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Solar radiation modification (SRM) (or solar radiation management or solar geoengineering), is a group of large-scale approaches to limit global warming by increasing the amount of sunlight (solar radiation) that is reflected away from Earth and back to space. Among the potential approaches, stratospheric aerosol injection (SAI) is the most-studied, followed by marine cloud brightening (MCB); others such as ground- and space-based show less potential or feasibility and receive less attention. SRM could be a supplement to climate change mitigation and adaptation measures, but would not be a substitute for reducing greenhouse gas emissions. SRM is a form of climate engineering or geoengineering.

Scientific studies, based on evidence from climate models, have consistently shown that some forms of SRM could reduce global warming and many effects of climate change. 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. SRM would therefore pose environmental risks, as would a warmed world without SRM. Confidence in the current projections of how SRM would affect regional climate and ecosystems is low. Furthermore, a suboptimal implementation of SRM--such as starting or stopping suddenly, or intervening too strongly in the Earth's energy balance--would increase environmental risks.

SRM presents political, social and ethical challenges. A common concern is that attention to it would lessen efforts to reduce greenhouse gas emissions. Because some SRM approaches appear to be technically feasible and have relatively low direct financial costs, some countries could be capable of deploying it on their own, raising questions of international relations. Although some existing applicable governance instruments and institutions are applicable, there is currently no formal international framework designed to regulate SRM. Issues of governance and effectiveness are intertwined, as poorly governed use of SRM might lead to its suboptimal implementation. For these reasons and more, SRM is often a contested topic among environmentalists.

In the face of ongoing global warming and insufficient reductions to greenhouse gas emissions, SRM receives increasing attention. Climate scientists and other experts from around the world research and publish academic articles, while more nongovernmental and intergovernmental organizations, as well as national governments, are examining and developing views.

Context

The context for the interest in solar radiation modification (SRM) options is continued high global emissions of greenhouse gases, rising global temperatures, and worsening climate impacts. Human's greenhouse gas emissions have disrupted the Earth's energy budget. 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/m in 1980, to 3.1 W/m in 2019. This imbalance, or "radiative forcing," means that the Earth absorbs more energy than it emits, causing global temperatures to rise which will, in turn, have negative impacts on humans and nature.

In principle, net emissions could be reduced and even eliminated achieved through a combination of emission cuts and carbon dioxide removal (together called "mitigation"). However, emissions have persisted, consistently exceeding targets, and experts have raised serious questions regarding the feasibility of large-scale removals. 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.

SRM would increase Earth's reflection of sunlight 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/m, 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 could theoretically buy time by slowing the rate of climate change or to eliminate the worst climate impacts until net negative emissions reduce atmospheric greenhouse gas concentrations sufficiently. This is because SRM could, unlike the other responses, cool the planet within months after deployment.

SRM is generally intended to complement, not replace, emissions reduction and carbon dioxide removal. For example, the IPCC Sixth Assessment Report says: "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".

Major reports on SRM that have investigated advantages and disadvantages of SRM (sometimes grouped with carbon dioxide removal and under the title of climate engineering) include those by the Royal Society (2009), the US National Academies (2015 and 2021), the UN Environment Programme (2023), and the European Union's Scientific Advice Mechanism (2024).

History

In 1965, during the administration of U.S. President Lyndon B. Johnson, the President's Science Advisory Committee delivered Restoring the Quality of Our Environment, the first report which warned of the harmful effects of carbon dioxide emissions from fossil fuel. To counteract global warming, the report mentioned "deliberately bringing about countervailing climatic changes", including "raising the albedo, or reflectivity, of the Earth".

In 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.

David Keith, an American physicist, has worked on solar geoengineering since 1992, when he and Hadi Dowlatabadi published one of the first assessments of the technology and its policy implications, introducing a structured comparison of cost and risk. Keith has consistently argued that geoengineering needs a "systematic research program" to determine whether or not its approaches are feasible. He has also appealed for international standards of governance and oversight for how such research might proceed.

The first modeled results of SRM were published in 2000. In 2006 Nobel Laureate Paul Crutzen published an influential scholarly paper where he said, "Given the grossly disappointing international political response to the required greenhouse gas emissions, and further considering some drastic results of recent studies, research on the feasibility and environmental consequences of climate engineering should not be tabooed."

Atmospheric methods

The atmospheric methods for SRM include stratospheric aerosol injection (SAI), marine cloud brightening (MCB) and cirrus cloud thinning (CCT).

Stratospheric aerosol injection (SAI)

For stratospheric aerosol injection (SAI) small particles would be injected into the upper atmosphere to cool the planet with both global dimming and increased albedo. Of all the proposed SRM methods, SAI has received the most sustained attention: The IPCC concluded in 2018 that SAI "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.

This technique could give much more than 3.7 W/m of globally averaged negative forcing, which is sufficient to entirely offset the warming caused by a doubling of carbon dioxide.

The most recent Scientific Assessment of Ozone Depletion report in 2022 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 potential disadvantage of SAI is its potential to delay the regeneration of the stratospheric ozone layer (dependent on assumptions about which aerosols would be used to do the cooling).

Marine cloud brightening (MCB)

Marine cloud brightening (MCB) would involve spraying fine sea water to whiten clouds and thus increase cloud reflectivity. It would work by "seeding to promote nucleation, reducing optical thickness and cloud lifetime, to allow more outgoing longwave radiation to escape into space".

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/m of globally averaged negative forcing, which is sufficient to reverse the warming effect of a doubling of atmospheric carbon dioxide concentration.

Cirrus cloud thinning (CCT)

Cirrus cloud thinning (CCT) involves "seeding to promote nucleation, reducing optical thickness and cloud lifetime, to allow more outgoing longwave radiation to escape into space."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.

Other methods

Ground-based albedo modification

The IPCC describes ground-based albedo modification as "whitening roofs, changes in land use management (e.g., no-till farming), change of albedo at a larger scale (covering glaciers or deserts with reflective sheeting and changes in ocean albedo)." It is a method of enhancing Earth's albedo, i.e. the ability to reflect the visible, infrared, and ultraviolet wavelengths of the Sun, reducing heat transfer to the surface.

Space-based

Space-based approaches could be advantageous compared to stratospheric aerosol injection because they do not interfere directly with the biosphere and ecosystems. However, space-based approaches would cost about 1000 times more than their terrestrial alternatives. In 2022, the IPCC Sixth Assessment Report discussed SAI, MCB, CCT and even attempts to alter albedo on the ground or in the ocean but did not address space-based approaches.

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.

When summarizing these spaced-based 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.

Costs

Cost estimates for SAI

A study in 2020 looked at the cost of SAI through to the year 2100. It found that relative to other climate interventions and solutions, SAI remains inexpensive. However, at about $18 billion per year per degree Celsius of warming avoided (in 2020 USD), a solar geoengineering program with substantial climate impact would lie well beyond the financial reach of individuals, small states, or other non-state potential rogue actors. The annual cost of delivering a sufficient amount of sulfur to counteract expected greenhouse warming is estimated at $5–10 billion US dollars.

Technical problem areas

Aspects of regional scales and seasonal timescales

Modelling studies have consistently concluded that moderate SRM use would significantly reduce many of the impacts of global warming —for example, average and extreme temperature, water availability, and cyclone intensity Furthermore, SRM's effect would occur rapidly, unlike those of other responses to climate change. However, even under optimal implementation, some climatic anomalies—especially regarding precipitation—would persist, although mostly at lesser magnitudes than without SRM.

However, SRM has significant potential risks and uncertainties. The IPCC Sixth Assessment Report explains some of the risks and uncertainties as follows: " 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."

Likewise, a 2023 report from the UN Environment Programme stated, "Climate model results indicate that an operational SRM deployment could fully or partially offset the global mean warming caused by anthropogenic GHG emissions and reduce some climate change hazards in most regions. There could be substantial residual or possible overcompensating climate change at regional scales and seasonal timescales." The report also said: "An operational SRM deployment would introduce new risks to people and ecosystems".

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.

Impacts on precipitation

Models indicate that SRM would reverse warming-induced changes to precipitation more rapidly than changes to temperature. 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 misleading claims that SRM "would disrupt the Asian and African summer monsoons", something that has been repeatedly challenged by climate scientists who study SRM. Ultimately the impact would depend on the particular implementation regime.

Deployment length

A modeling study in 2023 showed that the range of possible deployment timescales is vast even if pathways start at a similar point at the beginning of SRM deployment. This is because the evolution of mitigation under SRM, the availability of carbon removal technologies and the effects of climate reversibility are not precisely known. Since these effects will be mostly uncertain at the time of SRM initialization, a precedent prediction of deployment length seems unlikely, with possibilities ranging from decades to multiple centuries. This is a knowledge gap that must be considered before any SRM proposal is seriously considered.

For all realizations that follow current NDC (nationally determined contributions) median 2100 warming projections (2.4 ∘C), none deploy SRM for a shorter period than 100 years.

The direct climatic effects of SRM are reversible within short timescales. Models project that SRM interventions would take effect rapidly, but would also quickly fade out if not sustained.

Termination shock

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 risk might be manageable because it would be in states' interest to resume any terminated deployment, and maintaining back-up SRM infrastructure would increase the resilience of an SRM system.

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 affect the growth rates of phytoplankton, trees, and crops between now and the end of the century. Uniformly reduced net shortwave radiation would affect solar photovoltaics, but the real-world impact is complex and is affected by temperature and cloud fraction, and interacts with demand-side factors (especially heating and cooling load).

Uncertainty regarding effects

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.

Climate change has various effects on agriculture. One of them is the CO₂ fertilization effect which affects different crops in different ways. A net increase in agricultural productivity from SRM (in combination with raised carbon dioxide levels) has 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.

There have also been proposals to focus SRM at the poles, in order to combat sea level rise or regional marine cloud brightening (MCB) in order to protect coral reefs from bleaching. However, there is low confidence about the ability to control geographical boundaries of the effect.

SRM might be used in ways that are not optimal. In particular, SRM's climatic effects would be rapid and reversible, which would bring the disadvantage of sudden warming if it were to be stopped suddenly. Similarly, if SRM was very heterogenous, then the climatic responses could be severe and uncertain.

Governance and policy risks

Global governance issues

The potential use of SRM poses several governance 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 political scientists have argued that the current international political system is inadequate for the fair and inclusive governance of SRM deployment on a global scale. Other researchers have suggested that building a global agreement on SRM deployment would be very difficult, and speculated whether power blocs might emerge. However, there may be significant incentives for states to cooperate in choosing a specific SRM policy, which make unilateral deployment unlikely.

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.

Although climate models of SRM generally simulate 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 actual or perceived local negative impacts from SRM could be a source of international tensions.

There is a risk that countries may start using SRM without proper research and evaluation. 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.

Lessened climate change mitigation

A common concern is that the use of SRM, or even the idea, might reduce the political and social impetus for climate change mitigation. This has often been called a potential "moral hazard", although such language is not precise. However, some engagement work has suggested that SRM may in fact increase the likelihood of emissions reduction because the pursuit of such a risky approach underlines the seriousness of global warming.

Support for SRM research

Support for SRM research has come from scientists, NGOs, international organisations, and governments. The leading argument in support of SRM research is that there are large and immediate risks from climate change, and SRM is the only known way to quickly stop (or reverse) warming. 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 and other large organizations that have called for further research on SRM include:

• In the UK in 2009: the Royal Society, the Institution of Mechanical Engineers (UK)
• In Australia in 2012: Australia's Office of the Chief Scientist
• In the Netherlands in 2013: Netherlands' scientific assessment institute
• In the United States from 2015 to 2022: the US National Academies, the American Geophysical Union, the American Meteorological Society, the U.S. Global Change Research Program, the Council on Foreign Relations
• Global organizations from 2023 to 2024: the World Climate Research Programme and reports from the UN Environment Programme and the UN Educational, Scientific and Cultural Organization
• In the European Union: the Group of Chief Scientific Advisors (the report from 2024 specifically examines "how the EU can address the risks and opportunities associated with research on solar radiation modification and with its potential deployment".)

Two sign-on letters in 2023 from scientists and other experts have called for expanded "responsible SRM research". One wants to "objectively evaluate the potential for SRM to reduce climate risks and impacts, to understand and minimize the risks of SRM approaches, and to identify the information required for governance". It was endorsed by "more than 110 physical and biological scientists studying climate and climate impacts about the role of physical sciences research." Another called for "balance in research and assessment of solar radiation modification" and was endorsed by about 150 experts, mostly scientists.

Some nongovernmental organizations actively support SRM research and governance dialogues. The Degrees Initiative is a UK registered charity, established to build capacity in developing countries to evaluate SRM. It works toward "changing the global environment in which SRM is evaluated, ensuring informed and confident representation from developing countries."

Operaatio Arktis is a Finnish youth climate organisation that supports research into solar radiation modification alongside mitigation and carbon sequestration as a potential means to preserve polar ice caps and prevent tipping points.

SilverLining is an American organization that advances SRM research as part of "climate interventions to reduce near-term climate risks and impacts." It is funded by "philanthropic foundations and individual donors focused on climate change". One of their funders is Quadrature Climate Foundation which "plans to provide $40 million for work in this field over the next three years" (as of 2024).

The Alliance for Just Deliberation on Solar Geoengineering advances "just and inclusive deliberation" regarding SRM, in particular by engaging civil society organisations in the Global South and supporting a broader conversation on SRM governance. The Carnegie Climate Governance Initiative catalyzed governance of SRM and carbon dioxide removal, although it ended operations in 2023.

The Climate Overshoot Commission is a group of global, eminent, and independent figures. It investigated and developed a comprehensive strategy to reduce climate risks. The Commission recommended additional research on SRM alongside a moratorium on deployment and large-scale outdoor experiments. It also concluded that "governance of SRM research should be expanded".

Campaigners have claimed that the fossil fuels lobby advocates for SRM research. However, researchers have pointed out the lack of evidence in support of this claim.

Opposition to deployment and research

Opposition to SRM has come from various academics and NGOs. Common concerns are that SRM could lessen climate change mitigation efforts, that SRM is ultimately ungovernable, and that SRM would cause tensions, or even conflict, between nations. Opponents of SRM research often emphasize that reductions of greenhouse gas emissions would also bring co-benefits (for example reduced air pollution) and that consideration of SRM could prevent these outcomes.

The ETC Group, an environmental justice organization, has been a pioneer in opposing SRM research. It was later joined by the Heinrich Böll Foundation (affiliated with the German Green Party) and the Center for International Environmental Law.

In 2021, researchers at Harvard put plans for an SRM-related field experiment 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."

In 2022, a scientific journal Wiley Interdisciplinary Reviews: Climate Change published "Solar geoengineering: The case for an international non-use agreement". The authors argued that geoengineering cannot be used in a responsible manner under the current system of international relations, so the only option is for as many governments as possible to make a commitment they would neither deploy such technologies, nor fund research into them, grant intellectual property rights or host such experiments when conducted by third parties. In 2024, the same journal had published a commentary from a different group of scientists, which criticized the proposed non-use agreement and argued for a more permissive research framework.. The academic paper launched a campaign which, as of December 2024, has been supported by nearly 540 academics and 60 advocacy organizations have endorsed the proposal.

By 2024, U.S. government agencies were allegedly operating an airborne early warning system for detecting small concentrations of aerosols to determine where other countries might be carrying out geoengineering attempts, thought to have unpredictable effects on climate.

Research funding

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.

A study from 2022 investigated where the funding for SRM research came from globally. The authors concluded "the primary funders of research do not emanate from fossil capital" and that there are "close ties to mostly US financial and technological capital as well as a number of billionaire philanthropists".

Under the World Climate Research Programme there is a Lighthouse Activity called Research on Climate Intervention as of 2024. This will include research on all possible climate interventions (another term for climate engineering): "large-scale Carbon Dioxide Removal (CDR; also known as Greenhouse Gas Removal, or Negative Emissions Technologies) and Solar Radiation Modification (SRM; also known as Solar Reflection Modification, Albedo Modification, or Radiative Forcing Management)".

Government funding

Few countries have an explicit governmental position on SRM. Those that do, such as the United Kingdom and Germany, support some SRM research even if they do not see it as a current climate policy option. For example, the German Federal Government does have an explicit position on SRM and stated in 2023 in a strategy document climate foreign policy: "Due to the uncertainties, implications and risks, the German Government is not currently considering solar radiation management (SRM) as a climate policy option". The document also stated: "Nonetheless, in accordance with the precautionary principle we will continue to analyse and assess the extensive scientific, technological, political, social and ethical risks and implications of SRM, in the context of technology-neutral basic research as distinguished from technology development for use at scale".

Some countries, such as the U.S., U.K., Argentina, Germany, China, Finland, Norway, and Japan, as well as the European Union, have funded SRM research. NOAA in the United States has spent $22 million USD from 2019 to 2022, with only a few outdoor tests carried out to date. As of 2024, NOAA provides about $11 million USD a year through their solar geoengineering research program.

In 2021, the National Academies of Sciences, Engineering, and Medicine released their consensus study report Recommendations for Solar Geoengineering Research and Research Governance. The report recommended an initial investment into SRM research of $100–200 million over five years.

In late 2024, the Advanced Research and Invention Agency, a British funding agency, announced that research funds totaling 57 million pounds (about $75 million USD) will be made available to support projects which explore "Climate Cooling". This includes outdoor experiments: "This programme aims to answer fundamental questions as to the practicality, measurability, controllability and possible (side-)effects of such approaches through indoor and (where necessary) small, controlled, outdoor experiments." Successful applicants will be announced in 2025.

Non-profits and philanthropic support for research

There are also research activities on SRM that are funded by philanthropy. According to Bloomberg News, as of 2024 several American billionaires are funding research into SRM: "A growing number of Silicon Valley founders and investors are backing research into blocking the sun by spraying reflective particles high in the atmosphere or making clouds brighter." The article listed the following billionaires as being notable geoengineering research supporters: Mike Schroepfer, Sam Altman, Matt Cohler, Rachel Pritzker, Bill Gates, Dustin Moskovitz.

SRM research initiatives, or non-profit knowledge hubs, include for example SRM360 which is "supporting an informed, evidence-based discussion of sunlight reflection methods (SRM)". Funding comes from the LAD Climate Fund. David Keith, a long-term proponent of SRM research, is one of the members of the advisory board.

Another example is Reflective, which is "a philanthropically-funded initiative focused on sunlight reflection research and technology development". Their funding is "entirely by grants or donations from a number of leading philanthropies focused on addressing climate change": Outlier Projects, Navigation Fund, Astera Institute, Open Philanthropy, Crankstart, Matt Cohler, Richard and Sabine Wood.

Deployment activities

Make Sunsets

At least one startup in the private sector has tried to sell "cooling credits" for SRM activities. Make Sunsets launches balloons containing helium and sulfur dioxide. The company sells cooling credits, making the contested claim that each US$10 credit would offset the warming effect of one ton of carbon dioxide warming for a year. Based in California, Make Sunsets conducted some of its first activities in Mexico. In response to these activities, which were conducted without prior notification or consent, the Mexican government announced measures to prohibit SRM experiments within its borders, although it is unclear whether this became actual policy. Even people who advocate for more research into SRM have criticized Make Sunsets' undertaking.

Society and culture

Studies into opinions about SRM have found low levels of awareness, uneasiness with the implementation of SRM, cautious support of research, and a preference for greenhouse gas emissions reduction. 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.

See also

• Cloud seeding – Weather modification that condenses clouds to cause rainfall
• Passive daytime radiative cooling – Management strategy for global warming
• Weather modification – Act of intentionally altering or manipulating the weather

References

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• Climate change policy
• Planetary engineering
• Climate engineering
• Atmospheric radiation