Solar radiation modification



Solar radiation modification (SRM), or solar geoengineering, refers to a range of approaches to limit global warming by increasing the amount of sunlight (solar radiation) that the atmosphere reflects back to space or by reducing the trapping of outgoing thermal radiation. Among the multiple potential approaches, stratospheric aerosol injection is the most-studied, followed by marine cloud brightening. SRM could be a temporary measure to limit climate-change impacts while greenhouse gas emissions are reduced and carbon dioxide is removed, but would not be a substitute for reducing emissions.

Multiple authoritative international scientific assessments, based on evidence from climate models and natural analogues, have generally shown that some forms of SRM could reduce global warming and many adverse effects of climate change. Specifically, controlled stratospheric aerosol injection appears able to greatly moderate most environmental impacts—especially warming—and consequently most ecological, economic, and other impacts of climate change across most regions. However, because warming from greenhouse gases and cooling from SRM would operate differently across latitudes and seasons, a world where global warming would be offset by SRM would have a different climate from one where this warming did not occur in the first place. Furthermore, confidence in the current projections of how SRM would affect regional climate and ecosystems is low.

SRM would pose environmental risks. In addition to its imperfect reduction of climate-change impacts, stratospheric aerosol injection could, for example, slow the recovery of stratospheric ozone. If a significant SRM intervention were to suddenly stop and not be resumed, the cooling would end relatively rapidly, posing serious environmental risks. Some environmental risks remain unknown.

Governing SRM is challenging for multiple reasons, including that several countries would likely be capable of doing it alone. For now, there is no formal international framework designed to regulate SRM, although aspects of existing international law would be applicable. The most common concern about SRM is that its research and evaluation might undermine reductions of greenhouse gas emissions. Issues of governance and effectiveness are intertwined, as poorly governed use of SRM might lead to its highly suboptimal implementation. Thus, many questions regarding the acceptable deployment of SRM, or even its research and development, are currently unanswered. There are many controversies surrounding this topic and hence, SRM has become a very political issue.

Overview
Averaged over the year and location, the Earth's atmosphere receives 340 W/m2 of solar irradiance from the sun. 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. This imbalance means that the Earth absorbs more energy than it emits, causing global temperatures to rise.

SRM would increase Earth's ability to deflect sunlight, such as by increasing the albedo of the atmosphere or the surface. An increase in planetary albedo of 1% would reduce radiative forcing by 2.35 W/m2, eliminating most of global warming from current anthropogenically elevated greenhouse gas concentrations, while a 2% albedo increase would negate the warming effect of doubling the atmospheric carbon dioxide concentration.

SRM methods include:


 * Stratospheric aerosol injection (SAI), in which small particles would be injected into the upper atmosphere to cool the planet with both global dimming and increased albedo
 * Marine cloud brightening (MCB), which would spray fine sea water to whiten clouds and thus increase cloud reflectivity
 * Albedo enhancement, in which cool roofs and reflectors are would increase the albedo or reflectivity of the Earth's surface to deflect solar radiation back into space
 * Cirrus cloud thinning (CCT), which is strictly not SRM but shares many of characteristics as the other methods.

SRM's climatic effects would be rapid and reversible, which would bring the obvious advantage of speed but the serious disadvantage of sudden warming if it were to be stopped suddenly and not resumed.

Potential roles
The context of research and evaluation of SRM is continued high emissions of greenhouse gases. The 2023 Emissions Gap Report from the UN Environment Programme estimated that even the most optimistic assumptions regarding countries' current conditional emissions policies and pledges has only a 14% chance of limiting global warming to 1.5 °C.

Regardless of the method used, there is a wide range of potential deployment scenarios for SRM, which differ both in the scale of warming they would offset and their target endpoint.

SRM is generally intended to complement, not replace, greenhouse gas emissions reduction and carbon dioxide removal. For example, 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 emission levels globally". However, SRM's actual role may differ from this, such as being used as an emergency response to sudden climate change impacts.

Initially, the majority of studies considered relatively extreme scenarios where modeled global emissions were very high and are offset with similarly high levels of SRM. In later years, research explored using SRM to partially offset global warming and aid to avoid failing the Paris Agreement goals of 1.5 C-change and 2 C-change or to halve warming.

SRM's speed of effect gives it two potential roles in managing risks from climate change. First, if mitigation (that is, emissions reduction and carbon dioxide removal) 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 SRM could reduce these unexpectedly severe impacts. In this way, the knowledge to implement SRM as a backup plan would serve as a sort of risk diversification or insurance. Second, SRM 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.)

SRM has been suggested as a means of stabilizing regional climates. There have also been proposals to focus SRM at the poles, in order to combat sea level rise or regional MCB in order to protect coral reefs from bleaching. However, there is low confidence about the ability to control geographical boundaries of the effect.

Potential 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, but the temperature will only go back down if we remove more carbon dioxide than we emit. SRM on the other hand could cool the planet within months after deployment, 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, relative to the expected costs of both unabated climate change and aggressive mitigation. Finally, the direct climatic effects of SRM are reversible within short timescales.

Benefits
Climate models consistently indicate that a moderate magnitude of SRM 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. (See figure.)

The Intergovernmental Panel on Climate Change (IPCC) concluded in its Sixth Assessment Report: ".... 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 sinks, but this would not stop 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  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."Other authoritative international scientific assessments concur. The most recent Scientific Assessment of Ozone Depletion report from the World Meteorological Organization concluded "Stratospheric Aerosol Injection (SAI) has the potential to limit the rise in global surface temperatures by increasing the concentrations of particles in the stratosphere... . However, SAI comes with significant risks and can cause unintended consequences." A 2023 independent expert review from the UN Environment Programme concluded "In current climate model simulations, well-designed SRM deployments offset some effects of greenhouse gases (GHG) on global and regional climate change by reflecting more sunlight into space. SRM is the only option that could cool the planet within years... An operational SRM deployment would introduce new risks to people and ecosystems." A 2021 US National Academies of Sciences, Engineering, and Medicine report on SRM stated: "The available research indicates that [SRM] 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)."

SRM would imperfectly compensate for anthropogenic climate changes. Greenhouse gases warm throughout the globe and year, whereas SRM 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.

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 SRM would compensate both of these changes but would do more effectively for temperature than for precipitation. Therefore, using SRM 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 SRM. Furthermore, soil moisture is more important for plants than average annual precipitation. Because SRM would reduce evaporation, it more precisely compensates for changes to soil moisture than for average annual precipitation. Likewise, the intensity of tropical monsoons is increased by climate change and decreased by SRM. A net reduction in tropical monsoon intensity might manifest at moderate use of SRM, 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 SRM "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 SRM has also been predicted by some studies due to the combination of more diffuse light and carbon dioxide's fertilization effect. Other studies suggest that SRM would have little net effect on agriculture. Understanding of SRM'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.

Stratospheric aerosol injection


Injecting reflective aerosols into the stratosphere is the proposed SRM 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." This technique would mimic a cooling phenomenon that occurs naturally by the eruption of volcanoes. 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. Delivery by custom aircraft appears most feasible, with artillery and balloons sometimes discussed. The annual cost of delivering a sufficient amount of sulfur to counteract expected greenhouse warming is estimated at $5–10 billion US dollars. This technique could give much more than 3.7 W/m2 of globally averaged negative forcing, which is sufficient to entirely offset the warming caused by a doubling of carbon dioxide.

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. The extra condensation nuclei created by the spray would change the size distribution of the drops in existing clouds to make them whiter. 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. 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, which is sufficient to reverse the warming effect of a doubling of atmospheric carbon dioxide concentration.

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 SRM, 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 SRM methods, it is often included.

Ocean sulfur cycle enhancement
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. Such fertilization, especially in the Southern Ocean, would enhance dimethyl sulfide production and consequently cloud reflectivity. This could potentially be used as regional SRM, 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


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). 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. 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." 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 $1.25–2.5 trillion. While low next to the estimated $20 trillion saved by limiting the warming to 1.5 C-change rather than 2 C-change, it does not include any maintenance costs.

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.

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

Vegetation
Reforestation in tropical areas has a cooling effect. Changes to grassland have been proposed to increase albedo. This technique can give 0.64 W/m2 of globally averaged negative forcing, 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. 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. This technique is an example of bio-geoengineering. This technique can give 0.44 W/m2 of globally averaged negative forcing, 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


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. 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. and this was followed by other books in 1929, 1957 and 1978. 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. 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.

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. In 1997, a single, very large mesh of aluminium wires "about one millionth of a millimetre thick" was also proposed. 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. 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.

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. 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. 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. 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. The authors admit that they lack a background in either climate or rocket science, and the proposal may not be logistically feasible.

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 estimated 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 for the most widely used launch vehicles. 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 ), 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-change threshold.

Limitations and risks
As well as imperfect and geographically uneven cancellation of the climatic effect of greenhouse gases, described above, SRM has other significant limitations and risks.

Lessened mitigation
The existence of SRM may reduce the political and social impetus for mitigation. This has often been called a potential "moral hazard", although such language is not precise. Some modelling work suggests that the threat of SRM may in fact increase the likelihood of emissions reduction.

Maintenance and termination shock
Models project that SRM interventions would take effect rapidly, but would also quickly fade out if not sustained. If SRM masked significant warming, stopped abruptly, and was not resumed within a year or so, the climate would rapidly warm towards levels which would have existed without the use of SRM, sometimes known as termination shock. 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 appears preventable because it would be in states' interest to resume any terminated deployment regime, and because infrastructure and knowledge could be made redundant and resilient.

Uncertainty
Much uncertainty remains about SRM's likely effects. Most of the evidence regarding SRM's expected effects comes from climate models and volcanic eruptions. Some uncertainties in climate models (such as aerosol microphysics, stratospheric dynamics, and sub-grid scale mixing) are particularly relevant to SRM and are a target for future research. Volcanoes are an imperfect analogue as they release the material in the stratosphere in a single pulse, as opposed to sustained injection.

Disagreement and control
Although climate models of SRM rely on some optimal or consistent implementation, leaders of countries and other actors may disagree as to whether, how, and to what degree SRM be used. This could result in suboptimal deployments and exacerbate international tensions. Likewise, blame for perceived local negative impacts from SRM could be a source of international tensions.

Unwanted or premature use
There is a risk that countries may start using SRM without proper precaution or research. SRM, at least by stratospheric aerosol injection, appears to have low direct implementation costs relative to its potential impact, and many countries have the financial and technical resources to undertake SRM. Some have suggested that SRM 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". Others argue that states will insist on maintaining control of SRM.

Slowing stratospheric ozone recovery
Stratospheric aerosol injection, the most studied SRM technique, using sulphates appears likely to catalyze the destruction of the protective stratospheric ozone layer.

Failure to reduce ocean acidification
SRM does not directly influence atmospheric carbon dioxide concentration and thus does not reduce ocean acidification. While not a risk of SRM per se, this points to the limitations of relying on it to the exclusion of emissions reduction.

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 and solar energy. 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. As a result, deployment of atmospheric SRM would reduce by at least 2–5% the growth rates of phytoplankton, trees, and crops between now and the end of the century. Uniformly reduced net shortwave radiation would hurt solar photovoltaics by the same >2–5% because of the bandgap of silicon photovoltaics.

Global governance issues
The governance of SRM contains many relevant aspects. The potential use of SRM poses several challenges because of its high leverage, low apparent direct costs, and technical feasibility as well as issues of power and jurisdiction. Because international law is generally consensual, this creates a challenge of widespread participation being required. Key issues include who will have control over the deployment of SRM and under what governance regime the deployment can be monitored and supervised. A governance framework for SRM 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.

Some researchers have suggested that building a global agreement on SRM deployment will be very difficult, and instead power blocs are likely to emerge. There are, however, significant incentives for states to cooperate in choosing a specific SRM policy, which make unilateral deployment a rather unlikely event.

Other relevant aspects of the governance of SRM include supporting research, ensuring that it is conducted responsibly, regulating the roles of the private sector and (if any) the military, public engagement, setting and coordinating research priorities, undertaking trusted scientific assessment, building trust, and compensating for possible harms.

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

"[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."

Advocacy for and against SRM research
There is no meaningful public advocacy for the use of SRM. In the private sector several startups have secured funding for potential SRM deployment, and in the case of Make Sunsets, have begun implementing their solution. Inspired by the novel Termination Shock, Make Sunsets launched balloons containing helium and sulfur dioxide, a form of SAI expected to reflect solar energy back into space. Based in California, they conducted early work in Mexico, though this resulted in the Mexican government soon after mentioning Make Sunset while announcing a ban on solar geoengineering. Make Sunsets' undertaking has been criticised even by some who advocate for more research into SRM. The most salient political issues thus regard research.

Few countries have an explicit governmental position on SRM. Most of those that do, such as the United Kingdom and Germany, support SRM research. Other countries, such as the U.S., Germany, China, Finland, Norway, and Japan, as well as the European Union, have funded SRM research. In contrast, Mexico announced that it will prohibit "experimental practices with solar geoengineering", although it remains unclear what this policy will include and whether the policy has actually been implemented. In 2024, Professor David Keith stated that in the last year or so, there has been far more engagement with SRM from senior political leaders than was previously the case. Other countries have expressed a range of views at intergovernmental forums such as the UN Environment Assembly.

The leading argument supportive of SRM research is that the risks of likely anthropogenic climate change are great and imminent enough to warrant research and evaluation of a wide range of responses, even one with limitations and risks of its own. Leading this effort have been some climate scientists (such as James Hansen), some of whom have endorsed one or both public letters that support further SRM research. Scientific organizations that have called for further research include the World Climate Research Programme, the Royal Society, the US National Academies, the American Geophysical Union, the American Meteorological Society, the U.S. Global Change Research Program, the Institution of Mechanical Engineers (UK), Australia's Office of the Chief Scientist, and the Netherlands' scientific assessment institute. Reports from the UN Environment Programme, the UN Educational, Scientific and Cultural Organization, and the Council on Foreign Relations have likewise called for further SRM research, as have a handful of relatively moderate American environmental nongovernmental organizations (Environmental Defense Fund, Union of Concerned Scientists, and the Natural Resources Defense Council).

A few nongovernmental organizations actively support SRM research and governance dialogues. The Degrees Initiative works toward "changing the global environment in which SRM is evaluated, ensuring informed and confident representation from developing countries." Among other activities, it provides grants to scientists in the Global South. SilverLining is an American organization that advances SRM research as part of "climate interventions to reduce near-term climate risks and impacts." The Alliance for Just Deliberation on Solar Geoengineering advances "just and inclusive deliberation" regarding SRM. The Carnegie Climate Governance Initiative catalyzed governance of SRM and carbon dioxide removal, although it ended operations in 2023.

Some critics claim that political conservatives, opponents of action to reduce greenhouse gas emissions, and fossil fuel firms are major advocates of SRM research. 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 SRM research. In fact, most conservative commentary on SRM has dismissed it as a radical but unnecessary response to the minor problem of climate change. Instead, claims of fossil-fuel industry support typically conflate SRM and carbon dioxide removal—where fossil fuel firms are involved—under the broader term "geoengineering".

Opposition to SRM research has largely come from opponents of emerging technologies, green environmental groups, and some academics, mostly from the social science and humanities but counting a few climate scientists. The most common concern is that SRM could lessen mitigation. Opponents of SRM research often emphasize that dramatic reductions of greenhouse gas emissions would also bring co-benefits including socio-economic transformations to sustainability and redistributive equity and that consideration of SRM could prevent these outcomes.

The radical anti-technology organization the ETC Group has been the pioneer in opposing SRM research, and was later joined by the Heinrich Böll Foundation (affiliated with the German Green Party) and the Center for International Environmental Law. In 2022, a dozen academics launched a political campaign for national policies of "no public funding, no outdoor experiments, no patents, no deployment, and no support in international institutions... including in assessments by the Intergovernmental Panel on Climate Change." The proponents call this a "non-use agreement", but others have asserted that these five policies, if enacted, would end all meaningful SRM research. The campaign has been endorsed by a few hundred fellow academics and environmental groups. Among the latter is the Climate Action Network, a coalition of hundreds of nongovernmental organizations. (The position from Climate Action Network included a footnote that excluded the Environmental Defense Fund and the Natural Resources Defense Council. )

In 2021, researchers at Harvard put plans for a SRM test on hold after Indigenous Sámi people objected to the test taking place in their homeland. Although the test would not have involved any atmospheric experiments, members of the Saami Council spoke out against the lack of consultation and SRM 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."

The Climate Overshoot Commission is an group of global, eminent, and independent figures. It investigated and developed a comprehensive strategy to reduce climate risks which includes SRM in its policy portfolio. The Commission's recommendation regarding SRM are:


 * 1) "a moratorium on the deployment of solar radiation modification (SRM) and large-scale outdoor experiments...
 * 2) governance of SRM research should be expanded...
 * 3) SRM research should also be strengthened...
 * 4) an international, independent scientific review and assessment of the best available evidence from SRM research should take place every few years...
 * 5) broad consultations and dialogues on these issues are needed."

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". 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. Along with carbon dioxide removal, SRM was discussed jointly as "geoengineering" in a 1992 climate change report from the US National Academies. The topic was essentially taboo in the climate science and policy communities until Nobel Laureate Paul Crutzen published an influential scholarly paper in 2006. Major reports by the Royal Society (2009), the US National Academies (2015, 2021), and the UN Environment Programme followed.

As of 2018, total research funding worldwide remained modest, at less than 10 million US dollars annually. Almost all research into SRM has to date consisted of computer modeling or laboratory tests, and there are calls for more research funding as the science is poorly understood. Major academic institutions, including Harvard University, have begun research into SRM, with NOAA alone investing $22 million from 2019 to 2022, though few outdoor tests have been run to date. The Degrees Initiative is a UK registered charity, established to build capacity in developing countries to evaluate SRM. The 2021 US National Academy of Sciences, Engineering, and Medicine report recommended an initial investment into SRM research of $100–200 million over five years.

Public attitudes
There have been a handful of studies into attitudes to and opinions of SRM. These generally find low levels of awareness, uneasiness with the implementation of SRM, cautious support of research, and a preference for greenhouse gas emissions reduction. 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.

The largest assessment of public opinion and perception of SRM, which had over 30,000 respondents in 30 countries, found that "Global South publics are significantly more favorable about potential benefits and express greater support for climate-intervention technologies." Though the assessment also found Global South publics had greater concern the technologies could undermine climate-mitigation.