaciers and for the Arctic melting.

As with the ERM techniques proposed in this article (Table 4), there is no more ocean acidification, acid deposition, ozone depletion, etc., many of Robock's [40] “20 Reasons why Geoengineering may be a Bad Idea” are invalid for ERM.

Table 4. Old [68] and new strategies proposed to stabilize the climate (⁎ GHGR refers to another paper in progress [317] and is not described here).


Ocean acidification due to anthropogenic CO2 emissions is a major problem [318] and current SRM geoengineering schemes do not address it, and on the contrary proposes to add acid rain by sulfate deliberate pollution. The ERM proposed here, as it is able to enhance longwave radiation out to space, and at the same time produce CO2-free energy, can prevent further CO2 emissions and help alleviating oceanic CO2-induced pH change. Even if some meteorological reactors like the AVEs were only used for ERM, not to produce renewable energy, their ability to transfer huge amounts of energy from the surface to high altitude atmospheric layers and then heat to the space would be worth tested rapidly. AVEs are already worth tested because cheaper, scalable, less dangerous or harmful and with more potential benefits than some SRM methods. For instance investing in the development of AVEs with no turbines to produce electricity, and equip all the thermal power plants to replace cooling towers and to disperse the waste heat in altitude will have an initial cost, but then for years it will cool the Earth surface for almost free, and save water.

Many of the UREs discussed in this paper not only could bring limitless clean energy but might also be able to reduce hurricane intensity and their destructive force, reducing insurance costs due to severe weather events, either due to climate change or not. If the $160 billion cost caused by the two hurricanes Katarina and Sandy, plus the $160 billion cost caused by eight other north Atlantic hurricanes of the last decade (Ike, Wilma, Ivan, Irene, Charley, Rita, Frances, and Jeanne) could have been be saved and instead invested in renewable energies and the UREs described here, the benefits for the climate, the Earth and the humanity could have been considerable. The possibility that the destructiveness and the number of causalities caused by the deadly tornado that hit El Reno, Oklahoma on 31 May 2013 could have been reduced by some ERM techniques, should encourage funding research in this area. If not only the UREs can help to equilibrate the energy budget of the Earth, but also avoid health costs and increase economic welfare factors as described by Shindell [250] that save lives and increase crop yields: these parameters have also to be taken into account.

So far geoengineering has been considered to have direct costs, with indirect benefits for avoiding several of the costs of global warming evaluated by the Stern Review [1]. Several risk assessments have been performed, comparing the different techniques [13] and options. The risk assessment performed by the Royal Society [16], concentrated on their cooling potential, effectiveness and affordability. Meanwhile the MR described in this review have not yet been assessed comparatively to each other, or towards their potential benefits for the climate. Maybe only two of them (SCPPs and DETs) have been assessed by some investors in terms of direct profitability and possible direct financial benefits. The ERM options described here deserve further risk assessment and potential climate and health benefits analysis. The main point is that these UREs can produce CO2-free renewable energy and be competitive with other existing energies, so allow investors to make profits and at the same time avoid future CO2 emissions and cool the Earth for free. Meanwhile sending sulfates in the stratosphere has a cost of several billions per year, that will not stop growing and SRM must be continued for decades or even perhaps for more than a century, without reducing the accumulation of CO2 in the atmosphere, nor solving the problem of ocean acidification.

Grandqvist and Smith [294] think that it is not unreasonable to imagine a world where clean power sources, using some combination of solar energy and sky cooling, become the backbone of a low-carbon economy. The prospect then is not only less pollution but, in due course, lower power cost. Also, as the proceeding global warming will lead to increasing demands on cooling that will soon escalate into a dominant problem for power supply and for the environment, they find fortunate that there is overhead an untapped and vast natural low-cost cooling resource: the clear night sky.

The ERM methods proposed in this review address the same type of outgoing longwave radiation that the GH effect keep trapped; meanwhile the SRM addresses incoming solar shortwave radiation. Thus even if SRM can on average reduce the temperature in similar proportions as the increase caused by the GH effect, the effects will be for instance more precipitation in some parts of the Earth and more droughts [112] elsewhere, with losers and winners. If unintended effects appear and if it becomes necessary to provide food to the local population, or if logistical support is needed, or if financial compensation or reconstruction are necessary, then the costs of SRM and of GE will keep growing up. Meanwhile building for instance SCPPs in desert countries with local labor and materials will provide work, growth, economic development and welfare and also benefits to the investors. Agricultural greenhouses in the middle of deserts can be filled with cold humid air coming out from DETs and irrigated with desalinated water produced in synergy at proximity.

This paper shows that geoengineering is not the last resort against global warming and that other strategies might help to solve the current problems without the need to just buy time by relieving GW symptoms without addressing the ocean acidification problem, nor the CO2 accumulation in the atmosphere. Instead of setting a sunshade for planet Earth like the techno-fix described in the 4th episode of the “Highlander” movies, other solutions might be possible. SRM try to solve the CO2 problem without decreasing CO2 releases. ERM proposes taking control of our planet's climate by unusual power generating systems producing CO2-free renewable energy allowing heat loss into space and helping to cool the Earth's surface. Instead of trying to counteract a longwave radiation trouble by different shortwave radiation strategies, ERM works on the longwave radiation part of the spectrum with MR based on thermodynamic properties of the Earth's atmosphere, reproducing several natural phenomenon and the water cycle. This timely composed review should stimulate many more research teams and hopefully it will be useful not only to the technical community, but also to policy makers and power industrial firms to enter the exciting field of ERM and to push it forward to diverse practical applications.

Investment in renewables, in UREs and in a sustainable economy is not only a worthwhile cause but has also economic value. The associated carbon credits would also help offset the carbon liabilities for normal operation and hence add to the economic viability of sky cooling, UREs and MR.

14. Conclusion
In this review the main GE methods proposed to perform SRM in order to reduce the effects of anthropogenic global warming were summarized, and some of their limitations introduced and ethical aspects reported. Before introducing the concept of ERM, a short review of the literature showing that anthropogenic waste heat release by thermal power plants might be important at a local scale was given, as well as a short overview of some drawbacks of several renewable energies, which makes them “not so green” or “not so neutral” for the climate change problem.

Then this review paper proposed several new concepts aimed to fight global warming by enhancing outgoing longwave radiation, and able to transfer heat out to space, prevent sea level rise and avoid future costs, for instance of hurricanes, or of CCS or CDR. In this purpose, power-generating systems able to transfer heat from Earth to upper layers of the atmosphere and then to the space are reviewed. The individual UREs presented in this paper were initially developed to become power plants for decarbonized electricity production. The principal new concept proposed in this paper is that it is possible to increase the longwave radiation transfer from the Earth surface to the outer space by increasing the direct energy transfer from the Earth to the space by the atmospheric window; transferring surface hot air in altitude; transferring cold air to the surface; transferring heat from the oceans in altitude; increase sea ice thickness. So these UREs can be named meteorological reactors. This article shows that a large family of MR exist, ant that these MR are able to manage longwave radiation in order to cool down the earth.

SRM is not intended to solve the climate change problem. SRM is intended to buy time to let our descendants or heirs find the solution for us, address it latter and pay for it. The world population growth, growth per capita, carbon intensity growth and the economic development require more energy. SRM does not provide more energy to humanity. SRM does not stop the inadvertent climatic change due to fossil fuels combustion. The IPCC conclusions can be summarized to the fact that all climate models demonstrate that the best way to stop climate change is to stop the introduction of CO2 in the atmosphere. SRM allows more CO2 accumulation in the atmosphere. SRM is a voluntary and targeted climate modification. On the contrary ERM consists in a voluntary reduction of the main cause of climate change as a large scale deployment of MR decarbonizes the economy, is able to provide the humans the energy they need and can help to cool the Earth surface and curb GW.

Hansen [106] performed computer simulations of the equilibrium responses in case forcings are introduced in the higher layers of the atmosphere: the higher the heating is introduced, the larger is the fraction of the energy that is radiated directly to the outer space without warming the surface. Simulations performed by Ban-Weiss [111] showed that for every 1 W m−2 that is transferred from sensible to latent heating, on average, as part of the fast response involving low cloud cover, there is approximately a 0.5 W m−2 change in the top-of-atmosphere energy balance (positive upward), driving a decrease in global mean surface air temperature. Other models [186] assimilate the tropical cyclones to Carnot heat engines that absorb heat from a warmer reservoir (the ocean surface) and reject a fraction of it to a colder reservoir (the highest atmospheric layers of the troposphere and thus the outer space) while doing work.

One of the ideas developed in this review is that GHGs are too good insulators that prevent normal interactions between the Earth atmosphere and the outer space and thus keep the Earth too hot, so atmospheric thermal bridges have to be created. For this purpose, the rupture technology concept of meteorological reactors is given: these are unusual power plants able at the same time to produce renewable and clean energy, avoiding future CO2 emissions, reducing hurricane intensity, preventing heat waste release at the surface level and cooling down the Earth by increased sensible heat transfer or latent heat transfer out to space.

A combination portfolio of techniques, an energy mix of a wide large bunch of methods that are free of CO2 emissions will be necessary to fight global warming. Meanwhile current power plants release heat at the surface, MR release it in altitude and can at least contribute to cool down the Earth surface and help it to keep a neutral global energy budget.

The accelerated technology scenarios explored by the IEA [95] suggest that even a major global mitigation program, based on successful development and deployment of several new technologies, will still allow substantial global warming by 2100.

Availability of key technologies will be necessary but not sufficient to limit CO2 emissions. Mitigation of three trillion tons of CO2 by 2100 is deemed a serious goal, thus a major increase in R&D resources is needed. Given the monumental challenge and uncertainties associated with a major mitigation program, the authors would like to advise to consider all available and emerging technologies. This suggests fundamental research on new MR energy technologies in addition to those already known in order to become part of the global research portfolio, since breakthroughs on today's embryonic technologies could yield tomorrow's alternatives.

The authors hope that the ideas exposed in this paper might help this purpose and that in the near future, by their capability to supply massive amounts of energy carbon emission-free and for their prospective for large-scale implementation some UREs described here will give a significant contribution to the overall solution to global warming, although they still require more research, but first and foremost much more investments to build the first industrial plants as the theoretical assessment is already rich and almost complete.

Acknowledgments
This research was supported by the National Natural Science Foundation of China (51106060) and the Natural Science Foundation of Hubei Province (2012FFB02214).

One of the authors (R K de_Richter) wishes to warmly acknowledge Dr. Denis Bonnelle for his support, guidance, advice, help and many fruitful and constructive scientific exchanges. Dr. D. Bonnelle has developed the ideas of the “radiative thermal bridges” and “meteorological reactors” described in this review.

The authors would like to thank the anonymous reviewers for their helpful and constructive comments and suggestions that greatly contributed to improve the quality of the paper.

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2015

The Dept. of Solar Radiation Management
Is attempting to block the sun’s heat a good idea—or a reflection on our pathetic efforts to cut carbon emissions?

January 25, 2015 Brian Palmer
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Solar Radiation Management (n.): a set of geo-engineering strategies intended to minimize global warming by reflecting the sun’s energy away from the earth.

Solar radiation management sounds like corporate-speak. (VP for Solar Radiation Management, anyone?) In fact, it’s one of the most controversial concepts in climate change mitigation.

The idea is fairly straightforward. First, some Climate Change 101: About 30 percent of the energy arriving from the sun is reflected back out into space, mostly by clouds and ice. The planet—its seas, lands, creatures, buildings, etc.—absorbs the remaining 70 percent, and gases such as carbon dioxide and methane trap some of that heat within the atmosphere. So here’s the pitch for SRM: What if we could bounce more of the sun’s energy back into space in the first place, preventing it from ever getting trapped by the greenhouse effect?


Illustrated by: Michael Jenkins
How might we make that happen? One wildly impractical proposal is a network of space mirrors. The problem with this idea isn’t just that it sounds like something a fourth-grader would dream up—it would take a tremendous amount of energy to launch those mirrors into space. (Wasting energy by burning more fossil fuels is precisely what we’re trying to avoid.) Another option is to mist seawater into the air. Those white clouds would reflect solar energy before it reaches the darker, energy-absorbing ocean. This strategy has moderate support among SRM advocates.

To understand the most popular approach, though, think back to the 1991 eruption of Mount Pinatubo in the Philippines, if you can remember it. (I envy your youth if you can’t.) The volcano spewed a huge amount of sulfur dioxide, a precursor of sulfuric acid. Within a year, the sulfuric acid layer stretched around the entire planet. Tiny droplets of sulfuric acid reflected sunlight back into space before it reached earth, and the planet’s temperature dropped by approximately 1 degree Fahrenheit for more than two years.


Photo: D. Harlow
SRM advocates say we could do something similar—and without killing hundreds of people and displacing thousands more, as Pinatubo’s eruption did. We could fly high-altitude planes or float balloons capable of spraying sulfur dioxide into the atmosphere. (Other chemicals, such as titanium dioxide, are also under consideration.) To offset a doubling of atmospheric CO2, which represents a global temperature increase of between 1.5 and 4.5 degrees Celsius, researchers think we would have to inject around five megatons of sulfur dioxide annually into the atmosphere.

Five megatons sounds like a lot—and it is—but SRM enthusiasts point out that coal-fired power plants already emit nearly 55 megatons of sulfur. This is just a 10 percent increase on that number, and for a good cause.

Are you sold? Not so fast. There are many, many complications, and even the most open-minded conservationists are skeptical of SRM. For one thing, it’s largely unproven. Just because a volcano can do something doesn’t mean we can do it—especially in a controlled fashion. In addition, the effects of SRM are likely to vary by region. The process could make some areas colder than they currently are, depressing agricultural production. Russian farmers, for example, might have something to say about this. SRM might also diminish our motivation to reduce carbon emissions. That’s an issue because excess carbon causes many problems beyond higher temperatures. The oceans would continue to acidify, for example, killing off species like clams, oysters, and corals, and disrupting marine ecosystems.

For now, think of solar radiation management as the panic room of climate change mitigation. Few people really want to go there, but it’s still tempting to some, especially since our current rate of progress on carbon reduction suggests things could get pretty bad. Let's hope we can turn things around before it comes to that.

onEarth provides reporting and analysis about environmental science, policy, and culture. All opinions expressed are those of the authors and do not necessarily reflect the policies or positions of NRDC. Learn more or follow us on Facebook and Twitter.
https://www.nrdc.org/onearth/dept-solar-radiation-management

Blocking out the sun won’t fix climate change – but it could buy us time
November 19, 2015 12.16am EST
Author
Hugh Hunt
Reader in Engineering Dynamics, University of Cambridge

Disclosure statement
Hugh Hunt received funding from the EPSRC for the SPICE project. He has worked as a consultant for Davidson Technology and is named on a patent "Atmospheric Delivery System" related to the use of tethered balloons for climate engineering http://www.google.co.uk/patents/WO2011073650A1?cl=en

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Volcanic eruptions lead to global cooling – could we mimic them? Beawiharta Beawiharta / Reuters
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The Paris climate talks hope to set out how we can reduce the amount of carbon we’re pumping into the atmosphere. But emissions cuts alone may not be enough. Atmospheric CO2 is the blanket that keeps our planet warm and any further emissions will mean more global warming. Observations in recent years show that warming is accelerating, that polar ice and glaciers are all melting, that sea level is rising … it all looks rather bleak.

Could we directly engineer the climate and refreeze the poles? The answer is probably yes, and it could be a cheap thing to achieve – maybe costing only a few billion dollars a year. But doing this – or even just talking about it – is controversial.

Some have suggested there is a good business case to be made. We could carefully engineer the climate for a few decades while we work out how to reduce our dependency on carbon, and by taking our time we can protect the global economy and avoid financial crises. I don’t believe this argument for a minute, but you can see it’s a tempting prospect.

Reflecting the sun
One option might be to reflect some of the sun’s energy back into space. This is known as Solar Radiation Management (SRM), and it is the most viable climate engineering technology explored so far.

We bring the expertise of academics to the public.
For instance we could spray sea water up out of the oceans to seed clouds and create more “whiteness”, which we know is a good way to reflect the heat of the sun. Others have proposed schemes to put mirrors in space, carefully located at the point between the sun and the Earth where gravity forces balance. These mirrors could reflect, say, 2% of the sun’s rays harmlessly into space, but the price tag puts them out of reach.


All those clouds reflect a lot of heat. Reuters
Perhaps a more immediate prospect for cooling the planet is to spray tiny particles high up into the stratosphere, at around 20km altitude – this is twice as high as normal commercial planes fly. To maximise reflectivity these particles would need to be around 0.5 micrometres across, like the finest of dust.

We know from large volcanic eruptions that particles injected at high altitude cool the planet. The 1991 eruption of Mount Pinatubo in the Philippines is the best recent example. It is estimated that more than 10m tonnes of sulphur dioxide were propelled into the high atmosphere and it quickly formed tiny droplets of sulphuric acid (yes, the same stuff found in acid rain) which reflected sunlight and caused global cooling. For about a year after Pinatubo the Earth cooled by around 0.4℃ and then temperatures reverted to normal.

I was involved recently in the SPICE project (Stratospheric Particle Injection for Climate Engineering) and we looked at the possibility of injecting all sorts of particles, including titanium dioxide, which is also used as the pigment in most paints and is the active ingredient in sun lotion.


The experiment to validate models of tether dynamics was cancelled. Hugh Hunt, CC BY-SA
The technology to deliver these particles is crazy – we looked at pumping them in a slurry up to 20km into the air using a giant hose suspended by a huge helium balloon. A small-scale experiment was cancelled because even it proved too controversial, too hot. Imagine if we demonstrate that this technology can work. Politicians could then claim there was a technical “fix” for climate change so there would be no need to cut emissions after all.

But this isn’t a ‘quick fix’
There are so many problems with climate engineering. The main one is that we have only one planet to work with (we have no Planet B) and if we screw this one up then what do we do? Say “sorry” I guess. But we’re already screwing it up by burning more than 10 billion tonnes of fossil fuels a year. We have to stop this carbon madness immediately.

Engineering the climate by reflecting sunlight doesn’t prevent more CO2 being pumped into the atmosphere, some of which dissolves in the oceans causing acidification which is a problem for delicate marine ecosystems.

There is therefore a strong imperative to remove the 600 billion tonnes of fossil carbon that we’ve already puffed into the air in just 250 years. This is known as Carbon Dioxide Removal (CDR).

We must work fast to cut our carbon emissions and at the same time we should explore as many climate engineering options as possible, simultaneously. However while reflecting sunlight may be an idea that buys us some time it is absolutely not a solution for climate change and it is still vital that we cut our emissions – we can’t use climate engineering as a get-out clause.

Geoengineering
solar radiation management
Climate engineering
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https://theconversation.com/blocking-out-the-sun-wont-fix-climate-change-but-it-could-buy-us-time-50818

2020

Can Space Mirrors Save Humanity from Climate Catastrophe?
From giant mirrors to sucking carbon dioxide out of the atmosphere, geoengineering could offer solutions to the climate crisis
Paul Abela, MSc

Oct 12, 2020 · 7 min read

When presented with a problem, human creativity, innovation, and problem-solving always rise to the challenge. We fix things, we always have, and always will. Many argue climate change is another technological challenge that needs to be overcome and geoengineering is the technology to rise to the challenge. The idea is by manipulating the climate, we can reduce the Earth’s temperature; balancing out any increases from global warming.
It sounds extreme because geoengineering projects are extreme.
But a recent political breakthrough may make the need for geoengineering redundant. China recently announced a bold target to reach net zero emissions by 2060, which sounds great. After all, China is the world’s largest emitter of greenhouse gases.
China’s announcement is a breakthrough, but it’s hardly a moment to celebrate. If anything, it provides an unsavoury picture of how bad things have become.
The Intergovernmental Panel on Climate Change (IPCC) argues we must limit global warming to 1.5°C to reduce the risks of climate change. The IPCC suggests “limiting warming to 1.5°C implies reaching net-zero CO2 emissions globally around 2050”. Net-zero means there is a balance between carbon emitted into the atmosphere and carbon removed from the atmosphere.
Based on the IPCC’s estimates, China’s target to reach net-zero will come ten years too late. And that’s just one country. When every country needs to hit net zero by 2050, you start to realise the enormity of the challenge we face.
Can technology save the day?
It’s also important to note China’s one-party political model provides it with the ability to make such a bold statement. In democracies where governments change every few years, any commitment a government makes isn’t set in stone.
In the United States, for example, Barack Obama’s administration agreed to the Paris Climate Agreement, only for this decision to be overturned by Donald Trump’s administration. So even if countries agree to a formal commitment, there is no guarantee they’ll keep their promise.
On top of this, global CO2 emissions continue to increase year-on-year. Last year, emissions hit 33 GtCO2, a record. At a time when global emissions need to decrease, the fact they’re increasing makes it seem optimistic, at best, that we’ll be able to meet the IPCC’s target. After all, 2050 is only 30 years away.
Looking at the political climate, we may have reached a point where geoengineering is our only option in solving the most significant challenge humanity has ever faced. If we don’t, we will face a catastrophe of never before seen proportions.
There are ideas, but as with everything about climate change, deploying them is far from black and white.
Solar radiation management (SRM)
Solar Radiation Management (SRM) is a concept that has received the most publicity. SRM “is a theoretical approach to reducing some of the impacts of climate change by reflecting a small amount of inbound sunlight back out into space.”
An idea of particular interest is Stratospheric Aerosol Injection (SAI). The reason why is because it occurs naturally.
The volcanic eruption at Mount Pinatubo in the Philippines released 20 million tonnes of sulphur dioxide into the atmosphere. Sulphur Dioxide works in the opposite way to greenhouse gases by reflecting (rather than absorbing) some of the Sun’s heat away from Earth.
When Mount Pinatubo erupted, the sulphur dioxide emitted from the volcanic eruption remained in the atmosphere for two to three years, reducing Earth’s average temperature during this period.
SAI would mimic this phenomenon. Aeroplanes or balloons would be deployed to spray sulphur dioxide into the upper atmosphere. By doing so, we would increase the number of sulphur particles in the atmosphere, helping to reflect sunlight away from Earth.
The issue with the idea is that it’s highly controversial. A study into the possible consequences of SRM found each solution “reduced temperatures but all also worsened floods or droughts for 25%-65% of the global population, compared to the expected impact of climate change.”
So SAI provides the desired result in decreasing temperatures. But with such massive unknowns, the risks may outweigh the benefits.
Space mirrors
Space mirrors are another example of SRM. The idea is to launch thousands of space mirrors into space.
While sounding pretty out there, the idea has gained popularity in the United Nations. If you had enough mirrors, you could reflect enough of the Sun’s light to reduce the Earth’s temperature. These mirrors would help to balance out the warming effect of greenhouse gases.
The major issue with the idea is that again; it’s risky. You couldn’t experiment to see how well it worked in the real world. Computer simulations could help in modelling the pros and cons. But you would never know for sure until you experimented on the Earth itself.
By launching mirrors into space, we would be propelling ourselves into a profoundly uncertain future. It would be like pulling the trigger of a gun with a blindfold on and hoping it hits the right target. If it didn’t, there would be no turning back.
Another stumbling block is any SRM project such as deploying space mirrors would need global consensus. It seems unlikely every country would agree to a real-world experiment.
Imagine if countries did agree to the project and some countries faced severe weather events as a result? To save face, it’s not hard to imagine politicians blaming one another, which could escalate into all-out war.
On top of this, the technology doesn’t exist yet. So any such project would be years in the making. Aside from it being pretty bonkers, the issue we would have is time. And time isn’t on our side.
Carbon capture and storage (CSS)
Carbon capture and storage (CSS) is “a technology that can capture up to 90% of the carbon dioxide (CO2) emissions pro­duced from the use of fossil fuels in electricity generation and industrial processes, preventing the carbon dioxide from entering the atmosphere.”
The primary benefit of using CSS compared to SRM is it doesn’t have any effect on the climate. It works to capture the carbon dioxide that would otherwise have entered the atmosphere. CSS offers a low-risk strategy to limit the amount of carbon emitted by industry.
“At the moment, CCS is the only technology that can help reduce emissions from large industrial installations. It could be an essential technology for tackling global climate change.”
The deployment of the technology wouldn’t involve any political wrangling. A country could invest in the technology, knowing it will reduce carbon emissions while having no adverse side effects on the climate.
CSS could be a game-changer for countries in helping them get to net-zero. There are currently 21 large scale CSS projects around the world. These projects store millions of tonnes of carbon dioxide, that would otherwise have gone into the atmosphere.
This sounds promising until one considers that we will need sixteen thousand more plants…to deal with current emissions. Emissions are still increasing so fast that we will need another thousand plants each year just to keep up with the annual increase. And transport emissions — which are growing even faster — can not be captured by any technology.
Marshall, G. Don’t even think about it, p. 179.
As George Marshall notes, we would need to scale the technology to an enormous level for it to have an impact. The issue with this is two-fold. The technology is expensive, and the only incentive for installing it is to prevent emissions entering the atmosphere.
From a financial perspective, any project would be a cost. And when the expense outweighs the rewards for installing the technology, it’s hard to imagine thousands of CSS projects being initiated spontaneously without any government legislation incentivising them to do so.
We only have one climate
Out of the three options, CSS has the most potential because it would have no impact on the climate.
While CSS is a promising technology, a drawback is it won’t help in dealing with the damage already done. It’s also specific to certain industries; for example, the travel or food industries couldn’t use CSS.
SRM would seek to rebalance the climate, helping to relieve the problem. But the most significant obstacle with SRM is deploying any project would impact each country in the world.
When the risks are so high, it feels unimaginable countries would agree to a project that could have a worse impact on the climate, then climate change itself.
We need a new approach
What’s staring us in the face is there are solutions without the need to deploy high-risk geoengineering projects. We can build a green economy which works in harmony with the environment.
What’s missing is the will to change.
As long as the will is lacking, humanity faces a profoundly uncertain future. That it’s reached a point where launching mirrors into space is a viable solution should give you an idea of how desperate the situation has become.
But these technological solutions are being used to justify the continuation of our current behaviour. Almost as if there’s nothing to worry about as when things do get bad, we’ll just use technology to get ourselves out of a hole.
If we’ve learnt nothing, it’s that our lack of respect for the Earth and its rules has led us into this mess in the first place. The way to get out of it is to respect nature and play by its rules. After all, if you play with fire, you can expect to get burnt.

https://medium.com/climate-conscious/can-space-mirrors-save-humanity-from-climate-catastrophe-6a3cb548b950
 

Fighting global warming by climate engineering: Is the Earth radiation
management and the solar radiation management any option
for fighting climate change?
Tingzhen Ming a
, Renaud de_Richter b,n
, Wei Liu a
, Sylvain Caillol b
a School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
b Institut Charles Gerhardt Montpellier UMR5253 CNRS-UM2 ENSCM-UM1 – Ecole Nationale Supérieure de Chimie de Montpellier,
8 rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France
article info
Article history:
Received 11 June 2013
Received in revised form
21 October 2013
Accepted 18 December 2013
Available online 22 January 2014
Keywords:
Earth radiation management
Geoengineering
Thermal shortcuts
Solar updraft chimney
Downdraft evaporative tower
Heat pipe
Clear-sky radiative cooling
abstract
The best way to reduce global warming is, without any doubt, cutting down our anthropogenic emissions
of greenhouse gases. But the world economy is addict to energy, which is mainly produced by fossil
carbon fuels. As economic growth and increasing world population require more and more energy, we
cannot stop using fossil fuels quickly, nor in a short term.
On the one hand, replacing this addiction with carbon dioxide-free renewable energies, and energy
efficiency will be long, expensive and difficult. On the other hand, meanwhile effective solutions are
developed (i.e. fusion energy), global warming can be alleviated by other methods.
Some geoengineering schemes propose solar radiation management technologies that modify
terrestrial albedo or reflect incoming shortwave solar radiation back to space.
In this paper we analyze the physical and technical potential of several disrupting technologies that
could combat climate change by enhancing outgoing longwave radiation and cooling down the Earth.
The technologies proposed are power-generating systems that are able to transfer heat from the Earth
surface to the upper layers of the troposphere and then to the space. The economical potential of some of
these technologies is analyzed as they can at the same time produce renewable energy, thus reduce and
prevent future greenhouse gases emissions, and also present a better societal acceptance comparatively
to geoengineering.
& 2014 The Authors. Published by Elsevier Ltd.
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 793
2. Overview of the major SRM geoengineering proposals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 795
2.1. Space mirrors [31,32] and science fiction-like proposals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 795
2.2. Sulfate aerosols [34,35] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 795
2.3. Cloud whitening [41,42] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 796
2.4. Other albedo changes [45,46] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 796
2.5. Some examples of small scale SRM experiments already performed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 797
2.6. Discussion about SRM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 798
3. Earth radiation management (ERM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 799
Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/rser
Renewable and Sustainable Energy Reviews
1364-0321 & 2014 The Authors. Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.rser.2013.12.032
Abbreviations: AVE, atmospheric vortex engine; BC, black carbon; CCS, carbon capture and sequestration; CDR, carbon dioxide removal; CE, climate engineering; CSP,
concentrated solar power; DET, downdraft energy towers; ERM, earth radiation management; GE, geoengineering; GH, greenhouse; GHG, greenhouse gases; GW, global
warming; HMPT, Hoos mega power tower; IPCC, Intergovernmental Panel on Climate Change; MR, meteorological reactors; OTEC, ocean thermal energy conversion; PCM,
phase change materials; SCPP, solar chimney power plant; SRM, solar radiation management; SRM, sunlight reflection methods; URE, unusual renewable energies; UV,
ultraviolet
n Corresponding author. Tel.: þ33 4 67 52 52 22; fax: þ33 4 67 14 72 20.
E-mail address: renaud.derichter@gmail.com (R. de_Richter).
Renewable and Sustainable Energy Reviews 31 (2014) 792–834
Open access under CC BY license.
Open access under CC BY license.
3.1. Targeting high and cold cirrus clouds: not a SRM strategy but a ERM one . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 799
3.2. Preventing a possible weakening of the downwelling ocean currents: also an ERM strategy? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 800
3.3. Alternatives to SRM do exist . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 801
4. Why looking for energy removal methods? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 801
4.1. Waste heat and thermal emissions also warm Gaia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 801
4.2. Renewable energies have some dark side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 802
4.3. Can we enhance heat transfer?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 802
4.4. Earlier computer study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 802
4.5. Cooling by irrigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 803
5. ERM to produce thermal bridging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 804
6. Transferring surface hot air several kilometers higher in the troposphere. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 805
6.1. Solar updraft Chimneys: power plants that run on artificial hot air. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 805
6.2. Discussion about the cooling effects of kilometric high chimneys and towers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 806
6.3. The two hypotheses for the air released in altitude by SCPPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 807
6.4. Super chimney. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 808
6.5. The hot air balloon engine to release air in altitude . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 808
7. Transferring cold air to the Earth surface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 810
7.1. Downdraft evaporative cooling tower for arid regions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 810
8. Transferring latent (or sensible) heat to the top of the troposphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 811
8.1. Creating artificial tornadoes: the atmospheric vortex engine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 811
9. Transferring surface sensible heat to the troposphere. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 813
9.1. Heat pipes and thermo-siphons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 813
9.2. Super power station or mega thermo-siphon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 814
9.3. Mega thermo-siphon or ultra large scale heat-pipe. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 816
10. Other energy transfers to the troposphere to cool the earth surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 816
10.1. Polar chimney . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 816
10.2. Taking advantage of energy potential of the undersea level depressions to install other pipelines and ducts useful to produce electricity
and increase local albedo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 817
10.3. Examples of the endless possibilities of high towers use for global warming reduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 817
11. Clear sky radiative cooling or targeting the atmospheric window . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 819
12. Overview of the principal ERM techniques proposed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 822
13. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 824
14. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 828
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 828
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 828




1. Introduction
The most serious and important problem humankind has ever
had to face might be global warming with disastrous consequences and costly adverse effects [1]. Adaptation and mitigation
strategies might not be sufficient. In May 2013 the CO2 concentration in the Earth0
s atmosphere officially exceeded 400 ppm,
according to the Mauna Loa Observatory in Hawaii, which has
been monitoring atmospheric CO2 since 1958 when that figure
was around 320 ppm. At the time the Intergovernmental Panel on
Climate Change (IPCC) issued its 2007 assessment [2], it recommended to keep atmospheric greenhouse gases below 450 ppm in
order to keep the temperature rise under a 2 1C target [3].
Many scenarios have been considered in order to slowly
decrease our greenhouse gases (GHG) emissions to try to keep
the average temperature heat rise under þ2 1C. But without an
international agreement signed by the biggest polluters, this
o2 1C figure will remain only empty words and will not be
followed by actions and effects.
Human GHG emissions have already been so important and
some of these GHG have such extraordinarily long lifetimes that
even if by a magic wand we could stop all emissions overnight, the
average temperature of Earth would continue to rise or stay at
current levels for several hundred years [4].
Global warming results from the imbalance between the heat
received by the Earth and, the heat reradiated back to space. This
paper proposes methods to increase the IR radiation to space. The
surface outgoing longwave radiation is defined as the terrestrial
longwave radiative flux emitted by the Earth0
s surface beyond the
3–100 mm wavelength range. The shortwave incoming solar
radiation also called global irradiance or solar surface irradiance
[5] is the radiation flux density reaching a horizontal unit of Earth
surface in the 0.2–3 mm wavelength range. Both are expressed in
W m 2
.
The GHGs trap some heat and, by greenhouse effect, warm the
Earth surface. Incoming and reflected shortwave sunlight patterns
are represented on the right side of Fig. 1 from NOAA [6] (inspired
by Kiehl [7] and Trenberth [8]); outgoing infrared or longwave
radiation modes are symbolized on the left side. The Earth0
s
energy budget expressed in W m 2 is summarized in this figure.
The principal atmospheric gases ranked by their direct contribution to the greenhouse effect are [7] water vapor and clouds
(36–72%), carbon dioxide (9–26%), methane (4–9%) and ozone (3–7%).
Tackling climate change will require significant reductions in
the carbon intensity of the world economy. Developing new lowcarbon technologies and adopting them globally is therefore a
priority. But even moving relatively quickly toward a carbonneutral economy will still result in a net increase in CO2 in the
atmosphere for the foreseeable future. It seems that we are
nowhere close to moving quickly in this direction: gas and fossil
fuel reserves have effectively increased, due to improved technologies for extraction. Huge underwater oceanic reserves of methane
hydrates or clathrates [9,10] will possibly become extractible in
the near future. The recent shale gas boom in USA and the
methane reserves do not militate in favor of a reduction of the
energy consumption, nor in a reduction of CO2 and CH4 emissions.
With gas prices hitting rock bottom, the cost competitiveness of
renewable energies in the short- to mid-term will be harder to
meet than ever before. This has brought further uncertainty about
the future of solar projects and offshore wind technologies,

particularly solar ones. The innovation challenge spans the development of new unusual renewable energies based on low-carbon
technologies as well as – and possibly even more pressing –
improving the performance, the efficiency, and particularly lowering the costs of the existing ones.
This review intends to be an element that provides an update
on proposed solutions to the control and the management of the
climate, and to propose a tool of choice among new and innovative ones.
Geoengineering aims at stabilizing the global climate, reducing
global warming and fighting anthropogenic climate change owing
to two strategies: shortwave (0.3–3 μm) sunlight reflection methods and carbon dioxide removal technologies. After a short overview of a set of geoengineering strategies, this paper then
proposes innovative methods for increasing outgoing terrestrial
(4–100 μm, and most often 4–25 μm) radiant energy fluxes by
thermal longwave radiation methods. One of the main ideas
developed in this review is that GHGs are good insulators that
prevent normal interactions with the Earth atmosphere with the
space, and keep the Earth too hot, so “atmospheric thermal
bridges” have to be created. By analogy to the expression of
“thermal bride” used in civil engineering where heat is transferred
by conduction from one part of a building to another, with the
result of a cooling of the hotter part, we define an atmospheric
thermal bride has a way to transfer longwave radiation from one
part of the atmosphere (generally the Earth surface) to another
(generally in the higher troposphere, the stratosphere, or to the
open space). One natural phenomenon illustrating this concept
is the atmospheric window, by which IR radiation in the range
8–13 mm can escape directly to space.
After an overview of the principal geoengineering techniques of
solar radiation management (SRM or sunlight reflection methods),
we present in this review technological breakthrough alternatives,
many of them are little known, misunderstood or ignored, that can
decrease or decelerate global warming (GW), and also might help
to cool the Earth surface.
A 30 years power-purchase agreement of the Southern Californian
Public Power Authority [11] for the construction of the first solar
updraft chimney in La Paz County, Arizona, USA was announced in
February 2011. Another company [12] published plans to combine
downdraft evaporative cooling towers with wind towers to produce
electricity. The opportunity to take stock of similar disrupting
technologies and their benefits is examined in this paper.
These recent announcements for the construction of industrial
scale power plants of solar updraft chimneys and downdraft
energy towers have been made. These unusual renewable energy
power plants belong to the family of large scale power stations
called by us “meteorological reactors” which can convert heat into
artificial wind inside a duct and produce electricity by driving
turbines. Despite many interesting advantages such as the low cost
of the kWh produced, a long lifespan, clean energy production and
environmentally friendly operations with almost no maintenance,
their current commercial applications are limited because of their
large initial investment cost and low conversion yield.
Several energy-neutral ideas and techniques will be described,
followed by a description of a number of innovative and unusual
renewable energies (UREs), from the family of the meteorological
reactors (MR), which can at the same time help cooling the planet
by Earth radiation management (ERM), produce CO2-free electricity and prevent further CO2 emissions.
This review focuses on using several MR, night sky radiation
and giant heat pipes as active heat transfer tools to cool down the
Earth by artificial vertical wind generation and, at the same time,
production of sustainable CO2-free renewable energy without
the drawbacks of current climate engineering strategies. This
review sheds light on innovative activity and innovation dynamics
in heat-transfer technologies and CO2-free renewable energy
production.
Fig. 1. “Earth0
s Annual Global Mean Energy Budget” (from NOAA) [6].
794 Tingzhen Ming et al. / Renewable and Sustainable Energy Reviews 31 (2014) 792–834
2. Overview of the major SRM geoengineering proposals
Proposals for GE projects can mainly be divided into two categories: SRM and carbon dioxide removal (CDR) [13]. CDR techniques
(that curiously are considered as CE, but probably might not) are out
of the scope of this paper and thus will not be depicted. The IPCC
Fourth Assessment Report [14a] defines geoengineering (GE) as
“technological efforts to stabilize the climate system by direct intervention in the energy balance of the Earth for reducing global warming”.
Geoengineering [15] or climate engineering (CE) consists in a large
set of technologies that deliberately reduce solar insolation or
increase carbon removal directly from the atmosphere, on a large
scale, with the aim of minimizing, counteracting, mitigating, limiting,
counterbalancing or reversing anthropogenic climate change in order
to reduce GW or its consequences. The raise of geoengineering on the
scientific and policy agenda is no doubt at the international level, as it
has been assessed by the 5th IPCC working groups (WP) 1 and 3. The
5th IPCC report of WP1 issued in September 2013 [14b] cites
geoengineering 50 times only in its chapter 7 and 16 publications
on geoengineering are cited in Chapter 6.
In a Royal Society [16] report, geoengineering is defined as the
“deliberate large-scale manipulation of the planetary environment to
counteract anthropogenic climate change”. This Royal Society report
reviews a range of proposals aimed to reflect the Sun0
s rays back to
space, and, among means to remove CO2 from the air for instance
oceanic carbon sequestration, by injecting iron into the world0
s
seas to rapidly increase the amount of phytoplankton that feeds
itself from CO2.
An almost exhaustive list of proposed GE projects has been
established [17], a very large review of CE proposals has been
given by Vaughan [13], and numerous other strategies have
been listed [18,19]. Literature is now abundant about geoengineering proposals, describing them in detail and discussing their
advantages, effectiveness, potential side effects and drawbacks
[16,20,21,22], but also governance, legitimacy and ethical aspects
[23,24,25]. As a matter of fact, criticism about CE research focuses
on international consequences of possible unilateral use of GE
techniques [26,27].
SRM proposals aim to reduce GW by reducing the amount of
light received on the Earth and by its atmosphere [28]. It includes
(Fig. 2) several techniques like space solar reflectors; stratospheric
injection of aerosols; seeding tropospheric clouds by salt aerosols
or ice nucleation to make them whiter and also surface albedo
change (urban, rural, or atmospheric approaches). Numerous other
strategies have been proposed [29,30], but the aim of this review is
not to be exhaustive. GE has been quite studied since 2008 and is
envisioned as a plan B in case the governments do not succeed to
reduce CO2 emissions. At the international level of climate change
politics, the positioning of CE as an option between mitigation and
adaptation is taking concrete form. The elaboration of an alternative plan C developing the concept of Earth radiation management (ERM) is at least appealing and entailing and is the goal of
this review which has in mind the need for innovative breakthroughs. Those new strategies have the potential to address
2.2 times more energy flux (69%) than SRM (31%).
2.1. Space mirrors [31,32] and science fiction-like proposals
The idea of this GE scheme is to send into orbit giant mirrors
(55,000 orbiting mirrors each of 100 km2
) made of wire mesh; or
to send trillions of light and small mirrors (the size of a DVD), in
order to deflect sunlight back to space. In other words numerous
artificial mini-eclipses that will obscure the sun. This option is
widely considered unrealistic, as the expense is prohibitive, the
potential of unintended consequences is huge and a rapid reversibility is not granted.
Similarly, the reduction of incoming solar radiation was considered by placing a deflector of 1400-km diameter at the first
Lagrange Point, manufactured and launched from the Moon [33].
The idea to mine the moon [28] to create a shielding cloud of dust
is in the same league.
Several other proposals have been studied and discussed by
some scientists but, at our knowledge, not by the space industry
which probably fears that the thousands of orbiting debris could
damage the satellites in orbit.
2.2. Sulfate aerosols [34,35]
This scheme is inspired by studies of the Mount Pinatubo
volcano eruption in the Philippines in 1991 and by the cooling
Fig. 2. Overview of the principal SRM geoengineering techniques that attempt to increase the reflection back to space of the incoming solar radiation. These techniques are
often referred as acting by a “parasol or umbrella effect”.
Tingzhen Ming et al. / Renewable and Sustainable Energy Reviews 31 (2014) 792–834 795
effect of its sunlight blocking sulfur plume. This “artificial-volcano”
idea is one of the least costly, and very small sulfate particles
in the stratosphere could last for a couple of years. The two
main problems are acid rain creation and probable damage of the
ozone layer.
But, currently burning fossil fuels and coal in particular and
other anthropogenic emissions, already introduce every year
nearly 110 million tons of SO2 in the lowest levels of the atmosphere [36]. With other reflective tropospheric aerosols this has a
direct cooling effect evaluated by Hansen [37] to 1 W m 2
, plus an
indirect cooling effect of 0.8 W m 2
. Not all these aerosols are
anthropogenic and volcanic aerosols also tamped down Earth
warming: recent work from Neely [38] revealed that moderate
volcanic eruptions, rather than Asian anthropogenic influences,
are the primary source of the observed 2000–2010 increases in
stratospheric aerosol. Sulfates in the troposphere have a much
shorter resilience time than those in the stratosphere, that is why
1–5 million tons of small size particles of SO2 in the stratosphere
every year [39] would have a more efficient cooling effect than
current emissions in the troposphere.
Reducing the sulfates emissions from power plant, as is already
done in the US, Europe and Japan, is helpful for reducing acid rain,
but it removes the umbrella of sulfates protection that reflects
solar radiation back to space and shields the Earth from the
warming effect of GHGs and thus has a net warming effect. The
problem is complex but if by magic tomorrow it was possible to
stop completely burning coal, the result would be an immediate
major global warming effect.
This paradoxical existing incentive in favor of non-reduction of
pollution could be a possible rationale for promoting SRM in spite
of the moral dispute over GE. But to become morally acceptable,
SRM should be limited to the idea of compensating for the
warming effect of local air cleaning. SRM should not be aimed to
substitute to the needed efforts of GHG emissions down-curving,
as CO2 levels will continue to rise in the atmosphere soon breaking
the 450 ppmv level limit climatologists recommend, to eventually
reach 800 ppmv or even more.
Among several other critics [40] to the use of sulfates in the
stratosphere, there is the need to deliver every year at least one
million ton of SO2 using thousands of balloons, planes or rockets,
costing between $25 and $50 billion annually and having to be
maintained continuously. Also a change in overall rain patterns
and a non-uniform cooling effect obtained over the entire Earth
with winners and losers is among the drawbacks, togeth....

Table 2
Overview of principal ERM strategies and their characteristics.
Type of MR SCPP in deserts DET AVE Thermo-syphon Night sky radiation Tropical
SCPP
Polar SCPP
Outgoing radiation target Sensible heat Latent heat Latent heatþsensible heat Surface radiation Thermalsþsensible heat Latent heat
evaporation
Latent heat crystallization
Possible additional climate
benefitsa
Rain in deserts
Heat island effect
reduction in
urban areas
Low altitude
clouds (albedo)
Green the deserts
Increase
planetary albedo
Maintain thermohaline circulation
Re-ice the artic
Reduce hurricanes
intensity
Dew water collection
Heat island effect reduction
Cloud
cover
increase
Rain in
deserts
High albedo fresh snow
at poles
Sea ice cover increase
Research results available þþþ þ þ þ þþþ þþþ þ- þ
Small prototypes built þþþ þ þ þ þþþ þ no no
Renewable energy
production
Yes Yes Possible Possible No but can improve existing power
systems
Possible Possible
Useful without turbines Yes dry cooling csp Yes cooling GH for
agriculture in hot
deserts
Yes replace cooling towers Yes many industrial uses Not applicable Yes water
production
Yes to re-ice the
arcticþincrease polar albedo
with fresh snow
Possible synergies for cost
reduction b
CO2 capture
GHG removal
GH agriculture
CO2 capture Use waste heat of thermal
power plants
Dry cooling Cooling PV panels
Cool paints and coatings
With heat pipes
unknown unknown
Estimated cost for full
operational scale
$300–400 million
(750 m high)
$100–150 million
(750 m high)
$50–100 million or $10–20
million without turbines
[203b]
$100–150 million without
turbines (750 m high)
small covers, or coatings in
numerous locations
floating
$200–400
million
$200–300 million on
mountain side
Is a rapid implement-tation
possible? (a couple of
years)
Yes Yes Yes without turbines
No with turbines
Yes without turbines
No with turbines
Not at high altitude
Yes No No
Possible variants, (other
than mountain side)
Many: floating
urban ventilation
etc.
With
wind towers
With ETFE
textile shell
Variants [197,199] Multiple industrial uses:
ex. H2 production by
nuclear
Several to increase breezes from sea
or land, from valley or, mountain
Variant [143] Similar to thermo-syphon
a Additional to: avoided CO2 emissions; heat transfer out to space; and renewable energy production.
b Except for tropical SCPPs and thermosyphons, it may be advantageous to use the relief to support the chimney by the mountain side, which reduces the cost for building it; part of the duct structure can be in steel covered
https://www.sciencedirect.com/journal/renewable-and-sustainable-energy-reviews

 

Published: 13 September 2018
Evaluating climate geoengineering proposals in the context of the Paris Agreement temperature goals
Mark G. Lawrence, Stefan Schäfer, Helene Muri, Vivian Scott, Andreas Oschlies, Naomi E. Vaughan, Olivier Boucher, Hauke Schmidt, Jim Haywood & Jürgen Scheffran
Nature Communications volume 9, Article number: 3734 (2018) Cite this article

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Abstract
Current mitigation efforts and existing future commitments are inadequate to accomplish the Paris Agreement temperature goals. In light of this, research and debate are intensifying on the possibilities of additionally employing proposed climate geoengineering technologies, either through atmospheric carbon dioxide removal or farther-reaching interventions altering the Earth’s radiative energy budget. Although research indicates that several techniques may eventually have the physical potential to contribute to limiting climate change, all are in early stages of development, involve substantial uncertainties and risks, and raise ethical and governance dilemmas. Based on present knowledge, climate geoengineering techniques cannot be relied on to significantly contribute to meeting the Paris Agreement temperature goals.

Introduction
The Paris Agreement of the 21st UNFCCC Conference of Parties (COP21) in 2015 aims to limit “the increase in the global average temperature to well below 2 °C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5 °C above pre-industrial levels”. Various measures are specified in support of this, including efforts “to achieve a balance between anthropogenic emissions by sources and removals by sinks of greenhouse gases in the second half of this century”. To provide context, observations1 show that the global mean surface temperature increase above pre-industrial levels, ΔT¯s, was about 1.1 °C in 2015 and 2016, with El Niño contributing to the warming in these years, and about 1 °C in 2017, the warmest non-El Niño year on record.

Given that long-term global warming is simulated to scale approximately linearly with cumulative CO2 emissions2, this leaves only limited remaining budgets of anthropogenic CO2 emissions until an atmospheric CO2 burden consistent with ΔT¯s = 1.5 °C or 2 °C is reached. These budgets are uncertain and have proven challenging to compute3,4,5, as they depend on several complicating factors, such as the climate sensitivity to the radiative forcing by CO2 and the future relative roles of non-CO2 forcers, especially the intermediate and short-lived climate forcers (SLCFs) including greenhouse gases like methane and ozone, and aerosol particles containing soot, sulfate, and other components. Numerous approaches have yielded a wide range of remaining budget values for various temperature thresholds4. The IPCC3 found that ΔT¯s remains below 1.5 °C in the 21st century in 66% of Coupled Model Intercomparison Project phase five (CMIP5) simulations with a cumulative CO2 budget of 400 Gt(CO2) from 2011 onwards, which includes the effects of continued emissions of non-CO2 forcers. For a ΔT¯s of 2 °C, the corresponding remaining budget is 1000 Gt(CO2)3. At the current global emissions rate of just over 40 Gt(CO2)/yr6, these 1.5 °C and 2 °C budgets would already be exhausted by 2020 and 2035, respectively. In contrast, a recent analysis5 has suggested that the remaining budgets may be much larger—possibly exceeding 880 Gt(CO2) and 1870 Gt(CO2) from 2015 onwards for 1.5 °C and 2 °C, respectively, which would extend the time window to 2037 and 2062 at the current emissions rate. However, these higher values involve numerous assumptions, including that ΔT¯s is currently only 0.9 °C, implying a difference to 1.5 °C of 0.6 °C, which is at the high end of the range of estimates based on observational evidence7, as well as further assumptions such as extensive additional mitigation of SLCFs.

In the context of the Paris Agreement, planned mitigation efforts until 2030 are specified by the Nationally Determined Contributions (NDCs), here also including the Intended NDCs (INDCs) for parties which have not yet ratified the agreement. Analyses of the current NDCs indicate that while emissions in some regions of the world are likely to decrease in the coming decade, total global anthropogenic CO2 emissions from 2015 to 2030 are likely to remain constant8, or even increase by ~1%/yr9. Thus, given the estimated remaining budgets discussed above, limiting ΔT¯s to 1.5 °C would very likely require much more ambitious and rapid emissions reduction efforts than the current NDCs. For the 2 °C goal, if the current NDCs were to be followed until 2030, then a 66% probability of keeping ΔT¯s ≤ 2 °C has been calculated to require a decrease of CO2 emissions of about 5%/yr thereafter9. Such sustained reductions, proposed as a carbon law of halving global CO2 emissions every decade10, would require extensive efforts in the power, transport, agriculture and consumer goods sectors, far exceeding the current and planned efforts reflected in the NDCs. On the other hand, global warming exceeding 1.5 °C, and especially exceeding 2 °C, is expected to have highly detrimental consequences for societies and ecosystems around the world11, requiring extensive and costly adaptation measures, especially if low-probability, high-risk systemic transitions (e.g., collapsing ice sheets) are triggered by the increasing temperatures12,13.

Recognition of this impending challenge has given increased momentum to often controversial discussions about two additional possible approaches to limiting climate change (Fig. 1): removing greenhouse gases from the ambient atmosphere, particularly CO2 as the most important climate forcer; and intentionally modifying the atmosphere-Earth radiative energy budget to partly counteract unintended anthropogenic climate change. These proposed approaches have been referred to collectively under various names, including geoengineering, climate engineering, and climate interventions14,15,16,17; here we use climate geoengineering, i.e., geoengineering being done specifically for climate-related purposes. Although none of the proposed techniques exists yet at scales sufficient to affect the global climate, they have already taken up prominent roles in climate change scenarios and policy discussions. In particular, extensive application of techniques for removing CO2 from the atmosphere is assumed in the widely used low-carbon RCP2.6 scenario18 of the Representative Concentration Pathways used by the Intergovernmental Panel for Climate Change (IPCC). Furthermore, an analysis19 of 116 scenarios which are consistent with limiting ΔT¯s to below 2 °C found that 87% of the scenarios require a transition to global net negative emissions, i.e., a CO2 removal rate exceeding gross emissions, during the second half of this century. In light of this situation, we assess the degree to which proposed climate geoengineering techniques could contribute significantly to achieving the Paris Agreement temperature goals during this century, which techniques can be largely disregarded in this context, and what the main open issues and research needs are, including the broader societal and political context.

Fig. 1
figure1
Proposed climate geoengineering techniques focused on in this review, placed in the context of mitigation efforts. a Mitigation is defined here as reducing the amount of CO2 and other climate forcers released into the atmosphere by either reducing the source activities (e.g., less energy consumption), increasing efficiency (thus reducing emissions per unit of the activity, e.g., kWh of energy produced), or removing forcers like CO2 directly at the source prior to their emission, e.g., from the concentrated stream of CO2 at power or industrial plants. For the latter, the captured CO2 can either be stored subsurface (CCS—carbon capture and storage), or utilized in long-lived materials such as carbonate-based cement (CCU—carbon capture and utilization). b In contrast to mitigation (including CCS and CCU), carbon dioxide removal (CDR) aims to reduce the amount of CO2 after it has been emitted into the ambient atmosphere, thus reducing greenhouse warming due to the absorption of terrestrial radiation (red arrows). The main proposed techniques are based on uptake of CO2 either by photosynthesis (techniques 1–5) or by abiotic chemical reactions (techniques 6 and 7), followed by storage of the carbon in various biosphere or geosphere reservoirs. c Radiative forcing geoengineering techniques aim to modify the atmosphere-surface radiative energy budget in order to partly counteract global warming, by two distinct approaches: increasing the amount of solar shortwave radiation (yellow arrows) that is reflected back to space (techniques 8, 9, 11, and 12), or increasing the amount of terrestrial longwave radiation which escapes to space (technique 10). The focus of this class of techniques is on inducing a negative radiative forcing (i.e., cooling). Thus, in place of the commonly used misnomers solar radiation management (SRM) and albedo modification14,15,17, which focus only on the solar radiation techniques and exclude terrestrial radiation modification by cirrus cloud thinning, we introduce the term radiative forcing based climate geoengineering, which we abbreviate to radiative forcing geoengineering (RFG)

Full size image
Types, metrics and budgets of proposed techniques
Carbon dioxide removal (CDR) techniques (Fig. 1b) are generally considered in terms of the cumulative amount or rate of CO2 removal from the atmosphere (Gt(CO2) or Gt(CO2)/yr), and compared to the current burden, remaining budgets, or global emissions of CO2. Most literature has focused on removal of CO2, rather than SLCFs20, due to its larger burden and longer lifetime, and thus comparatively slower response to mitigation efforts. This focus is further supported by the low-carbon RCP2.6 scenario18, wherein the emissions of the SLCFs methane and black carbon are already assumed to decrease significantly, meaning further measures to reduce their emissions or remove them from the atmosphere would have a limited additional effect21.

Efforts to modify the radiative energy budget of the atmosphere and Earth’s surface (Fig. 1c) are generally discussed in terms of reducing radiative forcing (in units of W/m2), defined as the change in the Earth’s net radiative energy balance at the tropopause that would occur if one climate system variable were changed while all other variables are held constant, while allowing stratospheric temperatures to equilibrate2. Given this focus and metric, we call this approach radiative forcing geoengineering (RFG), which we define as the cooling term, i.e., the magnitude of the negative radiative forcing. RFG and CDR are not entirely independent, since each can have indirect effects on the other, e.g., afforestation changes the surface albedo, while changes in temperature and light due to RFG techniques could affect biophysical processes, and thus CO2 uptake by oceans and ecosystems22,23,24.

To quantitatively assess the potential of CDR and/or RFG to compensate for a shortfall in the reduction of emissions of climate forcers, we start with emissions scenario data9 which is based on the assumption that the current NDCs will be fulfilled by 2030, and build on this with a parametric analysis (similar to ref. 25 but starting with the NDCs rather than the RCP scenarios). Failure to fulfil the NDCs—or mitigation in excess of the commitments—would accordingly either increase or reduce the expected emissions and gaps to specific targets, as illustrated in one parametric scenario with extensive mitigation starting already in 2021. Figure 2 and Supplementary Table 1 shows results for a range of annual decrease rates for CO2 emissions (see the Methods section for assumptions and computations).

Fig. 2
Gaps to the Paris Agreement temperature goals. The emissions gaps [Gt(CO2)] between computed cumulative CO2 emissions between 2015 and 2100 and the remaining budgets to the cumulative emissions amounts that keep ΔT¯s below 1.5 °C (green line) and 2 °C (blue line) with a 66% likelihood are shown for several scenarios. The remaining budgets are based on data from an IPCC analysis of model ensemble output3, yielding 650 ± 130 Gt(CO2) to 1.5 °C and 1300 ± 130 Gt(CO2) to 2 °C. The first four scenarios are based on fulfilment of the Paris Agreement NDCs by 2030 and various rates of annual emissions reductions thereafter. The last scenario is for an annual emissions reduction rate of 3%/yr starting already in 2021. The shading represents the lower and upper bound values computed for each temperature goal, and the solid line is the mean of these. The right side axis shows the implied negative radiative forcing [W/m2] associated with the CO2 budget gap values, using a conversion factor of 9.6 x 10–4 (W/m2)/(Gt(CO2). For further information and computations, see Methods and Supplementary Table 1

Full size image
Following the NDCs from 2015 to 2030 would result in cumulative emissions of 700 ± 37 Gt(CO2). This would already exceed our estimate of the remaining CO2 budget for the 1.5 °C goal, which is 650 ± 130 Gt(CO2) (see Methods). Even if a decrease rate of 3%/yr were to start in 2021, the cumulative emissions by 2030 would be ~600 Gt(CO2), requiring near-zero CO2 emissions thereafter to achieve the 1.5 °C goal without invoking CDR or RFG. Achieving the 2 °C goal by mitigation alone (i.e., requiring no emissions gap in 2100) would also be highly challenging, requiring fulfilling the current NDCs by 2030 and reducing emissions by over 5%/yr thereafter, or reducing CO2 emissions by more than 3%/yr starting already a decade earlier in 2021.

Defining clear threshold values for CDR and RFG techniques to be relevant for future climate policy is difficult due to a strong dependence on future emissions pathways. However, in the context of the Paris Agreement, useful reference values can be defined based on the difference between the 2 °C versus the 1.5 °C limits (see Methods): CDRref ≈ 650 Gt(CO2) for the cumulative CO2 budget, and RFGref ≈ 0.6 W/m2 for the equivalent radiative forcing. These reference values help provide orientation for the range of cases considered in Fig. 2 and Supplementary Table 1: they correspond to most of the gap in remaining emissions for the 1.5 °C limit in the case with a 5%/yr emissions reduction after 2030, and likewise for the 2 °C limit in the case of a 3%/yr reduction rate after 2030. In contrast, for the 1%/yr case these reference values only fill 38% of the gap to the 2 °C limit, and only 27% of the gap to the 1.5 °C limit. In these cases, a single technique would need to substantially exceed CDRref or RFGref, or a portfolio of techniques would be needed, each providing CDRref or RFGref or a significant fraction thereof.

Below we discuss the scalability and design challenges for any CDR or RFG technique to reach these values. While technical challenges are hereafter the main focus, we recognize that they cannot be viewed in isolation from the significant ethical, legal, political, and other social aspects that arise when discussing climate geoengineering, and provide an overview of these aspects in Box 1.

Box 1: Socio-political dimensions and governance issues
The significant ethical, legal, political and other social questions raised by hypothetical climate geoengineering interventions have been at the centre of attention since the early stages of the debate148,161. For instance, while a highly visible editorial on stratospheric aerosol injection (SAI) in 2006 (ref.81) discussed whether it might be a “contribution to resolve a policy dilemma”, the first major assessment report dedicated to climate geoengineering in 2009 (ref.14) pointed out in contrast that SAI and other forms of climate geoengineering are themselves likely to lead to further policy dilemmas.

This discourse has been primarily framed around the concept of moral hazard26,162,163—would climate geoengineering provide a false sense of insurance, potentially thwarting efforts to reduce emissions, and thus working counter to the Paris Agreement? Another frequently raised concern is a possible slippery slope dynamic164, in which research and vested interests are seen as precluding adequate independent assessment and appropriate consideration of alternatives. The moral hazard and slippery slope concerns have mostly been voiced with regard to RFG, but also apply to CDR. For example, the inclusion of large amounts of CDR in the low-carbon RCP2.6 scenario18 decreases the amount by which emissions need to be reduced to achieve the 2 °C target in computer models, by allowing for an overshoot that is assumed can later be compensated via a presently largely conceptual system of CDR technologies (a moral hazard). Following this pathway may increasingly lock in this technology option, crowding out other possible options (a slippery slope).

At present it is unclear whether any climate geoengineering technology could be implemented in a way that accounts for distributive, intergenerational, corrective, ecological, procedural and other forms of justice165. Furthermore, it has been argued that development and eventual deployment of climate geoengineering techniques, especially RFG, may place strains on human security and international relations166, resulting in conflict risks and societal instability167,168. A geoengineered climate would be the result of an intentional intervention attributable to identifiable actors, as opposed to the more ambiguous distribution of responsibility for damages from climate change induced by emissions of CO2 and other climate forcers. Thus a dynamic might unfold in which political tensions due to assigning and contestating blame for climate-related damages, such as those from extreme weather events, are exacerbated. The difficulties associated with detecting and attributing the effects of deployment of RFG may thereby lead to increased conflict potentials over liability and compensation169,170, questioning the very possibility of effective governance once deployment is under way169.

These concerns emphasize the importance of early development of effective governance for research and possible future deployment of climate geoengineering techniques. Outdoor field testing of RFG has particularly been met with calls for governance beyond the scientific and technical aspects and associated risks commonly focused on for such experimentation in other contexts161,171,172,173. Governance concerns apply to both RFG and CDR. For BECCS, for instance, governance and monitoring would be needed to minimize possible adverse effects that growing bioenergy crops would have on land and water use, and on food production and biodiversity (due to large scale monocultures). Furthermore, generally for any CDR method, the robust quantification and reliable reporting of removed CO2 would be essential174.

While some have called for governance via a single treaty that addresses all aspects of climate geoengineering (or CDR and RFG individually), others find this prohibitively difficult and argue for further developing existing instruments (ref.15 and citations therein). Some existing governance mechanisms apply at local, national and international levels, for example in the form of environmental regulation, professional norms, research funding procedures such as peer review and impact assessments, and international agreements175. In particular, land-based CDR methods are to some extent addressed in the Kyoto mechanisms (Clean Development Mechanism, Joint Implementation, and Emissions Trading). However, these mechanisms are contested due to concerns about accounting difficulties, risk of fraud, and lack of efficiency. Thus far, two multilateral treaty bodies, the London Convention/London Protocol (LC/LP) and the Convention on Biological Diversity (CBD), have directly addressed different types of climate geoengineering by issuing specific resolutions and decisions. The LC/LP has put in place restrictions on large scale deployment of marine geoengineering activities, while the CBD requests that “no climate-related geo-engineering activities that may affect biodiversity take place”, a decision that is non-binding upon treaty parties. Furthermore, it has been suggested that the UNFCCC could contribute to regulating individual techniques or aspects of climate geoengineering176, which may become an issue in the implementation of the Paris Agreement. An important first step towards developing governance has been proposals15,177 for applying overarching principles for guiding the research community and policymakers, including the principles of precaution and transparency, and considering research as a public good; these could be considered for formal adoption, e.g., by national and international funding bodies.

Show morefrom this box
Carbon dioxide removal
Numerous CDR techniques have been proposed (Fig. 1) and the surrounding literature indicates that some CDR techniques could contribute significantly to achieving net zero or net negative CO2 emissions15,16,26,27,28,29. While it is possible that CDR, together with mitigation, could eventually return atmospheric CO2 to previous levels, this would only partially return the climate and other Earth system parameters, such as ocean pH, to the corresponding previous state, due to hysteresis and other effects30,31. Here we examine the potential contributions of CDR towards achieving the Paris Agreement goals, and the challenges that would be faced, complementing previous analyses which have focused on issues like the assumed role of CDR in low-carbon scenarios18,19, or the ability to compensate sectors that are particularly difficult to mitigate (e.g., air travel, agriculture and certain industries).

Several CDR techniques have been developed as prototypes, and afforestation is already in widespread use, as are some of the components involved in other techniques, e.g., bioenergy (in BECCS). However, all of these are far from the scale of CDRref. Attempting to scale up any CDR technique would require addressing many technical and social issues, several of which are common across most or all of the techniques. One of the most important common technical issues is the total CO2 storage capacity (see Box 2). Further issues include limits of required chemical and biological resources, how the techniques would compete with each other and other sectors for resources, the time scales involved, and the economic costs and societal impacts (see Box 1).

Box 2: Carbon storage capacity and achievability
CO2 removal methods require adequate storage reservoirs, either directly for CO2 or for other forms of carbon (e.g., biomass, minerals and consumer products). A variety of reservoirs are possible, either quasi-permanent, confidently isolating CO2 from the atmosphere over long timescales (e.g., >10,000 years178), or temporary, where a non-negligible amount of the removed CO2 might return to the atmosphere within decades to centuries179. The achievability for nearly all reservoirs is qualitatively estimated (see Figure below) to be relatively high for small amounts (e.g., <1 Gt(CO2)), but challenging for larger amounts (e.g., >1000 Gt(CO2)), with considerable research needed, e.g., into ecological and economic implications, and development of adequate infrastructures for extensive deployment. For storage in the deep oceans, however, even relatively small amounts are likely to be challenging, given a lack of applicable practical experience. Deep-ocean storage, mainly via injection of liquefied CO2 into deep-ocean waters and seabed sediments180, is mostly considered temporary, since ocean circulation will return some of the CO2 to the atmosphere181. However, this occurs on timescales that are much longer than relevant for initial achievement of the Paris Agreement, with model simulations showing that for a CO2 discharge depth of 3000 m, slightly less than half of the CO2 would return to the atmosphere within 500 years182. The capacity for deep-ocean storage depicted in the Figure is based on a recent analysis179 and far exceeds CDRref.

Geological and geochemical storage capacity is considered large and quasi-permanent178,179. The main approach to geological storage is injection of CO2 (usually compressed as a supercritical fluid), via boreholes, into deep porous rock formations like oil and gas reservoirs and deep saline formations overlain by sealing layers37. Challenges include the lack of adequate geological data in some regions, as well as the trade-offs between efforts to co-locate CO2 capture sites and injection sites versus the development of CO2 pipelines and ship transport networks183. Enhanced weathering techniques apply geochemical storage, reacting CO2 with alkaline minerals, on either the land or ocean surface, and subsequently storing the weathering products55. Storage would likely be limited by logistical requirements (mining and transport) and ecological impacts rather than by mineral rock resource availability. Efforts are being made to combine geochemical storage with geological storage via in situ mineralization of liquid CO2 injected into boreholes with geochemical conditions conducive to rapid mineralization reactions184, but considerable work is needed to determine how well this could be scaled up to tens or hundreds of Gt(CO2).

Biosphere-based carbon stores in trees and soils are limited in total capacity179, though both likely exceed CDRref, with the storage capacity of soils estimated to be a few times larger than that of forests. Afforestation and soil carbon enrichment (e.g., terra preta) are well-established processes, and would be technically easier to implement in the near-term than geological and geochemical storage; however, these would compete against global trends of deforestation and top-soil degradation and loss. Challenges would likely grow rapidly at larger scales, with issues like land use competition, irrigation and fertilizer supply limits becoming increasingly significant32. In both cases, the biomass storage is temporary on timescales relevant to the Paris Agreement, and sustained ecosystem maintenance would be needed to prevent carbon from being returned to the atmosphere through changes in the local environment (e.g., disease), climate (e.g., drought, fire) or society (e.g., changing land use).

Carbon capture and utilization (CCU) could also be considered a form of storage reservoir. While products such as liquid fuels or polyurethane foams would return CO2 to the atmosphere via combustion or decay within years to decades, some products like construction materials could sequester CO2 for centuries. However, even with extensive policy and market support actions, the removal potential is likely less than 10 Gt(CO2) by 2100185.

Finally, it is important to bear in mind that even small amounts of carbon storage in some reservoirs may be very difficult or even unachievable if societal and political support is lacking.

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Biomass-based techniques
Numerous biomass-based CDR techniques have been proposed, all removing CO2 from the atmosphere by photosynthesis. Some then use the biomass for primary carbon storage (e.g., in trees, humus, peat, etc.), while others involve combustion and subsequent storage of the products (e.g., compressed CO2 and biochar).

Afforestation (here also including reforestation) involves increasing forest cover and/or density in previously non-forested or deforested areas. Principally the carbon storage potential is large compared to CDRref, given that historic deforestation was 2400±1000 Gt(CO2)16. However, since much of this deforestation was to make space for current agriculture and livestock, extensive land-use competition could be expected for such a degree of afforestation32. More realistic estimates therefore range from about 0.5–3.5 Gt(CO2)/yr by 2050, increasing to 4–12 Gt(CO2)/yr by 210027,28,33, implying a total removal potential of about 120–450 Gt(CO2) from 2015 to 2100 (assuming linear increases in the CO2 uptake rate, starting at zero in 2015).

Combining biomass energy with carbon capture and storage (BECCS), which can be used for either electricity generation or the production of hydrogen or liquid fuels34, is widely assumed in integrated assessment model scenarios to be able to provide sufficient CDR to keep ΔT¯s below 2 °C18,19. The range of estimates of the maximum removal potential of BECCS is large, again partly based on assumptions about land-use competition with agriculture, economic incentives for extensive development and deployment, and other factors, such as nature conservation. High-end estimates for BECCS in the literature involve underlying assumptions such as the use of forestry and agriculture residues35, the transition to lower meat diets, and the diversion of over half the current nitrogen and phosphate fertilizer inputs to BECCS, resulting in an uptake of ~10 Gt(CO2)/yr by 205032,33, with estimates for 2100 being similar or possibly even higher27,36. This would also depend on the development of both bioenergy and carbon capture and storage (CCS) technologies, infrastructures, and governance mechanisms to allow a capacity several orders of magnitude greater than current prototypes37,38,39. Assuming a linear development to 10 Gt(CO2)/yr until 2050 and constant thereafter would imply a cumulative removal potential by 2100 of ~700 Gt(CO2), i.e., exceeding CDRref. Various factors may reduce this, but it could also increase under the high-end assumptions mentioned above.

Biochar, a stable form of carbon produced by medium temperature pyrolysis (>350 °C) or high temperature gasification (~900 °C) of biomass in a low oxygen environment, can be buried or ploughed into agricultural soils, enriching their carbon content. Various gases or oils can also be produced by the pyrolysis process. While biochar production could principally be applied to a similar amount of biomass as assumed for BECCS (i.e., ~700 Gt(CO2) removal by 2100), many additional factors come into play40,41, including feedstock type and source, labile carbon fraction, char yield, required energy input, the mean soil residence time of the biochar carbon, sink saturation, and priming effects (i.e., accelerated organic matter decomposition). This results in a much lower estimated maximum removal potential for biochar, ~2–2.5 Gt(CO2)/yr28,41, or up to ~200 Gt(CO2) by 2100, although, as with BECCS, this could possibly be enhanced by additional use of residue biomass from agriculture and forestry41.

In addition to mixing biochar into soils, recent studies have focused on replenishing or enhancing organic carbon in cultivated soils through various agricultural practices42, such as limiting tilling, and composting (rather than burning) crop residues. While these ideas are generating considerable interest, including the COP21 4 per mille initiative43,44, their ability to be scaled up to being relevant for the Paris Agreement is poorly known, due to saturation and other effects. Earlier studies45 suggested a very limited possible role for soil enrichment; however, more recent analyses suggest a physical removal potential of ~200 Gt(CO2) by 210041, i.e., a significant fraction of CDRref, and this could possibly be increased up to 500 Gt(CO2) by practices such as soil carbon enrichment at greater depths43,44. Soil carbon enrichment may be more closely associated with co-benefits for agriculture than with trade-offs like competition for biomass, so that it might be seen as particularly attractive to pursue in the near term, while trade-offs and similar issues with other techniques are being resolved.

Ocean iron fertilization (OIF) is the proposal to fertilize iron-poor regions of the ocean to spur phytoplankton growth and increase the detritus carbon flux to the deep ocean46. The general conclusion emerging from modelling work, perturbative field studies, and analyses of natural iron enrichments downstream of islands, is that some oceanic carbon uptake could likely be achieved, particularly in the iron-limited Southern Ocean46. However, while early studies indicated that CO2 removal by OIF might be capable of far exceeding CDRref, later studies showed that this neglected many limiting factors, so that the removal capacity is likely less than 400 Gt(CO2) by 210047. Furthermore, this would likely result in significant side effects in the oceans, like disruption of regional nutrient cycling, and on the atmosphere, including production of climate-relevant gases like N2O15. Although there are reasons to encourage further research48, the limited removal potential and significant side effects, along with international legal developments that restrict large-scale deployment (see Box 1), make it unlikely that OIF will be employed to contribute significantly to the Paris Agreement goals. It seems similarly unlikely that related ocean carbon cycle techniques, such as using wave-driven pumps to enhance oceanic upwelling and thus increase the rate of mixing of fresh CO2 into deep-ocean waters, will contribute significantly49.

Many further biomass-based CDR techniques have been proposed, such as accelerating the formation of peatlands, or burying timber biomass in anoxic wetlands. A recent assessment15 has concluded that the expected CO2 removal capacity for each of these would likely be less than 100 Gt(CO2) by 2100, and several would have significant environmental side effects. Further research may reveal greater CO2 removal potentials, but current literature indicates that none would be capable of significantly contributing to achieving the Paris Agreement goals.

The biomass-based techniques share a wide range of research needs (Fig. 3), which are relevant to their possible roles in the Paris Agreement context, and can be grouped under three broad categories: (1) the technical carbon removal potential and how this can be increased; (2) social and environmental impacts and how trade-offs can be minimized while capitalizing on co-benefits and synergies; and (3) development and operational costs. Given the current state of research and development, it is not yet possible to generally prioritize any of these categories above the others, although this may be possible in dedicated studies of individual techniques. Several technique-specific aspects of the first two categories were discussed above.

Fig. 3
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Schematic of research needs for proposed biomass-based CDR techniques. A broad range of issues would need to be clarified to better understand the removal potentials, costs, trade-offs and risks prior to a possible implementation of any biomass-based CDR technique, as detailed in two recent assessments15,16, including: (1) the most effective biomass types to use for various techniques; (2) the applied technologies, especially for carbon capture and biomass pyrolysis; (3) the scalability, noting that modest deployment levels of biomass-based techniques could largely be constrained to local environmental and socio-economic impacts, while extensive deployment (e.g., at levels comparable to CDRref) could result in significant limitations due to land and biomass availability, biomass growth rates, and competition, e.g., for water and nutrient resources, with natural ecosystems, agriculture, and other biomass-based CDR techniques; (4) impacts of choices of biomass types and the extent of implementation on regional biodiversity, wildlife, and overall ecosystem resilience; (5) impacts of differences in the albedo of the respective biomass type (e.g., trees and energy crops) versus the albedo prior to the biomass growth; (6) the carbon payback, i.e., the temporary reduction in effectiveness of a terrestrial biomass CDR technique resulting from CO2 released due to disturbances to the ecosystem during biomass planting; (7) implications of the production of numerous non-CO2 gases with impacts on climate and air quality, such as volatile organic compounds (VOCs) like isoprene, and the long-lived greenhouse gas N2O; (8) the ability to co-locate biomass processing sites (BECCS plants and biochar pyrolysis facilities) with biomass growth locations and product storage and/or burial sites, as well as the necessary transport infrastructure if these are not co-located; (9) economic implications – not only the operational costs, but also the economic impacts, e.g., due to competition with agriculture

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For the third category, estimating development and operational costs has been particularly challenging, despite their importance in determining whether any technique could viably contribute to climate policy around the Paris Agreement. Published values for all of the techniques discussed above can presently only be taken as broadly indicative, and are typically of the order of $100/t(CO2), with the range of values given in the literature for each technique often being a factor of three or more27,28. This uncertainty is due to numerous factors, including extremely limited commercial experience with full-scale operations (e.g., for CCS or biochar), storage site properties and the details of CO2 transport or co-location of infrastructure for BECCS, land-use and resource competition with agriculture, and the compensating revenue from electricity or fuels produced by BECCS and biochar plants36. Complicating things further, land and resource competition might result in operational costs for biomass-based CDR techniques actually increasing as implementation scales grow, in contrast to the typical falling costs for most technologies as they grow in scale.

Mineralization-based and other abiotic techniques
Abiotic CDR techniques for removing CO2 from the atmosphere can be roughly distinguished into two main approaches: spreading weathering materials over large open spaces (enhanced weathering and ocean alkalinisation/liming); and capturing CO2 in some form of enclosure or on constructed machinery (direct air carbon capture and storage, abbreviated DACCS).

A review of proposals for terrestrial enhanced weathering50 divides these into (1) ex situ techniques, which involve dispersing mined, crushed and ground silicate rocks (e.g., olivine51,52) in order to increase the exposed surface area and thus allow a more rapid uptake of CO2, particularly in warm, humid regions where CO2 removal would be most rapid52, and (2) in situ techniques, which are forms of underground geological/geochemical sequestration (see Box 2). Similarly, ocean alkalinization has been proposed via distribution of crushed rock into coastal surface waters53, as slowly sinking micrometre-sized silicate particles deposited onto the open-ocean sea surface54,55, or via dispersion of limestone powder into upwelling regions56. Ocean alkalinization would contribute to counteracting ocean acidification, in turn allowing more uptake of CO2 from the atmosphere into the ocean surface waters. Terrestrial enhanced weathering could also enhance ocean alkalinity, via either riverine run-off, or mechanized transport and mixing of the alkaline weathering products into the oceans, though both may vary strongly regionally. Further proposals include combining enhanced weathering and ocean alkalinisation using silicates to neutralize hydrochloric acid produced from seawater57, or heating limestone to produce lime (combined with capture and storage of the by-product CO2), which has been a long-standing proposal for dispersal in the oceans to increase ocean alkalinity58, in turn allowing additional CO2 uptake from the atmosphere by the ocean.

Due to the abundance of the required raw materials, the physical CO2 removal potential of enhanced weathering is principally much larger than CDRref. However, since the current rate of anthropogenic CO2 emission is ~200 times the rate of CO2 removal by natural weathering59, the surface area available for reactions would need to be increased substantially via grinding and distribution of the weathering materials. This would imply large investments, including energy input, for the associated mining, grinding and distribution operations. Given that removing a certain mass of CO2 requires a similar mass of weathering material, the operations would need to be comparable to other current mining and mined-materials-processing industries, which could have significant impacts on sensitive ecosystems, as could the large amounts of alkaline weathering products that would be produced, especially in the runoff regions, about which very little is presently known.

DACCS could possibly be designed so that it requires a substantially reduced dedicated land or marine surface area compared to other CDR techniques, and might also allow the environmental impacts to be more limited and quantifiable. However, scaling up from small-scale applications of direct air capture technologies, such as controlling CO2 levels in submarines and spaceships60,61, to removing and storing hundreds of Gt(CO2) would involve substantial costs, especially due to the high energy requirements of three main technology components: (1) sustaining sufficient airflow through the systems to continually expose fresh air for CO2 separation; (2) overcoming the thermodynamic barrier required to capture CO2 at a dilute ambient mixing ratio of 0.04%; and (3) supplying additional energy for the compression of CO2 for underground storage.

While components (1) and (3) can be quantified using basic principles, and several studies61,62 indicate that combined they would probably require 300–500 MJ/t(CO2) (or ~80–140 kWh/t(CO2)), the energy and material requirements of the separation technology (2) are much more difficult to estimate. The theoretical thermodynamic minimum for separation of CO2 at current ambient mixing ratios is just under 500 MJ/t(CO2)62. However, thermodynamic minimum values are rarely achievable. Current estimates for the efficiency of DACCS are technology-dependent, ranging from at best 3 to likely 20 or more times the theoretical minimum61, or ~1500–10,000 MJ/t(CO2), implying that removing an amount equivalent to CDRref by 2100 would require a continuous power supply of approximately 400–2600 GW. Combined with the energy requirements for (1) and (3) (equivalent to about 100 GW), this represents about 20–100% of the current global electricity generation of ~2700 GW.

A wide range of chemical, thermal, and also some biological (algae and enzymes) techniques have been proposed for the separation technology, but the focus of research has been on two main approaches60,62,63,64,65: adsorption onto solids, e.g., amine-based resins that adsorb CO2 when ambient air moves across them, followed by release of concentrated CO2 by hydration of the resins in an otherwise evacuated enclosure; and absorption into high-alkalinity solutions with subsequent heating-induced release of the absorbed CO2. While the environmental and societal impacts of these technologies could likely be much better constrained in comparison to the other CDR techniques, they are still important to consider, and include environmental impacts due to placement of the capture devices and CO2 storage sites, mining and preparation of materials like resins that would be used in the systems, and the possible release of amines and other substances used in the separation process66.

The physical CO2 removal potential of DACCS far exceeds CDRref, provided the high energy requirements could be met; there are no significant principal limitations in terms of the material availability or CO2 storage capacity (see Box 2), and even the manufacture of millions of extraction devices annually would not be unfeasible (compared to, e.g., the annual global manufacturing of over 70 million automobiles). Large investments in DACCS might, however, be unlikely as long as large point sources (e.g., power or industrial plants) continue to be built and operated, since the same effective reduction of atmospheric CO2 levels via CCS applied to higher-concentration sources will generally be much less energy intensive and thus less expensive than CO2 capture from ambient air61. In general, for any possible longer-term application of CDR in climate policy, a major lynchpin will likely be development of CCS, both in terms of the carbon capture technologies and the storage infrastructure, since CCS is fundamental to both BECCS and DACCS, and since it is likely to be most economically favourable to first apply CCS to remaining large point sources.

The estimated development and operational costs for both enhanced weathering (including ocean alkalinisation) and DACCS at scales comparable to CDRref vary widely, even though the involved processes, especially for enhanced weathering (mining, processing and distribution), are nearly all well-established industrial activities. Published estimates cover a similar range to the biomass-based techniques, from about $20/t(CO2) to over $1000/t(CO2)27,28,60,65. Better estimates of the costs are particularly important for DACCS, since it essentially represents the cost ceiling for viability of any CDR measure due to its potential scalability and its likely constrainable environmental impacts. These potentially high costs, and the array of other associated challenges for both the abiotic and the biomass-based CDR techniques, provide important context for the discussions around further proposed measures for addressing climate change, namely RFG.

Radiative forcing geoengineering
Numerous RFG techniques have been proposed, which can fundamentally be divided into three vertical deployment regions (see Fig. 1): space-based (mirrors), atmospheric (stratospheric aerosol injection – SAI; marine sky brightening – MSB; and cirrus cloud thinning – CCT); and surface-based (urban areas, agricultural land, grasslands, deserts, oceans, etc.).

A key reason for interest in RFG techniques is that they might technically be able to stabilize or even reduce ΔT¯s within a few years, although there would be technique-specific differences in regional cooling (see Box 3). Proposed CDR techniques, on the other hand, would likely physically require much longer (decades) before they could lead to a notable stabilization or decrease in ΔT¯s, due to limits on the maximum rate of CO2 removal that could be achieved. Furthermore, although the operational costs for all proposed RFG techniques are currently very uncertain, considerable interest has been raised by the possibility67,68,69,70,71 that the operational costs to achieve a certain degree of cooling, e.g., RFGref, might be much lower than the operational costs for a comparable amount of CDR (e.g., achieving CDRref by 2100). However, comparing costs is difficult due to the different time horizons: CDR has no further operational costs once the desired amount of CO2 has been removed, whereas RFG would have ongoing costs to maintain the same cooling as long as the elevated CO2 levels persist (potentially over centuries). RFG has been considered under various complementary framings, including determining the forcing that would be needed to reduce ΔT¯s to zero72, and limiting the magnitude of future peaks in ΔT¯s while mitigation measures are implemented and CDR capacity is being developed73,74.

In the context of the Paris Agreement, we focus our discussion below on the three atmospheric RFG techniques (SAI, MSB, and CCT), which current literature indicates would have the most significant physical potential to contribute notably over the next few decades towards achieving the 1.5 or 2 °C temperature goals. Space mirror RFG could contribute considerable cooling from a climate physics perspective, based on model simulations using it as a proxy for RFG in general75,76; however, proposals for implementation77,78 rely on extensive future technology developments and a dramatic reduction in material transport costs from ~10,000$/kg79 to less than 100$/kg. Furthermore, there are significant, poorly understood risks including impacts from asteroids and space debris, and technical or communications failure. As such, while a future possibility, due to present challenges and associated times scales, space mirror RFG is not further considered here in the context of the Paris Agreement. Furthermore, for proposed surface-based RFG techniques, a recent literature assessment15 has shown that their potential maximum cooling effects are either too limited (i.e., well below RFGref), or are associated with substantial side effects, e.g., complete disruption of regional ecosystems such as in the deserts, so that it is also unlikely that any current proposed surface-based RFG techniques will be employed to contribute significantly to achieving the Paris Agreement goals.

All of the proposed RFG techniques generally share several aspects in common in terms of the anticipated climate responses and the uncertainties and risks involved (see Box 3). Furthermore, all three of the atmospheric RFG techniques would require generating an enhanced aerosol layer or modified clouds with geographical, optical, microphysical and chemical characteristics capable of producing the desired radiative forcing. This in turn requires consideration of the technique-specific issues around how well different particle composition types would work, how much would need to be injected, when and where, and what the expected cooling would be, as discussed in the following sections.

Box 3: RFG techniques: Key common effects, impacts and risks
An important component of the Paris Agreement framing is that there are many possible climatic manifestations of a world with the global mean surface temperature increase ΔT¯s = 1.5 °C or 2 °C, with regional differences in temperature and precipitation, and thus differing impacts on society and ecosystems. Implementation of RFG to complement mitigation would result in novel climates with regional climatic differences, since RFG has a different influence on the vertical and horizontal distributions of radiative forcing than CO2 and other anthropogenic climate forcers2,14,16. However, climate model simulations100,186 show that even for a relatively extreme case, e.g., wherein RFG were to reduce ΔT¯s from 3 °C to 1.5 °C, resulting temperature and precipitation distributions are almost universally closer than the ΔT¯s = 3 °C climate to the ΔT¯s = 1.5 °C climate achieved through mitigation alone, with only limited regional exceptions (mostly for maritime precipitation). Other model studies indicate that the simulated match to target climates could be made even better with appropriately designed geographical distributions of the introduced forcing187,188,190, e.g., by combining two or more techniques to capitalize on their regional differences in radiative forcing191,192. Under certain conditions, application of RFG in climate model ensembles can result in reduced simulated climate risks simultaneously in nearly all regions worldwide194, although this involves assumptions such as uninterrupted RFG deployment (i.e., no risk of failure or disruption).

The anticipated climate responses to most RFG techniques have been found to be similar in numerous climate modelling studies, particularly those within the Geoengineering Model Intercomparison Project (GeoMIP)72,76,193. Roughly well-distributed global forcing, via space mirrors or SAI, is expected to produce a pronounced latitudinal gradient in temperature response, with low latitudes cooling more than high latitudes. While this tendency is also present in simulations of MSB, the regionally applied forcing can result in temperature changes that dominate over the latitudinal gradient. Model simulations further show that RFG tends to cause the global mean precipitation rate to decrease disproportionately to temperature75,194. However, despite many broad similarities, specific techniques also exhibit notable differences in simulations124,195, e.g., MSB and desert brightening show very different precipitation responses relative to space mirrors and SAI117,119,121,196,197. Furthermore, in contrast to the solar radiation based techniques, simulations of CCT compute the strongest cooling at high latitudes, dependent on exact locations of cirrus thinning133,135, and an increase rather than a decrease in global precipitation136,137,191,195.

Despite a growing literature base of modelling studies such as those conducted within GeoMIP, understanding remains poor of the range of further positive and negative impacts that RFG would have on the Earth system, and by extension on society. Some impacts will be technique specific (e.g., risk of stratospheric ozone depletion caused by SAI151), but many will be common regardless of technique. Research from the impact assessment community on the topic of RFG has thus far been very limited, leaving impacts uncharacterized for several key sectors198, especially: health, for instance via changes in heatwaves, air quality and vector-borne diseases; food security, including crop yields and fish stocks; water resources, including effects of droughts and flooding; biodiversity and ecosystems (terrestrial and aquatic); and coasts, including inundation and erosion.

Finally, one of the most-discussed risks of RFG for Earth systems is the so-called termination shock. This refers to the rapid increase in temperature that would result should a significant amount of RFG (e.g., exceeding RFGref) be implemented and later stopped or scaled back over a short period of time199,200, returning the climate to the same warmed state as would have occurred in the absence of RFG. This would present a particular challenge for human populations and ecosystems, given that adaptation depends on both magnitude and rate of change200. Two measures have been proposed to ensure that such a rapid warming is improbable: first, significant mitigation combined with CDR to produce net negative CO2 emissions, thus reducing the amount of RFG that would be needed over time to keep ΔT¯s below a specific threshold (e.g., 2 °C); and second, careful development of backup systems and policies201.

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Stratospheric aerosol injection
Injecting reflecting aerosol particles or gaseous particle precursors into the lower stratosphere could increase the planetary albedo (reflectivity), in turn reducing surface temperatures. Discussions of SAI have a long history26,80,81,82,83,84, with the earliest studies focusing on enhancing the natural stratospheric sulfate aerosol layer. This could be done via injection of either sulfate particles, or sulphuric acid (H2SO4), which condenses into particles, or precursor gases like sulfur dioxide (SO2), hydrogen sulfide (H2S) or carbonyl sulfide (COS), which would then be oxidized to H2SO4. Numerous other possible particle compositions have been proposed and analyzed85,86,87,88,89,90,91,92, including: calcite (CaCO3, the main component of limestone); crystal forms of titanium dioxide (TiO2), zirconium dioxide (ZrO2), and aluminium oxide (Al2O3); silicon carbide (SiC); synthetic diamond; soot; and self-lofting nanoparticles. Each proposed material has its specific advantages and challenges (see Fig. 4), e.g., calcite particles91 are non-toxic, would not cause significant stratospheric heating, and may counteract stratospheric ozone loss, but their microphysics and chemistry under stratospheric conditions are poorly understood.

Fig. 4
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Key scientific and technical considerations and challenges for stratospheric aerosol injection (SAI). A wide range of scientific and technical factors would need to be considered in choosing which particle composition or combination of particle types to employ in possible implementation of SAI: a A high degree of control would be desired over the resulting aerosol particle size distribution, which influences the aerosol layer optical properties (for both solar and terrestrial radiation), the residence time, and the dispersion and transport of the aerosol layer. Such control would be more straightforward with manufactured particles such as TiO2, ZrO2, and Al2O3, than for H2SO4 or gaseous precursor injections (SO2, etc.). b Particle types that have limited effects on the stratospheric ozone layer would be preferable, which is a particular disadvantage of sulfate particles151,152. c Limited heating of the lower stratosphere would be preferable. Heating would depend on particle composition86, with some particle types, especially soot88 and small Al2O3 particles92, possibly heating the polar stratosphere by 10 °C or more, with significant but poorly understood impacts on stratospheric water vapour and dynamics90,92,93,153,154, including the possibility of increased stratospheric particle lifetime due to lofting95. d Particles with a high radiative forcing efficiency per unit mass would be preferable, as this would reduce the particle or precursor mass that needs to be transported to the stratosphere. e SAI would affect the ratio of direct and diffuse solar radiation, which would in turn impact photosynthesis, and thus crop yields155 and global net primary productivity156. Little is known yet about how this varies with particle type and size, or about other possible effects on ecosystems, as well as on solar energy production. f Human safety and environmental impact issues are of concern for several particle compositions, e.g., H2SO4 is a powerful acid, while aluminium and several other proposed particle components are well-known environmental contaminants, though their effective toxicity depends on their specific chemical forms; this is generally less of a concern for most proposed gaseous precursors like SO2

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Based on fundamental physical considerations, the radiative forcing by SAI would be expected to have an asymptotic limit, due to the growth of stratospheric particles to larger radii at greater injection rates, decreasing the residence time (due to increased sedimentation rates) and the optical efficiency. Estimates of this limit vary widely, especially due to differences in the representations of microphysics and dynamics in climate models. Model studies93,94,95, as well as evidence from past volcanic eruptions2, indicate a maximum potential cooling (negative radiative forcing) ranging from 2 W/m2 to over 5 W/m2, i.e., well above RFGref, though the upper end of the range would require extremely large injection amounts (comparable to the current global anthropogenic sulfur pollutant emissions of about 100 Tg(SO2)/yr).

SAI would require regular injections to maintain the aerosol layer, given the stratospheric particle residence time of about 1–3 years96,97,98. The injection amount needed would depend on the desired radiative forcing and the particle composition, size distribution, optical properties, and the vertical and horizontal injection location(s)94,96,98,99,100,101,102. Most model studies (as well as evidence from the volcanic record) show that the radiative forcing efficiency typically increases with the altitude of injection94,96,99, since a smaller fraction of the particle mass is lost due to sedimentation97; however, this is not found in all studies95, and may depend on the injection amount, even reversing sign for very large injection rates93. Geographically, injection in the tropics results in an effective dispersion towards the poles by the stratospheric Brewer–Dobson circulation, producing an aerosol layer with a broad global coverage99, but limited control over its regional distribution. On the other hand, model simulations have shown that high latitude injections aimed specifically at reducing Arctic warming would be relatively ineffective103,104,105,106,107, due to the shorter aerosol residence time and weaker solar radiation compared to the tropics.

Proposed injection mechanisms for SAI are via high-flying aircraft, stratospheric balloons, artillery shells, and rockets68,69,108,109, with studies to date indicating the first two are likely the most effective and economically feasible. All are in very early stages of research and development. Aircraft injections would require a new fleet of dedicated high-flying aircraft69, since civil aircraft fly too low and mostly too far north to be effective for global cooling109. Tethered balloons would require extensive technology development and testing to determine the feasibility and safety issues involved in annually transporting megatons of particles or precursors through hoses of over 20 km length68. Furthermore, for both platforms, coagulating particles or precursors like H2SO4 would likely require some mechanism to create turbulence in order to have sufficient control over the resulting particle size distribution (see Fig. 4a), and a large number of aircraft or tethered balloons would thus be needed in order to limit the local injection rate and prevent rapid coagulation to oversized particles98.

Marine sky brightening
MSB would involve seeding low-altitude clouds with cloud condensation nuclei particles to cause condensed water to spread over a greater number of smaller droplets, increasing the optical cross section and thus the cloud’s reflectivity110,111,112,113. This effect has been observed over oceangoing ships due to particles in their pollution plumes114, and in plumes of effusive volcanic eruptions115. Clouds with lower background particle concentrations, such as maritime stratiform clouds, are particularly susceptible to this effect. Modeling studies111,116,117,118 indicate the injected particles would also likely increase the clear-sky reflectivity, by an amount comparable to the marine cloud brightening (MCB), which has led to the combined term MSB.

Similar to SAI, the limited knowledge about key microphysical and dynamical processes involved results in a large uncertainty in the maximum cooling that could be achieved via MSB, with estimates111,113,117,119,120,121,122 ranging from 0.8 to 5.4 W/m2, i.e., likely well above RFGref. Analysis of satellite data113 and model simulations111,113,115 indicate that certain regions are more susceptible to MSB, in particular persistent stratocumulus cloud decks off the continental west coasts, especially South America, North America, southern Africa and Australia. However, there are considerable scientific uncertainties, such as the differences in responses of open and closed cell convection123. The cooling resulting from MSB would be more geographically heterogeneous than from SAI117,124, focused especially on the susceptible oceanic regions, leading to considerably different temperature and precipitation responses in comparison to more globally homogeneous forcing.

For implementation, the focus has been on injecting sea salt due to its availability, especially from autonomous ships67. Several challenges would need to be overcome, including: development of spray nozzles to form appropriately sized particles125; compensating for reduced lofting in the marine boundary layer due to cooling following evaporation of injected seawater126,127; and an ability to target suitable meteorological conditions, including low solar zenith angles, unpolluted air, and few or no overlying mid to high altitude clouds112,113,128. Efforts would also be needed to minimize environmental effects (i.e., corrosion and detriment to vegetation129) and chemical and microphysical effects (on ambient gases and particles130) of the injected sea-salt.

Cirrus cloud thinning
Cirrus, in contrast to most other forms of clouds, warm the Earth’s surface by absorption and re-radiation of terrestrial radiation on average more than they cool by reflecting solar radiation back to space2. CCT would aim to reduce this net warming by injecting highly effective ice nuclei into cirrus, causing the freezing of supercooled water droplets and inducing growth to large ice particles that sediment rapidly out of the clouds, reducing the mean cirrus cloud thickness and lifetime131,132,133,134. Since CCT would primarily target terrestrial radiation, in contrast to SAI and MSB, it may more directly counteract radiative forcing by anthropogenic greenhouse gases, though the degree of compensation would be limited by the geographical distribution of susceptible cirrus135.

The relatively close balance between a large gross warming and cooling by cirrus clouds, in contrast to the dominance of gross cooling for marine stratocumulus clouds and most aerosol particles under consideration for MSB and SAI, makes estimating a maximum radiative forcing potential even more challenging. A maximum net cooling in the range of 2–3.5 W/m2, considerably exceeding RFGref, has been computed based on model simulations131,132,135,136,137, though the high end of this range is accomplished by modifications in the models which are far removed from what could likely be achieved in reality (e.g., increasing the cirrus particle fall speeds 8–10-fold everywhere).

On the other hand, some studies138,139 have found that CCT might not work at all, or might even produce a net warming. In particular, there is a risk of ‘over-seeding’, i.e., forming new cirrus clouds due to seeding material being released in cloud-free regions, which would have a warming effect, working against the desired cooling132,138,139. Furthermore, recent findings of extensive heterogeneous nucleation in tropical anvil cirrus140 possibly rules out tropical cirrus for seeding134, since seeding would only be effective in an environment where a significant fraction of the natural freezing occurs via homogeneous nucleation (i.e., freezing of supercooled droplets without ice nuclei). Thus the focus of CCT studies is on the middle and high latitudes, where model simulations and satellite data indicate it would likely be most effective132,133,134.

Like SAI and MSB, CCT would require regular injection of seeding material, which would settle out with the cirrus cloud particles. Proposed seeding materials include bismuth tri-iodide (BiI3)131, which was historically investigated for weather modification programs and found to be a highly effective material for ice nuclei, though toxic. Sea salt may also be a candidate, as it is readily available and non-toxic, and has been found to function as an ice nuclei141, though considerably less effective than BiI3. The particle injections would likely require dedicated aircraft or unmanned drones to provide sufficient control over the seeding locations, which would need to be targeted at existing susceptible cirrus clouds. Due to the likelihood of over-seeding in cloud-free regions132, seeding via commercial aircraft can essentially be ruled out.

Research needs for RFG
While current scientific knowledge of the three atmospheric RFG techniques discussed above indicates that they might physically be able to contribute significantly towards reducing global mean temperatures, any large-scale implementation would likely require several decades, due to the considerable uncertainties and scientific research and development needs, along with the extensive considerations needed for a range of socio-political issues (see Box 1). Many of the research and development needs are generally in common across the atmospheric RFG techniques, in four broad categories. First, the associated geographical heterogeneities and side effects on various Earth systems (see Box 3) need to be much better characterized. Second, in terms of process understanding, perhaps the most significant general challenge in common to all three techniques is developing a greater understanding of the associated aerosol and cloud microphysics (Fig. 5).

Fig. 5
figure5
Key aerosol and cloud microphysics issues involved in atmospheric RFG techniques. For SAI, MSB and CCT, the aerosol and cloud microphysics involved are poorly understood and challenging to simulate – one of the main hindrances to confidently predicting the potential climate effects. a The size distribution of the injected or produced aerosol particles influences their effectiveness. As illustrated for SAI, initial studies indicate that there is an optimal particle size (estimated at r ≈ 0.25 μm98); much smaller particles do not effectively reflect sunlight, while much larger particles sediment out too quickly to produce a significant time-integrated radiative forcing. Simulations indicate that MSB has a smaller optimal particle size (r ≈ 0.13 μm), and oversized particles could even lead to a warming instead of cooling122,157. CCT is instead mainly affected by the injected particle concentration, with an optimum around 20/l, while excessive concentrations (greater than 100/l) could lead to warming132. The size distributions and particle concentrations in turn depend on the particle growth characteristics98,158,159, for which coagulation is a particularly important and uncertain factor. Chemical composition also influences the aerosol optical properties, but considerable research is needed to better understand this. Similarly, few studies have investigated the dependence on the ambient meteorological conditions, including turbulence, the susceptibility of clouds to formation of precipitation128,160 (for MSB), and the spectrum of vertical velocities, which affects the activation of cloud condensation nuclei and ice nuclei (for MSB and CCT). b An additional complexity for MSB and CCT is introduced by the aerosol-cloud interactions. The impacts of aerosol particles on cloud optical properties are very difficult to simulate in both cloud-resolving and global climate models, and have been repeatedly highlighted by the IPCC2 as one of the most significant uncertainties involved in climate change predictions. The chemical composition of the aerosol particles influences their effectiveness as cloud condensation nuclei (MSB) and ice nuclei (CCT). Finally, aerosol particles can affect cloud lifetimes, especially for MSB, since reducing the size of the cloud droplets can increase the lifetime of the clouds before they form precipitation, but can in turn reduce the lifetime by making them more susceptible to evaporation

Full size image
Third, a much better understanding is needed of the implementation costs, which have been proposed by some to be a factor of 10–1000 lower than the corresponding annual costs of CDR techniques. Initial estimates67,68,69,108 for development and installation costs for SAI via aircraft and tethered balloon injection systems and for MSB by unmanned ships are all in the range of $1–100 billion, with annual maintenance costs for SAI estimated at $20 billion or possibly even less. No published estimates are yet available for the operational costs of CCT by aircraft deployment, since the associated physical mechanism is still too poorly understood, pointing to an important future research need. An additional challenge to estimating the operational costs for RFG is the need to account for the long timescale over which it might be applied to uphold the Paris Agreement temperature goals, if not accompanied by simultaneous strong mitigation and CDR.

And fourth, establishing a more robust knowledge base for any of the proposed techniques would require eventually moving beyond theoretical, modelling, satellite-based and proxy data studies to also including in situ field experiments. Thus far, only two scientifically rigorous, dedicated, in situ, perturbative field experiments have been conducted related to the atmospheric RFG techniques142,143, focusing on marine stratus microphysics, though not explicitly focused on MSB. However, considerable work has been done recently on developing numerous concepts for a variety of field experiments112,144,145. These proposals have been anticipated to raise considerable public concern, and thus have been closely accompanied by governance development efforts (see Box 1).

Summary and outlook
Among the CDR techniques in Fig. 1, BECCS, DACCS, enhanced weathering and ocean alkalinisation are likely physically capable of removing more than CDRref (650 Gt(CO2)) in this century, while afforestation, biochar production and burial, soil carbon enrichment and OIF all have an upper bound for physical removal capacity that is a significant fraction of CDRref, though all would involve significant implementation costs and in most cases substantial negative side effects. For RFG, in the context and timescales of the Paris Agreement, likely only SAI, CCT and MSB have the technical potential to physically provide a global cooling that significantly exceeds RFGref (0.6 W/m2). Space mirrors and surface-based techniques would be anticipated to face prohibitive constraints including logistics, costs, timescales, and ecosystem side effects.

Any climate geoengineering technique would likely require several decades to develop to a scale comparable to CDRref or RFGref. For CDR, extensive global infrastructure development would be needed, along with resolving governance issues, including competition with other sectors like agriculture. For RFG, improving the scientific understanding (e.g., microphysical details) and developing delivery technologies and effective governance mechanisms would all be essential. For both CDR and RFG, these developments would require public and political support, especially for public investments given their technical and economic uncertainty at the scales of CDRref or RFGref. Given the meagre knowledge surrounding technique scalability, at present only indicative orders of magnitude can be given for costs: approximately $100/t(CO2) for CDR techniques (i.e., over $800 billion/yr to achieve CDRref between 2020 and 2100); and possibly as low as $10 billion/yr for the atmospheric RFG techniques to provide RFGref, though such low costs may never be achievable due to technological challenges upon scaling up. There are of course also numerous social and environmental impacts and associated costs (e.g., Figs. 3–4, and Box 1) that are currently only very roughly characterized in the literature.

In the context of their role in the Paris Agreement, and more generally in climate policy, climate geoengineering technologies may eventually become part of a significant socio-technical imaginary147,146,148, within which specific visions of the future are made to appear desirable, and which are influential on present political developments. Climate geoengineering is already entering the collective imagination149, e.g., as portrayed through media reports, and is also entering climate policy discussions, for instance through inclusion in the IPCC assessment reports2, and especially through the extensive reliance on CDR in low-carbon future scenarios analyzed by the IPCC18,19. Relatedly, the concept of the Anthropocene, with its emphasis on the planetary impact of human activities, may further normalize climate geoengineering technologies as potential tools for conscious planetary management. However, none of the proposed CDR or RFG techniques exist yet at a climate-relevant scale, and given the challenges discussed here, it is not yet certain that any of the individual techniques could ever be scaled up to the level of CDRref or RFGref. Avoiding a premature normalization of the hypothetical climate geoengineering techniques in science, society and politics would require actively opening up discussions to critical questioning and reframing.

We highlight three steps regarding future considerations of climate geoengineering in the context of the Paris Agreement. First, early development of effective governance—including for research—could be designed to reduce the likelihood and extent of potential injustices (see Box 1) and allow supporters and critics of climate geoengineering technologies to voice their concerns. Second, further disciplinary and interdisciplinary research could help to reduce the large uncertainties in the anticipated climate effects, side effects, costs, and technical implementation and societal aspects of the individual techniques. Legitimizing such research would require transdisciplinary processes involving stakeholders from the scientific and policy communities, civil society, and the public, especially in making decisions regarding potential large scale research programs. Ensuring such broad involvement is a major challenge for effective governance. National and international efforts to foster deliberation and coordinate any future large scale research may help to reduce some of the socio-political risks, especially the moral hazard risk of distracting from or deterring climate mitigation. Such coordination could also serve to reduce redundant work and channel research towards issues that are determined to be priorities for informing current and upcoming decision-making processes.

Finally, based on the current knowledge reviewed here, proposed climate geoengineering techniques cannot be relied on to be able to make significant contributions, e.g., at the levels of CDRref or RFGref, towards counteracting climate change in the context of the Paris Agreement. Even if climate geoengineering techniques were ever actively pursued, and eventually worked as envisioned on global scales, they would very unlikely be implementable prior to the second half of the century15. Given the rather modest degree of intended global mitigation efforts currently reflected in the NDCs (Fig. 2 and Supplementary Table 1), this would very likely be too late to sufficiently counteract the warming due to increasing levels of CO2 and other climate forcers to stay within the 1.5 °C temperature limit—and probably even the 2 °C limit—especially if mitigation efforts after 2030 do not substantially exceed the planned efforts of the next decade. Thus at present, the only reliable way to attain a high probability of achieving the Paris Agreement goals requires considerably increasing mitigation efforts beyond the current plans, including starting extensive emissions reductions much sooner than in the current NDCs.

Methods
Parameters in supplementary Table 1 and Fig. 2
Supplementary Table 1 provides values for four key parameters, based on the Paris Agreement Nationally Determined Contributions (NDCs) until 2030 and assumed annual decrease rates of the emissions beyond that (or starting in 2021 for one case): (1) the annual CO2 emissions rates in 2030 and 2100; (2) the cumulative CO2 emissions for 2015–2030, 2031–2100, and 2015–2100; (3) the gaps between the cumulative CO2 emissions and the remaining budgets of cumulative emissions (using 2015 as a reference starting date) that are consistent with the temperature limits of 1.5 and 2 °C; and (4) the approximate equivalent radiative forcing amounts that these emissions gaps represent. Figure 2 provides a graphical depiction of the cumulative CO2 emissions gaps and the equivalent radiative forcing gaps. The computations for these are described here, followed by a few overarching issues.

Annual CO2 emissions
The annual CO2 emissions rates in 2030 are based on a recent reassessment of the current NDCs9, which takes a more direct approach than several previous studies that are based on analyses with integrated assessment models8,150, arriving at a best estimate value of 51 Gt(CO2)/yr, which is 10–20% higher than most previous studies, while the lower bound value of 43 Gt(CO2)/yr computed by ref. 9 is similar to the best estimate values of most previous studies. To reflect this range of estimated future emissions, in Supplementary Table 1 we give a ± range that represents these lower bound and best estimate values based on the data from ref. 9.

The annual CO2 emissions rates, especially in 2100, are relevant for considering the subsidiary Paris Agreement goal of achieving net zero CO2 emissions during the second half of the century. Since the natural sink of CO2 (0.8–1.1 Gt(CO2)/yr52) is small compared to current anthropogenic emissions, and already largely balanced over longer time scales by natural CO2 sources such as volcanic activity2, achieving net zero CO2 emissions would require sufficient CDR to essentially completely compensate the anthropogenic CO2 emissions rate.

If actual CO2 emissions in 2030 are outside the range expected based on the current NDCs (i.e., the NDCs are either not achieved, or efforts exceed current commitments), then for the first four cases in Supplementary Table 1 (with emissions reductions starting in 2031), the subsequent emissions rates and cumulative emissions will scale linearly with the relative difference in 2030 (e.g., 10% lower emissions in 2030 imply 10% lower emissions in 2100 and 10% lower cumulative emissions from 2031–2100).

Cumulative CO2 emissions
The cumulative emissions for 2015–2030 in Supplementary Table 1 are computed based on the annual emissions data from ref.9, separately for the pathways based on the lower bound and best estimate values (from which the means and ± ranges are computed). For 2031–2100, for the case with constant annual emissions the cumulative emissions are simply computed as 70 times the lower and upper bound values for the annual emissions. For the cases with an annual decrease from 2031 onwards, individual pathways until 2100, starting from the lower and upper bound values in 2030, are calculated as Ey = (1−r) * Ey-1, where Ey is the emissions rate for the current year, Ey−1 for the previous year, and r is the annual emissions reduction factor (0.01, 0.03, or 0.05). The annual emissions along each pathway are then summed to obtain the cumulative emissions for 2031–2100. The same procedure is also applied to the fifth case in Fig. 2 and Supplementary Table 1, with a 3% annual reduction starting in 2021, for which the 2015–2030 cumulative emissions range is recalculated accordingly. In all cases, emissions reductions and the resultant cumulative emissions and implications for radiative forcing and global mean temperature increase are only considered until 2100.

Gaps to the remaining CO2 budgets
Various approaches have been applied to determine the remaining budgets of cumulative emissions of CO2 (and non-CO2 forcers) which are consistent with likely limiting global warming to various temperature thresholds. Each of these has various drawbacks. We describe a few of these here, as a background to why we have developed a simple, novel approach that is suited for this analysis. In one approach, the IPCC3 found that a cumulative CO2 budget of 400 Gt(CO2) from 2011 onwards likely keeps 21st century ΔT¯s below 1.5 °C, which can be adjusted to ~240 Gt(CO2) for 2015 onwards at the current global emissions rate of just over 40 Gt(CO2)/yr6. This would already be exhausted in 2020, which seems unlikely given that current global warming is approximately 1 °C, and the finding by the IPCC WG12 that if anthropogenic CO2 emissions were abruptly stopped, the global mean temperature would likely remain approximately constant for decades (due to a balancing of opposing factors). Using another approach3, the IPCC concluded that the cumulative emissions budget from 1870 onwards that is consistent with likely keeping ΔT¯s below 1.5 °C is 2250 Gt(CO2). Comparing this directly with the Global Carbon Project’s current estimate of historical cumulative emissions, which is 2235 ± 240 Gt(CO2) for 1870–2017, would also imply that the 1.5 °C budget has already been or will very soon be exhausted. This comparison makes one of the main problems with this approach clear: it is based on the small difference between two large and uncertain numbers. Recognizing this problem, the Global Carbon Project concludes6: “…extreme caution is needed if using our updated cumulative emission estimate to determine the ‘remaining carbon budget’ to stay below given temperature limits4. We suggest estimating the remaining carbon budget by integrating scenario data from the current time to some time in the future as proposed recently5.” The application of this alternate approach by ref.5 results in much higher estimates than the IPCC approaches: likely more than 880 Gt(CO2) and 1870 Gt(CO2) (from 2015 onwards) for 1.5 and 2 °C5. However, this is associated with several assumptions, which have been strongly criticized7, as noted in the main text.

For the purpose of our analysis we apply a similar though simpler approach, which is independent of the historical emissions and is straightforward to apply to any moderate temperature difference (e.g., 0.5 or 1 °C), and allows us to apply an uncertainty range to the current value of ΔT¯s. We first consider the cumulative budgets from 1870 onwards that were found by the IPCC3 to be consistent with likely limiting global warming to three temperature thresholds: 2250 Gt(CO2) for 1.5 °C, 2900 Gt(CO2) for 2 °C, and 4200 Gt(CO2) for 3 °C, where the simulated warming includes effects of co-emitted non-CO2 forcers. These three cumulative budget values make the quasi-linear response of simulated temperature to cumulative CO2 emissions very clear, with a slope of 1 °C for every 1300 Gt(CO2) between any pair of these temperature thresholds.

This slope is then applied to determine the remaining CO2 budget between any two values of ΔT¯s. There is a notable uncertainty in the current value of ΔT¯s7, given interannual and interdecadal variability, as well as the uneven geographical coverage of the global observations network, and other factors such as the reference starting date (i.e., what counts as pre-industrial), etc. Here we make use of the analysis in ref. 7 and apply a current value of ΔT¯s = 1.0 ± 0.1 °C, which accounts for the biased geographical coverage of the measurements network, especially the relatively few long-term temperature observations in the rapidly warming Arctic, and is thus higher than the value of ΔT¯s = 0.9 °C applied by ref. 5. Note that a small additional uncertainty is present in further factors, such as using the mid-1700s rather than the late 1800s as a reference period for pre-industrial temperatures. We do not take these additional factors into account, in order to remain comparable to the IPCC and other analysis that apply the late 1800s reference period; however, we note that this and other unaccounted factors could increase the current value of ΔT¯s by up to ~0.15 °C, reducing the remaining budgets by up to ~200 Gt(CO2), which is a comparatively small uncertainty in light of the broad ranges of values in Fig. 2 and Supplementary Table 1.

Applying a current value of ΔT¯s = 1.0 ± 0.1 °C and the slope of 1 °C per1300 Gt(CO2) cumulative emissions yields a value of 650 ± 130 Gt(CO2) from 2015 onwards for the remaining budget consistent with likely limiting ΔT¯s to 1.5 °C, and 1300 ± 130 Gt(CO2) for 2 °C. These remaining budget values are then subtracted from the projected emissions for the different cases to determine the gaps in the cumulative emissions budgets, giving an indication of how much CDR might be invoked in an attempt to compensate the emissions gaps in order to still achieve the Paris Agreement temperature goals. These resulting values are depicted in Fig. 2 and listed in Supplementary Table 1.

Equivalent radiative forcing amounts
Finally, in order to derive an indication of what these cumulative emissions gaps would imply for the amount of negative radiative forcing that would be needed to limit ΔT¯s to a given threshold, we can make use of the climate sensitivity simulated by model ensembles to convert from the CO2 emissions budget gaps (in Gt(CO2)) to equivalent radiative forcing in W/m2. For this, we use the slope noted above of 1 °C for every 1300 Gt(CO2), or 7.7 × 10–4 °C/Gt(CO2), and combine this with the best estimate value from the IPCC2 for the equilibrium climate sensitivity of λ ≈ 0.8 °C/(W/m2) (corresponding to a mean equilibrium warming of 3 °C for a radiative forcing of 3.7 W/m2 from a doubling of CO2 since preindustrial times), or inverted, 1.25 (W/m2)/°C. Together these give 9.6 × 10–4 (W/m2)/Gt(CO2) (or approximately 1 W/m2 for every thousand Gt(CO2)), which we apply to obtain the values listed in the final row of Supplementary Table 1. We note that this is only an approximate conversion, since the individual climate sensitivity components were derived from different model ensemble simulations designed for different purposes, but it is adequate for the purpose of providing orientation values for the radiative forcing that may be called for from proposed RFG techniques in the context of achieving the Paris Agreement goals, particularly in comparison to the possible equivalent CO2 budget contributions from CDR.

General issues
There are several general issues important for interpreting and applying Fig. 2 and Supplementary Table 1. First, it is important to note that a small amount of CDR is already assumed in some NDCs, in particular by China, India, Russia, the USA and Canada (in contrast to the statement by ref.29 that “none of the NDCs contains plans to develop negative emissions”). However, to the extent that information is available in the data used in ref.9, the amounts assumed by individual nations are very small, each less than 10 Gt(CO2) by 2030, so that only a few percent of the cumulative global emissions from now until 2030 are represented by CDR in the current NDCs. Given this small fraction, and the complexity of determining factors such as the exact amounts and timing that are assumed (which is often not transparent in the data), we do not attempt to account for these explicitly in our calculations. However, methodologically we note that the CDR amounts discussed in the present study are in addition to the small amounts of CDR that are already assumed in the current NDCs.

One of the most challenging issues to account for in such budget calculations is the role of non-CO2 forcers, including the short-lived climate forcers (SLCFs), which is often not possible to do consistently due to differences in their treatment between different studies, as well as frequently a lack of detailed information about how they were treated in individual studies. Throughout Fig. 2 and Supplementary Table 1 and the main text of this study, we work in units of CO2 emissions, which in many cases are converted from values given in the original publications in units of equivalent CO2 (“CO2e”), which accounts for the CO2-equivalent radiative forcing contribution of globally co-emitted SLCFs and other non-CO2 forcers. The radiative forcing from SLCFs has been shown by the IPCC2 and in numerous individual studies to be an important component in achieving ambitious temperature goals like those in the Paris Agreement. The SLCFs are also important due to the regional differences in their distributions: even though the non-CO2 forcers are responsible for a relatively small net radiative forcing, this is the result of considerably larger and regionally differing gross warming and cooling effects partly cancelling each other out globally. The notable regional differences in the radiative forcing may in turn be important in calculating the detailed impacts of proposed CDR and RFG techniques. Furthermore, focused efforts on the reductions of specific SLCFs, e.g., black carbon or sulfate aerosols, could result in rapid changes in their atmospheric levels, and thus could notably shift the net global radiative forcing by non-CO2 forcers in one direction or another, again impacting the calculated effects of climate geoengineering measures. On the other hand, since CO2 is much longer-lived, its levels accumulate in the atmosphere over centuries, while SLCFs are naturally removed within a timescale of a few days to a few decades after they are emitted. Thus, in the high-ambition, low-carbon pathways which are of most relevance for achieving the Paris Agreement goals with no or very limited use of CDR and/or RFG, the SLCFs play a particularly important role, whereas in the higher carbon pathways for which the discussion of CDR and possibly RFG will become more intense, the forcing by CO2 will tend to dominate more strongly over the SLCFs and other non-CO2 forcers. Thus, while it is important to account for the non-CO2 forcers, especially in consideration of achieving the 1.5 °C or 2 °C goals via mitigation alone, they will likely play a diminishing role specifically in the cases where CDR and possibly RFG will be most strongly invoked. This is important to bear in mind as background for the treatment of SLCFs in this study (i.e., for the data in Fig. 2 and Supplementary Table 1).

For the SLCFs, we find that in the emissions data for 2015–2030 from ref.9, the ratio of CO2e to CO2 is calculated to remain nearly constant during this period (ranging from 1.31 to 1.33). This allows a straightforward conversion using a factor of 1.32 between CO2e and CO2, but also reflects the uncertainty in the evolution of non-CO2 forcer emissions (especially SLCFs), which are often approached very simply (e.g., assuming they will either remain constant or their forcing ratio to CO2 will remain constant). This introduces an uncertainty in the results in Fig. 2 and Supplementary Table 1, since it is likely that the relative role of the non-CO2 forcers will change from what is currently reflected in the NDCs. Nevertheless, given that the factor of 1.32 implies a current net contribution of non-CO2 forcers to global radiative forcing of about 25%, along with the arguments above we do not expect the temporal changes in this ratio to make any qualitative differences in the results and ranges reflected in Fig. 2 and Supplementary Table 1.

Finally, for readability, and given the uncertainties described in the data and assumptions employed for the calculations for Fig. 2 and Supplementary Table 1, all values below 100 in Supplementary Table 1 are rounded to two significant figures, and above 100 are rounded to the nearest tens (while exact values are used for Fig. 2).

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Acknowledgements
We are grateful to many colleagues for enriching discussions and constructive comments that contributed to the development of this manuscript. We cannot name all, but key among them are the entire EuTRACE consortium, Peter Irvine for very valuable constructive comments on early drafts of this manuscript, and many other colleagues with whom we discussed specific topics and aspects of the manuscript, including David Keith, Alan Robock, Francois Benduhn, Phil Rasch, David Mitchell, Simone Tilmes, Ulrike Niemeier, Thomas Bruhn, Andrew Parker, Miranda Boettcher, Sean Low, and others. We thank Wera Wojtkiewicz and Kristina Steinmar for their great support, especially with the literature, and Sabine Zentek for the careful work and patient iterations on the graphics. The authors gratefully acknowledge the funding by the EU for the FP7 project EuTRACE (European Transdisciplinary Assessment of Climate Engineering). The IASS (M.L. and S.S.) is funded by the German Federal Ministry of Education and Research (BMBF) and the Brandenburg State Ministry of Science, Research and Culture (MWFK). H.M. was supported by Research Council of Norway project grants 261862/E10 and 229760/E10. V.S. was supported by UK Natural Environment Research Council grant NE/P019749/1. N.V. was supported by UK Natural Environment Research Council grant NE/P019951/1. A.O. and H.S. were supported by the German Science Foundation (DFG) projects ComparCE-2 and CELARIT within the Priority Program SPP 1689. J.S. was supported by the German Science Foundation (DFG) Cluster of Excellence CliSAP.

We would like to dedicate this paper to our dearly missed EuTRACE colleague and friend Jón Egill Kristjánsson, who not only provided valuable input to earlier drafts of the manuscript, but enriched all of our lives with many thoughtful discussions on the topic of climate geoengineering over the years that we were fortunate to have him among us.

Author information
Affiliations
Institute for Advanced Sustainability Studies (IASS), Potsdam, Germany

Mark G. Lawrence & Stefan Schäfer

University of Potsdam, Potsdam, Germany

Mark G. Lawrence

Institute for Science, Innovation and Society, University of Oxford, Oxford, UK

Stefan Schäfer

University of Oslo, Oslo, Norway

Helene Muri

Norwegian University of Science and Technology, Trondheim, Norway

Helene Muri

University of Edinburgh, Edinburgh, UK

Vivian Scott

GEOMAR, Kiel, Germany

Andreas Oschlies

University of East Anglia, Norwich, UK

Naomi E. Vaughan

Institut Pierre-Simon Laplace, CNRS / Sorbonne Université, Paris, France

Olivier Boucher

Max Planck Institute for Meteorology, Hamburg, Germany

Hauke Schmidt

University of Exeter, Exeter, UK

Jim Haywood

Met Office Hadley Centre, Exeter, UK

Jim Haywood

University of Hamburg, Hamburg, Germany

Jürgen Scheffran

Contributions
All authors were part of the EuTRACE consortium and co-authors of the EuTRACE assessment (available at www.eutrace.org), which was the original basis for this review. M.L. led the drafting and revising of the manuscript, and all authors contributed extensively to its development beyond the EuTRACE assessment. Three co-authors provided draft texts for topical sections (V.S. for CDR, H.M. for RFG and S.S. for the socio-political issues), and all co-authors edited and commented all sections, focusing especially on the sections representing their respective expertise (V.S., A.O. and N.V. for CDR; H.M., O.B., H.S. and J.H. for RFG; S.S. and J.S. for the socio-political issues).

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Correspondence to Mark G. Lawrence.

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Lawrence, M.G., Schäfer, S., Muri, H. et al. Evaluating climate geoengineering proposals in the context of the Paris Agreement temperature goals. Nat Commun 9, 3734 (2018). https://doi.org/10.1038/s41467-018-05938-3

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https://www.nature.com/articles/s41467-018-05938-3

In 2005asmallpilotprojectontheGurschenglacierofthe
Swiss Alpswasconductedtotrytostoptheicemeltingof
glaciers [52a] with a “ice protector” textilemadeofalight-
weightdual-layercompositewithpolyesterinthetopsideto
reflect light,andpolypropyleneonthebottomtoblockheat
and slowicemeltingduringthesummer.Itprovedsuccessful
as theblanketedareahad80%lessmeltthansurroundingice.
Coveringanareaof30,000m2 wasprojectedontheVorab
glacier.
In thePeruAndeanregion,alocalteamthatpaintedrocksin
whitewona$200,000prizefromtheWorldBankaspartofits
“100IdeastoSavethePlanet” competition [52b–c]. Meanwhile
the “Fund forInnovativeClimateandEnergyResearch” is
financed byGates [52d] and ismoredevotedtoSRMprojects,
the “Virgin EarthChallenge” financed byBrandson [52e] is
more concentratedonCDRandoffersa$25millionprizefora
commerciallyviableinventionabletopermanentlyremove
significant volumesofGHGsoutoftheEarth0s atmosphere,so
as tocontributemateriallytoavoidglobalwarming.
In coldcountries,withwinterfreezingrivers [53] and lakesit
can bedrilledboreholesintotheicethathasstartedtoform.
The waterwillbedischargedacrossthesurface,whereitwill
freeze andaddlayersoficerinks.Theicecapitselfbeingagood
insulator,ifnoholesweredrilledinit,muchlesswaterwould
freeze. Thisprocesscouldberepeatedatregularintervals
throughoutthewinterwiththeaimtoproduceabigblockof
ice severalmetersthickasrefrigerationstorage,tocooland
waterthecitiesasitmeltsduringsummer.Theinsulation
capacity oftheiceisbrokenbyoneofthe “thermal bridge”
strategiesthatwillbedevelopedlater.
Over thesummermonths,upto40–50% ofthewaterstoredin
small farmdamsmaybelosttoevaporation,butusingwhite
reflectivecoverstoreducethislossincreasesagriculturalwater
use efficiency andparticipatestoglobalcoolingbymodifying
albedo [54].
Rising saltygroundwatercurrentlythreatensmanyagricultural
lands, butasalinitymitigationstrategy[55] is alreadyappliedin
Australia.Theaimistopreventthecleargroundwatertomix
with thesaltyone,andconsistedduringthedryseasonin
pumping saltygroundwaterintoshallowevaporationbasinsto
form asaltpanwithhigherreflectance thanthesurrounding
farmland whichresultedinanimmediatemitigationoflocal
warmingbothbyevaporationandbyalbedomodification. The
main goalisachievedtoo:preventingsaltygroundwaterto
mix withtheclearone.
The SPICEproject [56]a–b (stratosphericparticleinjectionfor
climate engineering)consistedinusingasmallhose-aug-
mented balloonupjustoveronekmhigh,pumpingwaterinto
the air.Theaimwastotestthefeasibilityoflaterpipingsulfates
at 25kmhigh(see Fig. 4). Althoughonlywaterwastobe
sprayed,GEopponentssucceededtostopthisexperiment.
Partanen [57] showedthatmultiplyingthemass flux by5or
reducing theinjectedparticlesizefrom250nmto100nm
could havecomparableeffectsontheGEradiativeefficiency.
In 2002,anartificial cloudmakingmethodwaspatentedin
China [59]. ItreplicatesEarth0s HydrologicCycle,usingapipeline
facility constantlyconveyingair,fromaloweraltitudetoahigher
altitude, withwatervaporwhichcondensestoformapervasive
artificial cloud.MorerecentlyseveralUSpatents [59] from former
Microsoftscientistsdescribedaverysimilarconcept,witha
15–50 kmhighaltitudeduct “conduit” like in Fig. 5 for theaerosol
injection inthestratosphere.Thatsoundsquitehigh,butseveral
articles fromNASAdescribethefeasibilityofmulti-kilometer
height talltowers [61–65]. Laterinthisreview,someERM
strategiesproposetomakeuseofquitehighmeteorological
reactors,butcivilengineersandarchitectsareconfident ontheir
feasibility,asalreadyalmostkilometrichighbuildingshavebeen
successfullybuiltandnumerousprojectsallovertheworldtarget
taller ones.

2.6. DiscussionaboutSRM
SRM methodsmaybeabletoreducetemperaturesquicklyand
some ofthemlikestratosphericaerosolsatcomparativelylow
cost. However,eveniftheycouldreducesomeofthemost
significant effectsofglobalwarmingandlessensomeofits
harmful impacts,thesetechnologiescouldalsohavesignificant
unanticipatedharmfulsideeffects.Moreover,theywouldnot
eliminatethecauseofclimatechange,theemissionsofGHGs
and theassociatedthreatofoceanacidification. Formanyexperts
the wholeideaofpursuingthese “technical fixes” is controversial
since SRMcanprobablyrestoreonaveragetheEarth0s global
radiativebalance,butregionalclimatediscrepancieswillremain
[66].
Also, ifCO2 levels
continuetoriseduringSRM,thatmeansitmust bemaintainedindefinitelytoavoidabruptandcatastrophic
Fig. 6. SeveralgeoengineeringschemesasrepresentedbyMatthews [67].
798 Tingzhen Ming et al. / Renewable and Sustainable Energy Reviews 31 (2014) 792–834

warmingandtheremusthappennotechnological,economicalor
political failure.
In apositionwhereavoidanceofonedangerexposesoneto
another danger,CEhasbeenwidelyshunnedbythosecommitted
to reducingemissionsandbythepublicwhichfeelsthatSRMand
GE (oftenonlyassociatedtosulfateaerosols)isfartooriskyto
attempt,sincetamperingwithEarth0s andclimatesystemscould
lead tonewclimaticandecologicalproblems.
The principalGEschemesarerepresentedin Fig. 6 reproduced
from Matthews [67]. Inapaperwhosetitleis “Can wetest
geoengineering?”, MacMynowski [68] notedthatSRMtestscould
requireseveraldecadesorlongertoobtainaccurateresponse
estimates,asthehydrologicalandtemperatureresponseswill
differ fromashort-durationtestandalsofromwhathasbeen
observed afterlargevolcaniceruptions.Robock [40] found
“20 reasonswhygeoengineeringmaybeabadidea”.
By pumpingmassiveamountsofCO2 and otherGHGsintothe
atmosphere andbybuildingmega-citiesandthousandsofkilo-
metersofblackpavedhighways,humanshavealreadyengagedin
a dangerousgeophysicalexperiment.Theonlydifferencewith
CE isthatitwasunintentional.Thebestandsafeststrategyfor
reversingclimatechangeistohaltthisbuildupofatmospheric
GHGs andstopCO2 emissions, butthissolutionwilltaketime,and
it involvesamyriadofpracticalandpoliticaldifficulties. Mean-
while, thedangersaremountingandevenwithaseriouseffortto
control GHGsemissions,meaningfullyreducingtheminthevery
near termisanunattainablegoal.
As MyhrvoldandCaldeira [69] showed,therapiddeployment
of low-emissionenergysystemscandolittletodiminishthe
climate impactsinthe first halfofthiscentury:conservation,
wind, solar,nuclearpower,andpossiblyCCSappeartobeableto
achieve substantialclimatebenefits onlyinthesecondhalfofthis
century.
So maybeGEwillbeneeded,althoughseriousresearchonCEis
still initsinfancy,andtillrecentlyhasreceivedlittle financial
funding forscientific evaluationofbenefits andrisks.
But eveniftheethicsofgeoengineeringaswellaspolitical
aspects hasbeenwidelydiscussed [20–27], neitherinternational
nor public [70,71] consensus hasbeenyetobtainedevenfor
researchonthissubject:stoppingthe “spice experiment” previously
citedisanillustration.
It isworthnotingthatsince1977thereisanEnvironmental
Modification Convention,whichhassofarbeenratified by76
countries [72]. Itprohibitsthehostileuseoftechniquesthatmodify
the dynamics,composition,orstructureoftheEarth(includingthe
atmosphere)orofouterspace.Oneofthemainquestionsofthe
debateis:inafragileandglobalizeworld,whowouldgovern
geoengineeringactionsthatcanseverelyaffectclimateand,forthis
reason,mightbepotentiallyusedasweapons?
Also, tilldatethemostsuccessfulinternationalagreementis
the MontrealProtocolonSubstancesthatDepletetheOzoneLayer
[73] that wasagreedin1987.Itincludedtradesanctionstoachieve
the statedgoalsofthetreatyandoffersmajorincentivesfornon-
signatory nationstosigntheagreement.Asthedepletionofthe
ozone layerisanenvironmentalproblemmosteffectively
addressed onthegloballevelthetreatyincludepossibletrade
sanctions, becausewithoutthemtherewouldbeeconomicincen-
tivesfornon-signatoriestoincreaseproductionofcheapdepleting
substances, damagingthecompetitivenessofthesignatorynations
industries aswellasdecreasingthesearchforlessdamaging
alternatives.AllUNrecognizednationshaveratified thetreatyand
continue tophaseouttheproductionofchemicalsthatdepletethe
ozone layerwhilesearchingforozone-friendlyalternatives.Inthe
presence ofhalogenatedcompounds,thesulfateaerosolsinthe
stratospheremightdamagetheozonelayer [39] thus thisSRM
might beaviolationoftheMontrealProtocolspiritandgoal.
The intergenerationaltransferofatmosphericcarbonandGHGs
stocksandpollutionisalsopartofthediscussions [74,75] as thisis
equivalenttodelaycurrentgeneration0s abatementefforts.Future
generationswillhavetolimitthedamagesoftheatmospheric
carbon stockthattheywillinheritfromcurrentsociety.Together
with radioactivenuclearwastes,thisimpliesfuturecostsanda
poisoned chalicetoleavetoourheirsandsuccessors.
In theabsenceofadequatereductionsinanthropogenicCO2
emissions, GEhasbeenputforwardastheonlyremainingoption
that might fix ourrapidlychangingclimate,evenifscientistsare
reluctanttoencouragegovernmentstodeployCEratherthan
investincuttingemissionsandmakingeffortstocontrolthem.
CDR andCCStechniquesaddresstherootcause [16] of climate
changebyremovingthemostabundantGHGsfromtheatmo-
sphere, butwillrequiredecadestohavesignificant effects.SRM
techniquesaremuchfaster(months)andattempttooffsetthe
effects ofincreasedGHGsconcentrationsbyreducingtheabsorp-
tion ofsolarradiationbytheEarth.Bothmethodshavethesame
ultimate aimofreducingglobaltemperatures.
3. Earthradiationmanagement(ERM)
ProposedSRMGEschemesactbytheparasoleffect:reducing
solar incomingradiation.HoweverCO2 traps heatbothdayand
night overtheentireworldwhereasdiminishedsolarradiation
wouldbeexperiencedexclusivelyindaytimeandonaveragemost
stronglyattheequator.
The technologiesdescribedinthispaper,althoughseasonal,are
expectedtobelessintermittentandcovermorethanthediurnal
cycleandarewelldistributedfromequatortopoleastheyare
complementary. Fig. 7 showsonwhichradiation fluxesSRM
geoengineeringschemesmightbeusefulactingonshortwave
radiation(0.2–3 mm), whichrepresentslessthan1/3ofthetotal
incoming radiation.ERMproposedinthenextpartofthisreview
focuses onmorethan2/3oftheglobalradiativebudgetandis
possible nightanddayallovertheEarth.Thegoalofthispaperis
todemonstratethatseveralotherwaysofactionarepossible
acting onthelongwaveradiation(4–25 mm) flux.
3.1.Targetinghighandcoldcirrusclouds:notaSRMstrategy
but aERMone
Mitchell [76] proposedtocooltheEarthsurfacebyincreasing
outgoing longwaveradiationbyreducingthecoverageofhigh
cirrus clouds.
Cirrus cloudstendtotrapmoreoutgoingthermalradiation
than theyreflect incomingsolarradiationandhaveanoverall
warmingeffect.Astheyhaveagreaterimpactontheoutgoing
thermal radiation,itmakessensetotargetthecoldercirrusclouds.
This proposalconsistsinincreasingoutgoinglongwaveradiation
by dispersingcloudsoverthepolaricecaps.Thusbychangingice
crystalsizeinthecoldestcirrus,outgoinglongwaveradiation
might bemodified. AccordingtoMitchell,thecoldestcirrushave
the highesticesuper-saturationduetothedominanceofhomo-
geneousfreezingnucleation,soseedingcoldcirrus(highaltitude)
with efficient heterogeneousicenuclei(likebismuthtri-iodide
BiI3) shouldproducelargericecrystalsduetovaporcompetition
effects, thusincreasingoutgoinglongwaveradiationandsurface
cooling. BiI3 is non-toxicandBiisoneorderofmagnitudecheaper
than Ag(sometimesusedtoincreaserainfall).
PreliminaryestimatesbyMitchell [76a,b] and byStorelvmo
[76c] show thatglobalnetcloudforcingcouldneutralizethe
radiativeforcingduetoaCO2 doubling. Airlineindustryis
the potentialdeliverymechanismfortheseedingmaterialand
reversibilityshouldberapidafterstoppingseedingtheclouds.
Tingzhen Mingetal./RenewableandSustainableEnergyReviews31(2014)792–834 799

methods forenhancingdownwellingoceancurrents,includingthe
use ofexistingindustrialtechniquesforexchangeofheatbetween
waterandair.Theyproposedtheuseofsnow-cannonspoweredby
wind turbineson floating bargesduringthewintertohelpthe
formation ofthickerseaicebypumpingoceanwaterontothe
surface oficesheets.Seaicethatformsnaturallyintheoceandoes
so atthebottomofanicesheetandisnotverysalty(icerejectsthe
salt asitfreezes).Asseaiceformationincreasesthesalinity(salt
content)ofthesurroundingwater,thiscoldandsaltywaterisvery
dense, andsinkscreatingthe “global conveyorbelt”. Zhouand
Flynnmaketheassumptionthatifseawaterfreezesontopofan
ice sheet,saltwouldmainlybetrappedonthesurfaceorwithin
the ice,bothasbrinecellsandsolidsalts,especiallyifthe
thicknessoficebuiltuptoseveralmetersthick.Inthiscase,then
incrementaldownwellingcurrentwouldoccurwhentheseaice
meltedinthespring,sincethemeltingicewouldlowerthe
temperatureofseawaterandthesurroundingoceansalinitywould
be unchanged.Onthecontrary,ifbrinewasableto flow fromthe
topoftheicesheetbackintotheoceaninthewinteras
incrementalseaicewillbeformed,thenincrementaldownwelling
currentwouldoccurinthewinterdrivenbysalinity.Inbothcases
the goalofenhancingdownwellingoceancurrentsisreached.
Of courseontheonehandtheZhouandFlynnproposalcanbe
classified inthealbedomodifications schemesoftheSRMstrate-
gies (Fig. 8), asitparticipatesinmaintainingthepolaricecaps
which helptoregulateglobaltemperaturebyreflecting sunlight.
But ontheotherhandtheirstrategycanalsobeevaluated
differently.Asthe first layersof floating icearegoodthermal
insulators,naturalheattransferfromthewintercoldairtothe
liquidwaterundertheiceisnotveryefficient, sothegrowthofthe
ice capsisslowandtheincreaseofthethicknesslimited.TheZhou
and Flynnstrategyoverpassesthisproblem,andtheyobtaina
thicker icecapthatcanlastlongerinspringandthusreflect more
sunlight backtospace.Butalso,duringthewintermanufactureof
the icebysendingaseawaterspayinthecoldair,thelatentheatof
solidification (freezing)willbereleasedintheatmosphere:cold
ice iscreatedontheoceansurfacemeanwhilethehotair
generated,willbynaturalbuoyancygoupperinthetroposphere.
So aheattransferfromthesurfacetoahigherelevaheattransferfromthesurfacetoahigherelevationhas
occurred.Asimilarstrategywaspreviouslydescribedforrivers

....6.1.SolarupdraftChimneys:powerplantsthatrunonartificial
hot air
A solartower [117,78], alsocalledasolaraero-electricpower
plant, islikeaninvertedverticalfunnel.Theair,collectedatthe
bottomofthetower,iswarmedupbythesun,risesupanddrives
a turbinewhichproduceselectricity [118] (Fig. 12). Indeed,the
thermal radiationfromsunlightheatstheairbeneathaglassor
plastic cover,thehotairrisesupatallchimneywhichcausesa
decrease inpressure.Thus,coldairissuckedbytherisinghotair
within thechimney,whichcreatessurfacewindinsidetheGH.At
the bottomofthechimneythereareseveralturbines [119] that
catch theartificial windcomingintothechimney.Theturbines
generateelectricity.Thermalenergystorage [120] under the
collector allowspeakloadandnightproduction.Thepromoters
of thistechnologyexpectittobecost-competitivewithelectricity
from thegrid,meetingthedemandprofile andthusbeingthe first
non-intermittentrenewableenergysourcetoreachaprimary
providerstatus.Ofcourseseveralsolutionsexistorhavebeen
developed forenergystorageofotherintermittentrenewable
energies, asthermalstorageforCSP(hightemperaturemelted
salts intanks,for2or3h),chemicalbatteriesorhydrogen
production (fromwaterelectrolysis)forwindturbinesandPV.
All thesestoragesystemshaveforthemomentlowstorage
capacity andrequirehighinvestmentcosts.
Pumped-storagehydroelectricityisthemostestablishedtech-
nology forutility-scaleelectricitystorageandhasbeencommer-
cially deployedfordecades.Theworldpumpedstoragegenerating
capacity iscurrentlyabout130GW.Thisenergystoragemethodis
in theformofpotentialenergyofwater.Thefacilitiesgenerallyuse
the heightdifferencebetweentwonaturalorartificial water
reservoirsandjustshiftthewaterbetweenreservoirs.Low-cost
off-peak electricpowerfromnuclearpowerplantsorexcess
electricity generationcapacityfromwindturbinesisusedtorun
the pumpsandtransferwatertothehigherreservoir.Duringpeak
load orforloadbalancingwaterisreleasedbackintoalower
reservoirthroughaturbinegeneratingelectricity.Reversible
turbine/generatorassembliesactaspumpandturbine.
Compressed airenergystorageinundergroundcavernsorin
old saltminesisalsoanenergystoragepossibility,butfew
locations existandstoragecapacityislowerthanforpumped
storagehydroelectricity.Alsoasthecompressionofairgenerates
heat andtheairexpansionrequiresheat(theairiscolderafter
expansionifnoextraheatisadded)thesystemismoreefficient if
the heatgeneratedduringcompressioncanbestoredandused
during expansion,butthisincreasestheinvestmentcostsandthe
complexityofthesystem.
FortheSCPP,whichisalowtemperaturedifferencethermal
powergenerationsystem,gravel,waterinplasticbagsortubes,
and eventhesoilcanbenaturalenergystoragematerials.Adding
the storagecapacityisrelativelycheap.Consideringthelargearea
of thecollector,theSCPPcangenerateoutputpowercontinuously
and steadilydayandnight.Theuseoflowtemperaturesolid/liquid
phase changematerials(PCM)willconsiderablyincreasetheinitial
investmentofbuildingacommercialscaleSCPP.
Quite numerousprototypeshavebeenbuiltindifferentcoun-
tries, butonlyauniquelargeSCPPprototypewasbuiltinthe1980s
in Manzanares,SpainbySchlaich [122–124] and produced50kW.
AccordingtoanannouncementfromtheprivatecompanyEnvir-
oMission attheendofDecember2011,the200MWLaPazSolar
TowerProjectinArizona,USA,shouldbeonlineduringthe first
quarterof2015 [125]. However,the200MW figure seemedover
estimated,asthesamecompanyannouncedfor2006asimilar
powerplantinBuronga,Australia,whichtargetsthesamepower
output with a1.3timestallerchimneyandanalmost2times
largerGH,butneverthelessa30yearpowerpurchaseagreement
wassignedwiththeArizonapowerauthority [125].
Anotherprivatenewcompetitorappeared [126] which intendsto
develop200MWprojectsandannouncedhavingalreadypurchased
a 127,000hasitesurroundingthetownshipofTuckanarra,inthe
Mid-WestregionofWesternAustralia.
Two yearsearlier,inDecember2009,itwasannouncedthata
much smaller200kWSCPPdemonstrationpilotwascompletedin
Jinshawan,Wuhai,InnerMongolia,China,andthata25.1MW
SCPP wasscheduledforDecember2013,theconstructionbeing
expectedtoaccountfor2.510.000m2 of desertareaand1.26
billion RMBinvestment [127] ($200million).
The effectsofwatervaporandpossiblecondensationinalarge
SCPP areanimportantissueandwereinvestigatedbyseveral
researchers,particularlybyKröger [128]. Ofcourse,watershould
not beevaporatedundertheGHasitwillreducethepoweroutput
because ofthelatentheatofvaporizationneeded,andasaresult
the airtemperaturedifferentialwilldecrease;butifmoistair
enters insidetheGH,itimprovestheplantdrivingpotentialand
condensation mayoccurinsidethechimneyoftheplantunder
certain conditions,releasinginsideitthelatentheatofcondensa-
tion. Pretorius [129] described aplantmodelthattakesinto
account theeffectofwatervaporintheairinsideandoutside
the plant,andconsidersthepossiblecondensationoftheairinside
the chimneyoftheplant.
Ninic [130] studied theimpactofairhumidityontheheight
potential(theheightatwhichdisappearsthebuoyancyforceof
the collectorairascendingwithnosolidchimney)andonthe
increaseoftheoperatingpotentialandefficiency ofthewhole
plant. Theheightpotentialcouldbeconsiderablyincreasedifthe
air enteringthecollectorisalreadymoistened.
The cloudformationintheplumesofSCPPswasstudiedby
VanReken [131] and theresultsindicatethatforveryhighwater
vaporconcentrations,cloudwouldprobablyformdirectlyinside
the chimney;withpossibleprecipitationinsomecases.Formore
moderatewatervaporenhancements,thepotentialforcloud
formation variedseasonallyandwassensitivetotheassumed
entrainmentrate.Inseveralcasestherewascloudformationinthe
plume afteritexitedthechimney.Thepowerplantperformance
can probablyslightlybereducedbytheseclouds,buttheselow
altitude cloudscouldalsohaveabeneficial effectonGWbyalbedo
modification.
Zhou [132] studied thespecialclimatearoundaSCPPandthen,
using athree-dimensionalnumericalsimulationmodel,investi-
gatedtheplumeofaSCPPinanatmosphericcross flow [133], with
severalwindspeedsandinitialhumidityhypothesis.Itwasfound
by Zhouthatrelativehumidityoftheplumeisgreatlyincreased,
due totheplumejetintothecoldersurroundings.Inaddition,a
great amountoftinygranulesintheplume,originatingfromthe
ground orcontainedintheairsucked,actaseffectivecondensa-
tion nucleiformoisture,andcondensationwouldoccur.Acloud
systemandprecipitationwouldbeformedaroundtheplume
when vaporissupersaturated,withmaybesomebeneficial effects
in thedesertswhereSCPPsareintendedtobebuilt.
Furthermore,thelatentheatreleasedfromthecondensationof
supersaturatedvaporcanhelptheplumetokeeponrisingat
higher altitudes.Evenifitdependsonwindconditions [134], the
plume oftenreachesmorethan3kmupto4kmwhichwasthe
upper limitoftheZhousimulationmodel(Fig. 13). Thenumerical
Tingzhen Mingetal./RenewableandSustainableEnergyReviews31(2014)792–834 805