Solar geoengineering

Solar geoengineering, or solar radiation modification (SRM), is a type of climate engineering in which sunlight (solar radiation) would be reflected back to outer space to limit or offset human-caused climate change. There are multiple potential approaches, with stratospheric aerosol injection (SAI) being the most-studied method, followed by marine cloud brightening (MCB).[1] Other methods have been proposed, including a variety of space-based approaches, but they are generally considered less viable,[2] and are not taken seriously by the Intergovernmental Panel on Climate Change.[3] SRM methods could have a rapid cooling effect on atmospheric temperature, but if the intervention were to suddenly stop for any reason, the cooling would soon stop as well. It is estimated that the cooling impact from SAI would cease 1–3 years after the last aerosol injection, while the impact from marine cloud brightening would disappear in just 10 days. Contrastingly, once any carbon dioxide is added to the atmosphere and not removed, its warming impact does not decrease for a century, and some of it will persist for hundreds to thousands of years. As such, solar geoengineering is not a substitute for reducing greenhouse gas emissions but would act as a temporary measure to limit warming while emissions of greenhouse gases are reduced and carbon dioxide is removed.[3]

refer to caption and image description
Illustration of the different proposed methods of reflecting more sunlight to reduce Earth's temperature

If solar geoengineering were to cease while greenhouse gas levels remained high, it would lead to "large and extremely rapid" warming and similarly abrupt changes to the water cycle. Rapid termination would significantly increase the threats to biodiversity from climate change.[4] In spite of this risk, solar geoengineering is frequently discussed as a policy option because it is much faster and (in the short run) cheaper than any form of climate change mitigation. While cooling the atmosphere by 1 °C (1.8 °F) through stratospheric aerosol injection would cost at least $18 billion annually (at 2020 USD value),[5] and other approaches also cost tens of billions of dollars or more annually,[6] this would still be "orders of magnitude" cheaper than greenhouse gas mitigation,[7] and the unmitigated effects of climate change would cost far more than that.[2]

As of 2022, hundreds of studies have used climate models to simulate the impacts of SRM on the various aspects of the Earth's climate. In general, they show that it can combat many of the adverse effects of climate change, such as the increase in extreme weather, the decrease in soil moisture, slowdown of Atlantic meridional overturning circulation, Arctic sea ice decline and the melting of mountain glaciers. However, they concur that is impossible for SRM to fully reverse climate change and return the world to its preindustrial state, because the scale of any intervention required to completely offset the recent warming would substantially alter the weather patterns and the water cycle compared to the past, while ocean acidification would proceed until CO2 concentrations stop increasing. For the same reason, simply using SRM to maintain present-day temperature would still alter the climate to some extent.[3] Climate models often struggle to correctly estimate regional impacts of global dimming caused by historical sulfate air pollution,[8][9][10] and so there is only low confidence in the current projections of how solar geoengineering would affect regional climate and ecosystems.[3]

Governing solar geoengineering is challenging for multiple reasons, including that few countries would likely be capable of doing it alone.[11] For now, there is no formal international framework designed to regulate SRM, with aspects of the UN Convention on Biological Diversity or the Vienna Convention for the Protection of the Ozone Layer coming the closest out of the existing agreements. Thus, many questions regarding the acceptable deployment of SRM, or even its research and development, are currently unanswered.

Overview
Solar geoengineering can be deployed on different scales. This graph shows the baseline radiative forcing under three different Representative Concentration Pathway scenarios, and how it would be affected by the deployment of SAI, starting from 2034, to either halve the speed of warming by 2100, to halt the warming, or to reverse it entirely.[5]

Solar geoengineering (SG, or SRM) increases Earth's ability to deflect sunlight, e.g., by increasing the albedo of the atmosphere or the surface. While reducing the average temperature, it would not address ocean acidification.[12] Climate models project that SRN interventions would take effect rapidly, but would also quickly fade out if not sustained. This means that their direct effects are effectively reversible, but also risks a rapid rebound after a prolonged interruption, sometimes known as termination shock. The US National Academy of Sciences, Engineering, and Medicine stated in a 2021 report: "The available research indicates that SG could reduce surface temperatures and potentially ameliorate some risks posed by climate change (e.g., to avoid crossing critical climate “tipping points”; to reduce harmful impacts of weather extremes)."[13]

Solar geoengineering methods include:[2]

Regardless of the method used, there is a wide range of potential deployment scenarios for solar geoengineering, which differ both in the scale of warming they must offset, and their target endpoint. Historically, the majority of studies consider relatively extreme scenarios where global emissions are very high and are offset with similarly high levels of SRM. More recently, research began exploring alternatives like using SRM as an aid to avoid failing the Paris Agreement goals of 1.5 °C (2.7 °F) and 2 °C (3.6 °F). It has also been suggested that SRM is deployed to halve the current warming, as this may be less disruptive to societies and ecosystems than attempting to reach the preindustrial levels.[15] However, this approach may also increase flood and wildfire risk in Europe.[3] There have also been proposals to focus the use of SRM at the poles, in order to combat polar amplification of warming and the associated Arctic sea ice decline, permafrost thaw and ice sheet melt leading to increased sea level rise.[16] However, actual deployment of even the cheapest proposals is projected to cost tens of billions of US dollars annually, so the decision to deploy these interventions would not be taken lightly.[12][16]

Means of operation

Averaged over the year and location, the Earth's atmosphere receives 340 W/m2 of solar irradiance from the sun.[17] Due to elevated atmospheric greenhouse gas concentrations, the net difference between the amount of sunlight absorbed by the Earth and the amount of energy radiated back to space has risen from 1.7 W/m2 in 1980, to 3.1 W/m2 in 2019.[18] This imbalance - called radiative forcing - means that the Earth absorbs more energy than it lets off, causing global temperatures to rise.[19] The goal of solar geoengineering would be to reduce radiative forcing by increasing Earth's albedo (reflectivity). An increase in planetary albedo of 1% would reduce radiative forcing by 2.35 W/m2, eliminating most of global warming from anthropogenic greenhouse gas emissions, while a 2% albedo increase would negate the warming effect of doubling the atmospheric carbon dioxide concentration[2] However, because warming from greenhouse gases and cooling from solar geoengineering would operate differently across latitudes and seasons, a world where global warming is offset would still have a different climate from the world where this warming did not occur in the first place, mainly as the result of an altered hydrological cycle.[3]

Potential roles

Solar geoengineering may end up being deployed as an emergency solution to climate change, but in the long run, it is intended to complement, not replace, greenhouse gas emissions reduction and carbon dioxide removal. For example, the Royal Society stated in its landmark 2009 report: "Geoengineering methods are not a substitute for climate change mitigation, and should only be considered as part of a wider package of options for addressing climate change.[2] The IPCC Sixth Assessment Report concurs: "There is high agreement in the literature that for addressing climate change risks SRM cannot be the main policy response to climate change and is, at best, a supplement to achieving sustained net zero or net negative CO2 emission levels globally".[3]

Potential complementary responses to climate change: greenhouse gas emissions abatement, carbon dioxide removal, solar geoengineering, and adaptation. Originally called the "napkin diagram" and drawn by John Shepherd.[20]

Solar geoengineering's speed of effect gives it two potential roles in managing risks from climate change. First, if mitigation and adaptation continue to be insufficient, and/or if climate change impacts are severe due to greater-than-expected climate sensitivity, tipping points, or vulnerability, then solar geoengineering could reduce these unexpectedly severe impacts. In this way, the knowledge to implement solar geoengineering as a backup plan would serve as a sort of risk diversification or insurance. Second, solar geoengineering could be implemented along with aggressive mitigation and adaptation in order "buy time" by slowing the rate of climate change and/or to eliminate the worst climate impacts until net negative emissions reduce atmospheric greenhouse gas concentrations. (See diagram.)

Solar geoengineering has been suggested as a means of stabilizing regional climates - such as limiting heatwaves[21] or Arctic sea ice decline and permafrost thaw,[16] but there's low confidence about the ability to control geographical boundaries of the effect.[3]

History

In 1965, during the administration of U.S. President Lyndon B. Johnson, President's Science Advisory Committee delivered "Restoring the Quality of Our Environment", a landmark report which warned of the harmful effects of carbon dioxide emissions from fossil fuel and mentioned "deliberately bringing about countervailing climatic changes," including "raising the albedo, or reflectivity, of the Earth."[22] As early as 1974, Russian climatologist Mikhail Budyko suggested that if global warming ever became a serious threat, it could be countered with airplane flights in the stratosphere, burning sulfur to make aerosols that would reflect sunlight away.[23] Along with carbon dioxide removal, solar geoengineering was discussed jointly as "geoengineering" in a 1992 climate change report from the US National Academies.[24] The topic was essentially taboo in the climate science and policy communities until Nobel Laureate Paul Crutzen published an influential scholarly paper in 2006.[25] Major reports by the Royal Society (2009)[2] and the US National Academies (2015, 2021)[12][13] followed.

As of 2018, total research funding worldwide remained modest, at less than 10 million US dollars annually.[26] Almost all research into solar geoengineering has to date consisted of computer modeling or laboratory tests,[27] and there are calls for more research funding as the science is poorly understood.[28][29] Major academic institutions, including Harvard University, have begun research into solar geoengineering,[30] with NOAA alone investing $22 million from 2019 to 2022, though few outdoor tests have been run to date.[31] The Degrees Initiative is a UK registered charity,[32] established to build capacity in developing countries to evaluate solar geoengineering.[33] The 2021 US National Academy of Sciences, Engineering, and Medicine report recommended an initial investment into solar geoengineering research of $100–$200 million over five years.[29] In May 2022, the Climate Overshoot Commission was launched to recommend a comprehensive strategy to reduce climate risk which includes sunlight reflection methods in its policy portfolio, and will issue a final report prior to the 2023 UN Climate Change Conference.[34]

Evidence of effectiveness and impacts
Modeling evidence of the effect of greenhouse gases and solar geoengineering on average annual temperature (left column) and precipitation (right column).[35] The first row (a) is moderately high continued greenhouse gas emissions (RCP4.5) at the end of the century. The second row (b) is the same emissions scenario and time, with solar geoengineering to reduce global warming to 1.5 degrees C. The third row (c) is the same emissions scenario but in the near future, when global warming would be 1.5 degrees C, with no solar geoengineering. The similarity between the second and third rows suggests that solar geoengineering could reduce climate change reasonably well.

Climate models consistently indicate that a moderate magnitude of solar geoengineering would bring important aspects of the climate - for example, average and extreme temperature, water availability, cyclone intensity - closer to their preindustrial values at a subregional resolution.[15] (See figure.)

The Intergovernmental Panel on Climate Change (IPCC) concluded in its Sixth Assessment Report:[36]: 69 

.... SRM could offset some of the effects of increasing GHGs on global and regional climate, including the carbon and water cycles. However, there would be substantial residual or overcompensating climate change at the regional scales and seasonal time scales, and large uncertainties associated with aerosol–cloud–radiation interactions persist. The cooling caused by SRM would increase the global land and ocean CO2 sinks, but this would not stop CO2 from increasing in the atmosphere or affect the resulting ocean acidification under continued anthropogenic emissions. It is likely that abrupt water cycle changes will occur if SRM techniques are implemented rapidly. A sudden and sustained termination of SRM in a high CO2 emissions scenario would cause rapid climate change. However, a gradual phase-out of SRM combined with emission reduction and CDR would avoid these termination effects.

The 2021 US National Academy of Sciences, Engineering, and Medicine report states: "The available research indicates that SG could reduce surface temperatures and potentially ameliorate some risks posed by climate change (e.g., to avoid crossing critical climate 'tipping points'; to reduce harmful impacts of weather extremes)."[13]

Solar geoengineering would imperfectly compensate for anthropogenic climate changes. Greenhouse gases warm throughout the globe and year, whereas solar geoengineering reflects light more effectively at low latitudes and in the hemispheric summer (due to the sunlight's angle of incidence) and only during daytime. Deployment regimes could compensate for this heterogeneity by changing and optimizing injection rates by latitude and season.[37][38]

In general, greenhouse gases warm the entire planet and are expected to change precipitation patterns heterogeneously, both spatially and temporally, with an overall increase in precipitation. Models indicate that solar geoengineering would compensate both of these changes but would do more effectively for temperature than for precipitation. Therefore, using solar geoengineering to fully return global mean temperature to a preindustrial level would overcorrect for precipitation changes. This has led to claims that it would dry the planet or even cause drought, but this would depend on the intensity (i.e. radiative forcing) of solar geoengineering. Furthermore, soil moisture is more important for plants than average annual precipitation. Because solar geoengineering would reduce evaporation, it more precisely compensates for changes to soil moisture than for average annual precipitation.[39] Likewise, the intensity of tropical monsoons is increased by climate change and decreased by solar geoengineering.[40] A net reduction in tropical monsoon intensity might manifest at moderate use of solar geoengineering, although to some degree the effect of this on humans and ecosystems would be mitigated by greater net precipitation outside of the monsoon system. This has led to claims that solar geoengineering "would disrupt the Asian and African summer monsoons," but the impact would depend on the particular implementation regime.

People are concerned about climate change largely because of its impacts on people and ecosystems. In the case of the former, agriculture is particularly important. A net increase in agricultural productivity from elevated atmospheric carbon dioxide concentrations and solar geoengineering has also been predicted by some studies due to the combination of more diffuse light and carbon dioxide's fertilization effect.[41] Other studies suggest that solar geoengineering would have little net effect on agriculture.[42] Understanding of solar geoengineering's effects on ecosystems remains at an early stage. Its reduction of climate change would generally help maintain ecosystems, although the resulting more diffuse incoming sunlight would favor undergrowth relative to canopy growth.

Advantages

The target of net zero greenhouse gas emissions can be achieved through a combination of emission cuts and carbon dioxide removal, after which global warming stops,[43] but the temperature will only go back down if we remove more carbon dioxide than we emit. Solar geoengineering on the other hand could cool the planet within months after deployment,[12] thus can act to reduce climate risk while we cut emissions and scale up carbon dioxide removal. Stratospheric aerosol injection is expected to have low direct financial costs of implementation,[44] relative to the expected costs of both unabated climate change and aggressive mitigation. Finally, the direct climatic effects of solar geoengineering are reversible within short timescales.[12]

Limitations and risks

As well as the imperfect cancellation of the climatic effect of greenhouse gases, described above, there are other significant problems with solar geoengineering.

Incomplete solution to elevated carbon dioxide concentrations
Change in sea surface pH caused by anthropogenic CO2 between the 1700s and the 1990s. This ocean acidification will still be a major problem unless atmospheric CO2 is reduced.

Solar geoengineering does not remove greenhouse gases from the atmosphere and thus does not reduce other effects from these gases, such as ocean acidification.[45] While not an argument against solar geoengineering per se, this is an argument against reliance on it to the exclusion of emissions reduction.

Uncertainty

Most of the information on solar geoengineering comes from climate models and volcanic eruptions, which are both imperfect analogues of stratospheric aerosol injection. The climate models used in impact assessments are the same that scientists use to predict the impacts of anthropogenic climate change. Some uncertainties in these climate models (such as aerosol microphysics, stratospheric dynamics, and sub-grid scale mixing) are particularly relevant to solar geoengineering and are a target for future research.[46] Volcanoes are an imperfect analogue as they release the material in the stratosphere in a single pulse, as opposed to sustained injection.[47] Modelling is uncertain as little practical research has been done.[11]

Maintenance and termination shock

Solar geoengineering effects would be temporary, and thus long-term climate restoration would rely on long-term deployment until sufficient carbon dioxide is removed.[48][49] If solar geoengineering masked significant warming, stopped abruptly, and was not resumed within a year or so, the climate would rapidly warm.[50] Global temperatures would rapidly rise towards levels which would have existed without the use of solar geoengineering. The rapid rise in temperature might lead to more severe consequences than a gradual rise of the same magnitude. However, some scholars have argued that this termination shock appears reasonably easy to prevent because it would be in states' interest to resume any terminated deployment regime; and because infrastructure and knowledge could be made redundant and resilient, allowing states to act on this interest and gradually phase out unwanted solar geoengineering.[51][52]

Some claim that solar geoengineering "would basically be impossible to stop."[53][54] This is true only of a long-term deployment strategy. A short-term, temporary strategy would limit implementation to decades.[55]

Disagreement and control

Although climate models of solar geoengineering rely on some optimal or consistent implementation, leaders of countries and other actors may disagree as to whether, how, and to what degree solar geoengineering be used. This could result in suboptimal deployments and exacerbate international tensions.[56]

Some observers claim that solar geoengineering is likely to be militarized or weaponized. However, weaponization is disputed because solar geoengineering would be imprecise.[57] Regardless, the U.N. Convention on the Prohibition of Military or Any Other Hostile Use of Environmental Modification Techniques, which prohibits weaponizing solar geoengineering, came into force in 1978.[6]

Unwanted or premature use

There is a risk that countries may start using solar geoengineering without proper precaution or research. Solar geoengineering, at least by stratospheric aerosol injection, appears to have low direct implementation costs relative to its potential impact. This creates a different problem structure.[58][59] Whereas the provision of emissions reduction and carbon dioxide removal present collective action problems (because ensuring a lower atmospheric carbon dioxide concentration is a public good), a single country or a handful of countries could implement solar geoengineering. Many countries have the financial and technical resources to undertake solar geoengineering.[11]

In 2000s, some have suggested that solar geoengineering could be within reach of a lone "Greenfinger," a wealthy individual who takes it upon him or herself to be the "self-appointed protector of the planet".[60][61] Others disagree and argue that states will insist on maintaining control of solar geoengineering.[62] Subsequent research had dimmed this notion, as the annual costs of around $18 billion per 1 °C (1.8 °F) of cooling are likely to be prohibitive for even the wealthiest individuals.[5]

Distribution of effects

Both climate change and solar geoengineering would affect various groups of people differently. Some observers describe solar geoengineering as necessarily creating "winners and losers." However, models indicate that solar geoengineering at a moderate intensity would return important climatic values of almost all regions of the planet closer to preindustrial conditions. That is, if all people prefer preindustrial conditions, such a moderate use could be a Pareto improvement.

Developing countries are particularly important, as they are more vulnerable to climate change. All else equal, they therefore have the most to gain from a judicious use of solar geoengineering. Observers sometimes claim that solar geoengineering poses greater risks to developing countries. There is no evidence that the unwanted environmental impacts of solar geoengineering would be significantly greater in developing countries, although potential disruptions to tropical monsoons are a concern. But in one sense, this claim of greater risk is true for the same reason that they are more vulnerable to greenhouse gas-induced climate change: developing countries have weaker infrastructure and institutions, and their economies rely to a greater degree on agriculture. They are thus more vulnerable to all climate changes, whether from greenhouse gases or solar geoengineering.

Lessened mitigation

The existence of solar geoengineering may reduce the political and social impetus for mitigation.[63] This has generally been called a potential "moral hazard," although risk compensation may be a more accurate term. This concern causes many environmental groups and campaigners to be reluctant to advocate or discuss solar geoengineering.[64] However, several public opinion surveys and focus groups have found evidence of either assertion of a desire to increase emission cuts in the face of solar geoengineering, or of no effect.[2][65][66][67][68][69][70] Likewise, some modelling work suggests that the threat of solar geoengineering may in fact increase the likelihood of emissions reduction.[71][72][73][74]

Effect on sky and clouds

Managing solar radiation using aerosols or cloud cover would involve changing the ratio between direct and indirect solar radiation. This would affect plant life[75] and solar energy.[76] Visible light, useful for photosynthesis, is reduced proportionally more than is the infrared portion of the solar spectrum due to the mechanism of Mie scattering.[77] As a result, deployment of atmospheric solar geoengineering would reduce by at least 2-5% the growth rates of phytoplankton, trees, and crops [78] between now and the end of the century.[79] Uniformly reduced net shortwave radiation would hurt solar photovoltaics by the same >2-5% because of the bandgap of silicon photovoltaics.[80]

Proposed forms

Stratospheric aerosol injection
Stratospheric Particle Injection for Climate Engineering
Main article: Stratospheric aerosol injection

Injecting reflective aerosols into the stratosphere is the proposed solar geoengineering method that has received the most sustained attention. The Intergovernmental Panel on Climate Change concluded that Stratospheric aerosol injection "is the most-researched SRM method, with high agreement that it could limit warming to below 1.5°C."[81] This technique would mimic a cooling phenomenon that occurs naturally by the eruption of volcanoes.[82] Sulfates are the most commonly proposed aerosol, since there is a natural analogue with (and evidence from) volcanic eruptions. Alternative materials such as using photophoretic particles, titanium dioxide, and diamond have been proposed.[83][84][85][86][87] Delivery by custom aircraft appears most feasible, with artillery and balloons sometimes discussed.[88][89][90] The annual cost of delivering a sufficient amount of sulfur to counteract expected greenhouse warming is estimated at $5 to 10 billion US dollars.[91] This technique could give much more than 3.7 W/m2 of globally averaged negative forcing,[92] which is sufficient to entirely offset the warming caused by a doubling of carbon dioxide.

Marine cloud brightening
Main article: Marine cloud brightening

Various cloud reflectivity methods have been suggested, such as that proposed by John Latham and Stephen Salter, which works by spraying seawater in the atmosphere to increase the reflectivity of clouds.[93] The extra condensation nuclei created by the spray would change the size distribution of the drops in existing clouds to make them whiter.[94] The sprayers would use fleets of unmanned rotor ships known as Flettner vessels to spray mist created from seawater into the air to thicken clouds and thus reflect more radiation from the Earth.[95] The whitening effect is created by using very small cloud condensation nuclei, which whiten the clouds due to the Twomey effect.

This technique can give more than 3.7 W/m2 of globally averaged negative forcing,[92] which is sufficient to reverse the warming effect of a doubling of atmospheric carbon dioxide concentration.

Cirrus cloud thinning
Main article: Cirrus cloud thinning

Natural cirrus clouds are believed to have a net warming effect. These could be dispersed by the injection of various materials. This method is strictly not solar geoengineering, as it increases outgoing longwave radiation instead of decreasing incoming shortwave radiation. However, because it shares some of the physical and especially governance characteristics as the other solar geoengineering methods, it is often included.[96]

Ocean sulfur cycle enhancement
Main article: Ocean fertilization

Enhancing the natural marine sulfur cycle by fertilizing a small portion with iron—typically considered to be a greenhouse gas remediation method—may also increase the reflection of sunlight.[97][98] Such fertilization, especially in the Southern Ocean, would enhance dimethyl sulfide production and consequently cloud reflectivity. This could potentially be used as regional solar geoengineering, to slow Antarctic ice from melting. Such techniques also tend to sequester carbon, but the enhancement of cloud albedo also appears to be a likely effect.

Cool roof
The albedo of several types of roofs (lower = hotter)
Main article: Reflective surfaces (climate engineering)

Painting roof materials in white or pale colors to reflect solar radiation, known as 'cool roof' technology, is encouraged by legislation in some areas (notably California).[99] This technique is limited in its ultimate effectiveness by the constrained surface area available for treatment. This technique can give between 0.01 and 0.19 W/m2 of globally averaged negative forcing, depending on whether cities or all settlements are so treated.[92] This is small relative to the 3.7 W/m2 of positive forcing from a doubling of atmospheric carbon dioxide. Moreover, while in small cases it can be achieved at little or no cost by simply selecting different materials, it can be costly if implemented on a larger scale. A 2009 Royal Society report states that, "the overall cost of a 'white roof method' covering an area of 1% of the land surface (about 1012 m2) would be about $300 billion/yr, making this one of the least effective and most expensive methods considered."[2] However, it can reduce the need for air conditioning, which emits carbon dioxide and contributes to global warming.

Radiative cooling

Some papers have proposed the deployment of specific thermal emitters (whether via advanced paint, or printed rolls of material) which would simultaneously reflect sunlight and also emit energy at longwave infrared (LWIR) lengths of 8–20 μm, which is too short to be trapped by the greenhouse effect and would radiate into outer space. It has been suggested that to stabilize Earth's energy budget and thus cease warming, 1–2% of the Earth's surface (area equivalent to over half of Sahara) would need to be covered with these emitters, at the deployment cost of $1.25 to $2.5 trillion. While low next to the estimated $20 trillion saved by limiting the warming to 1.5 °C (2.7 °F) rather than 2 °C (3.6 °F), it does not include any maintenance costs.[100][101]

Ocean and ice changes

Oceanic foams have also been suggested, using microscopic bubbles suspended in the upper layers of the photic zone. A less costly proposal is to simply lengthen and brighten existing ship wakes.[102]

Arctic sea ice formation could be increased by pumping deep cooler water to the surface.[103] Sea ice (and terrestrial) ice can be thickened by increasing albedo with silica spheres.[104] Glaciers flowing into the sea may be stabilized by blocking the flow of warm water to the glacier.[105] Salt water could be pumped out of the ocean and snowed onto the West Antarctic ice sheet.[106][107]

Vegetation

Reforestation in tropical areas has a cooling effect. Changes to grassland have been proposed to increase albedo.[108] This technique can give 0.64 W/m2 of globally averaged negative forcing,[92] which is insufficient to offset the 3.7 W/m2 of positive forcing from a doubling of carbon dioxide, but could make a minor contribution. Selecting or genetically modifying commercial crops with high albedo has been suggested.[109] This has the advantage of being relatively simple to implement, with farmers simply switching from one variety to another. Temperate areas may experience a 1 °C cooling as a result of this technique.[110] This technique is an example of bio-geoengineering. This technique can give 0.44 W/m2 of globally averaged negative forcing,[92] which is insufficient to offset the 3.7 W/m2 of positive forcing from a doubling of carbon dioxide, but could make a minor contribution.

Space-based
The basic function of a space lens to mitigate global warming. The image is simplified, as a 1000 kilometre diameter lens is considered sufficient by most proposals, and would be much smaller than shown. Additionally, a zone plate would only be a few nanometers thick.
Main articles: Space sunshade and Space mirror (climate engineering)

There has been a range of proposals to reflect or deflect solar radiation from space, before it even reaches the atmosphere, commonly described as a space sunshade.[84] The most straightforward is to have mirrors orbiting around the Earth - an idea first suggested even before the wider awareness of climate change, with rocketry pioneer Hermann Oberth considering it a way to facilitate terraforming projects in 1923.[111] and this was followed by other books in 1929, 1957 and 1978.[112][113][114] By 1992, the U.S. National Academy of Sciences described a plan to suspend 55,000 mirrors with an individual area of 100 square meters in a Low Earth orbit.[2] Another contemporary plan was to use space dust to replicate Rings of Saturn around the equator, although a large number of satellites would have been necessary to prevent it from dissipating. A 2006 variation on this idea suggested relying entirely on a ring of satellites electromagnetically tethered in the same location. In all cases, sunlight exerts pressure which can displace these reflectors from orbit over time, unless stabilized by enough mass. Yet, higher mass immediately drives up launch costs.[2]

In an attempt to deal with this problem, other researchers have proposed Inner lagrangian point between the Earth and the Sun as an alternative to near-Earth orbits, even though this tends to increase manufacturing or delivery costs instead. In 1989, a paper suggested founding a lunar colony, which would produce and deploy diffraction grating made out of a hundred million tonnes of glass.[115] In 1997, a single, very large mesh of aluminium wires "about one millionth of a millimetre thick" was also proposed.[116] Two other proposals from the early 2000s advocated the use of thin metallic disks 50–60 cm in diameter, which would either be launched from the Earth at a rate of once per minute over several decades, or be manufactured from asteroids directly in orbit.[2] When summarizing these options in 2009, the Royal Society concluded that their deployment times are measured in decades and costs in the trillions of USD, meaning that they are "not realistic potential contributors to short-term, temporary measures for avoiding dangerous climate change", and may only be competitive with the other geoengineering approaches when viewed from a genuinely long (a century or more) perspective, as the long lifetime of L1-based approaches could make them cheaper than the need to continually renew atmospheric-based measures over that timeframe.[2]

Relatively few researchers have revisited the subject since that Royal Society review, as it became accepted that space-based approaches would cost about 1000 times more than their terrestrial alternatives.[117] In 2022, the IPCC Sixth Assessment Report had discussed SAI, MCB, CCT and even attempts to alter albedo on the ground or in the ocean, yet completely ignored space-based approaches.[3] There are still some proponents, who argue that unlike stratospheric aerosol injection, space-based approaches are advantageous because they do not interfere directly with the biosphere and ecosystems.[118] After the IPCC report was published, three astronomers have revisited the space dust concept, instead advocating for a lunar colony which would continuously mine the Moon in order to eject lunar dust into space on a trajectory where it would interfere with sunlight streaming towards the Earth. Ejections would have to be near-continuous, as since the dust would scatter in a matter of days, and about 10 million tons would have to be dug out and launched annually.[119] The authors admit that they lack a background in either climate or rocket science, and the proposal may not be logistically feasible.[120]

In 2021, researchers in Sweden considered building solar sails in the near-Earth orbit, which would then arrive to L1 point over 600 days one by one. Once they all form an array in situ, the combined 1.5 billion sails would have total area of 3.75 million square kilometers, while their combined mass is esimated in a range between 83 million tons (present-day technology) and 34 million tons (optimal advancements). This proposal would cost between five and ten trillion dollars, but only once launch cost has been reduced to US$50/kg, which represents a massive reduction from the present-day costs of $4400-$2700/kg[121] for the most widely used launch vehicles.[122] In July 2022, a pair of researchers from MIT Senseable City Lab, Olivia Borgue and Andreas M. Hein, have instead proposed integrating nanotubes made out of silicon dioxide into ultra-thin polymeric films (described as "space bubbles" in the media [118]), whose semi-transparent nature would allow them to resist the pressure of solar wind at L1 point better than any alternative with the same weight. The use of these "bubbles" would limit the mass of a distributed sunshade roughly the size of Brazil to about 100,000 tons, much lower than the earlier proposals. However, it would still require between 399 and 899 yearly launches of a vehicle such as SpaceX Starship for a period of around 10 years, even though the production of the bubbles themselves would have to be done in space. The flights would not begin until research into production and maintenance of these bubbles is completed, which the authors estimate would require a minimum of 10–15 years. After that, the space shield may be large enough by 2050 to prevent crossing of the 2 °C (3.6 °F) threshold.[117][118][123]

Governance

Solar geoengineering poses several governance challenges because of its high leverage, low apparent direct costs, and technical feasibility as well as issues of power and jurisdiction.[124] Solar geoengineering does require widespread engagement with community and stakeholders, not to incur in a multitude of challenges and barriers to the research, testing and deployment of novel technology.[125] Because international law is generally consensual, this creates a challenge of participation that is the inverse of that of mitigation to reduce climate change, where widespread participation is required. Discussions are broadly on who will have control over the deployment of solar geoengineering and under what governance regime the deployment can be monitored and supervised. A governance framework for solar geoengineering must be sustainable enough to contain a multilateral commitment over a long period of time and yet be flexible as information is acquired, the techniques evolve, and interests change through time.

Legal and regulatory systems may face a significant challenge in effectively regulating solar geoengineering in a manner that allows for an acceptable result for society. Some researchers have suggested that building a global agreement on solar geoengineering deployment will be very difficult, and instead power blocs are likely to emerge.[126] There are, however, significant incentives for states to cooperate in choosing a specific solar geoengineering policy, which make unilateral deployment a rather unlikely event.[127]

In 2021, the National Academies of Sciences, Engineering, and Medicine released their consensus study report Recommendations for Solar Geoengineering Research and Research Governance, concluding:"[13]

[A] strategic investment in research is needed to enhance policymakers' understanding of climate response options. The United States should develop a transdisciplinary research program, in collaboration with other nations, to advance understanding of solar geoengineering's technical feasibility and effectiveness, possible impacts on society and the environment, and social dimensions such as public perceptions, political and economic dynamics, and ethical and equity considerations. The program should operate under robust research governance that includes such elements as a research code of conduct, a public registry for research, permitting systems for outdoor experiments, guidance on intellectual property, and inclusive public and stakeholder engagement processes.[13]

Public attitudes and politics

There have been a handful of studies into attitudes to and opinions of solar geoengineering. These generally find low levels of awareness, uneasiness with the implementation of solar geoengineering, cautious support of research, and a preference for greenhouse gas emissions reduction.[128][129] As is often the case with public opinions regarding emerging issues, the responses are highly sensitive to the questions' particular wording and context. Although most public opinion studies have polled residents of developed countries, those that have examined residents of developing countries—which tend to be more vulnerable to climate change impacts—find slightly greater levels of support there.[130][131][132]

There are many controversies surrounding this topic and hence, solar geoengineering has become a very political issue. No countries have an explicit government position on solar geoengineering.

Support for solar geoengineering research comes almost entirely from those who are concerned about climate change. Some observers claim that political conservatives, opponents of action to reduce climate change, and fossil fuel firms are major advocates of solar geoengineering research.[133] However, only a handful of conservatives and opponents of climate action have expressed support, and there is no evidence that fossil fuel firms are involved in solar geoengineering research.[134] Instead, these claims often conflate solar geoengineering and carbon dioxide removal—where fossil fuel firms are involved—under the broader term "geoengineering."

As noted, the interests and roles of developing countries are particularly important.[135] The Degrees Initiative works toward "changing the global environment in which SRM is evaluated, ensuring informed and confident representation from developing countries."[136] Among other activities, it provides grants to researchers in the Global South.

In 2021, researchers at Harvard were forced to put plans for a solar geoengineering test on hold after Indigenous Sámi people objected to the test taking place in their homeland.[137][138] Although the test would not have involved any immediate atmospheric experiments, members of the Saami Council spoke out against the lack of consultation and solar geoengineering more broadly. Speaking at a panel organized by the Center for International Environmental Law and other groups, Saami Council Vice President Åsa Larsson Blind said, "This goes against our worldview that we as humans should live and adapt to nature."

See also

References
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