Solar radiation modification

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,^[1]: 350 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,^[2] 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 SRM could reduce global warming and many effects of climate change.^[3][4][5] 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.^[2] 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.^[6] 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.^[7] For these reasons and more, SRM is often a contested topic.

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.

The interest in solar radiation modification (SRM) arises from ongoing high global greenhouse gas emissions, increasing global temperatures, and worsening climate impacts. Human activities have disrupted the Earth's energy budget, causing an increase in radiative forcing—the imbalance between energy absorbed by the Earth and energy radiated back into space. This imbalance has risen from 1.7 W/m² in 1980 to 3.1 W/m² in 2019, primarily due to elevated atmospheric greenhouse gas concentrations.^[9] As a result, global warming continues, increasing risks to both human and natural systems.^[10]

In principle, achieving net-zero emissions through emissions reductions and carbon dioxide removal (CDR) could halt global warming. However, emissions reductions have consistently fallen short of targets, and large-scale CDR remains uncertain due to feasibility concerns.^[11][12] The 2023 UN Environment Programme (UNEP) Emissions Gap Report estimated that even under the most optimistic emissions reduction pledges, there is only a 14% chance of limiting global warming to 1.5°C.^[13]

SRM aims to increase Earth's albedo by modifying the atmosphere or surface to reflect more sunlight. A 1% increase in planetary albedo could reduce radiative forcing by 2.35 W/m², offsetting most of the warming from current greenhouse gas concentrations. A 2% increase could counteract the warming effect of a doubling of atmospheric carbon dioxide.^[14]

Unlike emissions reduction or CDR, SRM could reduce global temperatures within months of deployment.^[15] This rapid effect means SRM could help limit the worst climate impacts while emissions reductions and CDR are scaled up. However, SRM would not reduce atmospheric carbon dioxide concentrations, meaning that ocean acidification and other climate change effects would persist.

The IPCC Sixth Assessment Report emphasizes that SRM is not a substitute for emissions reductions or CDR, stating: > "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." ^[2]

Major reports evaluating the potential benefits and risks of SRM include those by:

• The Royal Society (2009),^[14]
• The US National Academies (2015, 2021),^[15][16]
• The United Nations Environment Programme (2023),^[4]
• The European Union Scientific Advice Mechanism (2024).^[17]

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 use. To counteract global warming, the report mentioned "deliberately bringing about countervailing climatic changes," including "raising the albedo, or reflectivity, of the Earth".^[18][19]

In 1974, Russian climatologist Mikhail Budyko suggested that if global warming ever became a serious threat, it could be countered by releasing aerosols into the stratosphere. He proposed that aircraft burning sulfur could generate aerosols that would reflect sunlight away from the Earth, cooling the planet.^[20][21]

Along with carbon dioxide removal, SRM was discussed under the broader concept of geoengineering in a 1992 climate change report from the US National Academies.^[22]

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. Their research introduced a structured comparison of cost and risk. Keith has consistently argued that geoengineering requires a "systematic research program" to determine its feasibility.^[23][24][25] He has also advocated for international standards of governance and oversight for such research.^[26]

The first modeled results of SRM were published in 2000.^[27]

In 2006, Nobel Laureate Paul Crutzen published an influential paper arguing that, given the lack of adequate greenhouse gas emissions reductions, research on the feasibility and environmental consequences of climate engineering should not be dismissed.^[28]

The atmospheric methods for SRM include stratospheric aerosol injection (SAI), marine cloud brightening (MCB), and cirrus cloud thinning (CCT).^[29]

For stratospheric aerosol injection (SAI), small particles would be introduced into the upper atmosphere to reflect sunlight and induce global dimming. Of all the proposed SRM methods, SAI has received the most sustained attention: The IPCC concluded in 2021 that SAI "is the most-researched SRM method, with high agreement that it could limit warming to below 1.5 °C."^[29] This technique would replicate natural cooling phenomena observed following large volcano eruptions.^[30]

Sulfates are the most commonly proposed aerosol due to their natural occurrence in volcanic eruptions. Alternative substances, including photophoretic particles, titanium dioxide, and diamond dust, have also been suggested.^[31]

Custom-designed aircraft are considered the most feasible delivery method, with artillery and balloons occasionally proposed.^[32]

SAI could produce more than 3.7 W/m² of negative radiative forcing,^[29] potentially offsetting the warming effect of a doubling of atmospheric carbon dioxide.

The World Meteorological Organization's 2022 Scientific Assessment of Ozone Depletion stated that "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."^[32]

A key concern with SAI is its potential to delay the recovery of the ozone layer, depending on which aerosols are used.^[32]

Marine cloud brightening (MCB) involves spraying fine sea water droplets to increase cloud reflectivity. This process enhances cloud condensation nuclei, altering the size distribution of cloud droplets to make them more reflective.^[29]

MCB could be implemented using fleets of unmanned rotor ships, such as Flettner vessels, to disperse seawater mist into the air, increasing cloud albedo and reflecting more radiation.^[33] The process relies on the Twomey effect to enhance cloud whitening.

This technique could produce more than 3.7 W/m² of negative forcing,^[29] potentially reversing the warming effect of a doubling of atmospheric carbon dioxide.

Cirrus cloud thinning (CCT) involves seeding cirrus clouds to reduce their optical thickness and decrease cloud lifetime, allowing more outgoing longwave radiation to escape into space.^[29]

Cirrus clouds generally have a net warming effect. By dispersing them through targeted interventions, CCT could enhance Earth's ability to radiate heat away.

This method is often grouped with SRM despite working primarily by increasing outgoing radiation rather than reducing incoming shortwave radiation.^[32]

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)."^[29]

Ground-based approaches are considered localized and would have limited global impact compared to atmospheric or space-based methods.^[29] While urban cooling could be achieved through reflective roofs and pavement, large-scale desert albedo modification could significantly alter regional precipitation patterns.^[29] Covering glaciers with reflective materials has been proposed to slow melting, though feasibility at scale remains uncertain.^[31]

Space-based solar radiation modification involves deploying mirrors, reflective particles, or shading structures at or near the L1 Lagrange point between Earth and the Sun.^[34] Unlike atmospheric methods, space-based approaches would not directly interfere with Earth's climate systems, but they require significant financial investment and decades of technological development.

Historically, proposals have included orbiting mirrors, space dust clouds, and electromagnetically tethered reflectors. The Royal Society (2009) and later assessments concluded that while space-based methods may be viable in the distant future, costs and deployment challenges make them infeasible for near-term climate intervention.^[14]

SRM could have relatively low direct financial costs of deployment compared to the projected economic damages of unmitigated climate change.^[29] The cost varies widely depending on the method used. Stratospheric aerosol injection (SAI) is the most studied and has the most cost estimates, while marine cloud brightening (MCB) and cirrus cloud thinning (CCT) remain less understood, with only preliminary assessments available.

Studies estimate that SAI deployment could cost between $5 billion to $10 billion per year to deliver sufficient aerosol quantities to counteract anticipated global warming.^[35] Another estimate suggests $18 billion per year per 1°C of cooling, meaning that substantial deployment would be beyond the financial capacity of small states or non-state actors.^[32]

A 2021 report by the US National Academies estimated that developing the technology for SAI could cost several billion dollars over a decade, while annual deployment could cost in the tens of billions of dollars.^[36] Costs are expected to increase over time due to reduced efficiency from larger aerosol particles, requiring greater mass injections to maintain cooling levels.^[37]

Compared to SAI, MCB and CCT have fewer cost estimates. The US National Academies (2021) report suggests that MCB could cost several billion dollars per year, but no definitive analysis exists.^[16] One earlier estimate from the suggested an MCB deployment cost of around $5 billion per year.^[38]

Cirrus cloud thinning (CCT) is even less studied, and no formal cost estimates exist.^[29] The method remains highly uncertain, as some studies suggest CCT could cause net warming rather than cooling due to complex cloud-aerosol interactions.^[39]

Modelling studies have consistently concluded that moderate SRM use would significantly reduce many of the impacts of global warming, including average and extreme temperature, water availability, and cyclone intensity.^[40] SRM would take effect rapidly, unlike mitigation or carbon dioxide removal, making it the only known method to lower global temperatures within months.^[29]

However, SRM would not perfectly reverse climate change effects. Differences in regional precipitation patterns, cloud cover, and atmospheric circulation could persist, with some regions experiencing overcompensation or residual warming and cooling effects.^[41]

The IPCC Sixth Assessment Report states:

• "SRM could offset some of the effects of increasing greenhouse gases 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 timescales, 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."*^[29]: 69

A 2023 UNEP report similarly concluded that an operational SRM deployment could reduce some climate hazards but would also introduce new risks to ecosystems and human societies.^[41]

SRM would be highly effective in reducing global mean temperature. Model results indicate that stratospheric aerosol injection (SAI) at a moderate level could bring global temperatures down to 1.5°C or 2°C above pre-industrial levels.^[42]

SRM could also reduce the frequency and severity of heatwaves and limit the intensification of tropical cyclones, which are projected to worsen under continued global warming.^[43] However, regional disparities could emerge, meaning some areas may warm more than others or experience shifts in extreme weather patterns.

SRM would alter global precipitation patterns, as greenhouse gases warm the planet year-round, whereas SRM primarily reflects sunlight during daylight hours and varies by latitude. This asymmetry could lead to some regions receiving too much or too little rainfall.^[43]

Studies suggest that SRM would more precisely maintain soil moisture than precipitation levels, since it reduces evaporation rates.^[44] While some models indicate that SRM could reduce overall precipitation, others find that the effects depend on the specific deployment strategy.

SRM could benefit agriculture by reducing extreme heat stress and preventing drought-induced crop failures.^[45]

The CO₂ fertilization effect, which enhances plant growth under high CO₂ levels, would continue under SRM. Some studies indicate that diffuse sunlight under SRM might improve crop yields, while others suggest that reducing overall sunlight could slightly decrease agricultural productivity.^[46][47]

Ecosystem impacts are less well understood. Some studies suggest that SRM could prevent mass coral bleaching events by reducing sea surface temperatures, but it would not address ocean acidification.^[43] Terrestrial ecosystems could experience shifts in plant productivity, with forests and grasslands responding differently to changes in temperature and precipitation.

SRM poses significant uncertainties, particularly regarding regional climate impacts, atmospheric interactions, and long-term effectiveness. While climate models suggest that SRM could reduce many global warming hazards, limitations in model accuracy, aerosol-cloud interactions, and the response of regional climate systems remain key uncertainties.^[48] Furthermore, 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 might be able to compensate for some of this heterogeneity by changing and optimizing injection rates by latitude and season.^[49][50]

Models indicate that SRM would reverse warming-induced changes to precipitation more rapidly than changes to temperature.^{[citation needed]} 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,^{[citation needed]} 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.^[51] Likewise, the intensity of tropical monsoons is increased by climate change and decreased by SRM.^[52]

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.^{[citation needed]} 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.^{[citation needed]}

SRM does not directly influence atmospheric carbon dioxide concentration and thus does not reduce ocean acidification.^[53] While not a risk of SRM per se, this points to the limitations of relying on it to the exclusion of emissions reduction.

Managing solar radiation using aerosols or cloud cover would involve changing the ratio between direct and indirect solar radiation. This would affect plant life^[54] and solar energy.^[55] 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.^[56] As a result, deployment of atmospheric SRM would affect the growth rates of phytoplankton, trees, and crops ^[57] between now and the end of the century.^[58] 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).

Much uncertainty remains about SRM's likely effects.^[53] 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.^[59] Volcanoes are an imperfect analogue as they release the material in the stratosphere in a single pulse, as opposed to sustained injection.^[60]

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.^[61] 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.^[62] Other researchers have suggested that building a global agreement on SRM deployment would be very difficult, and speculated whether power blocs might emerge.^[63] However, there may be significant incentives for states to cooperate in choosing a specific SRM policy, which make unilateral deployment unlikely.^[64]

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.^[65] Likewise, blame for actual or perceived local negative impacts from SRM could be a source of international tensions.^[66]

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.^[67] 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".^[68] Others argue that states will insist on maintaining control of SRM.^[69]

A common concern is that the use of SRM, or even the idea, might reduce the political and social impetus for climate change mitigation.^[70] 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.^{[71][72][73][74]}

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.^[75] 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.^[75]

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.^[75]

The direct climatic effects of SRM are reversible within short timescales.^[15] Models project that SRM interventions would take effect rapidly, but would also quickly fade out if not sustained.^[76]

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.^[77] 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.^[78][79]

SRM was raised as a possible counter for Climate Change by Mikhail Budyko in 1974. Yet in the 20th and early 21st century even calls for further SRM research have been controversial, and few scientists called for its actual use. ^{[80] [81]}

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.^[82]

In 2025, James Hansen and others published a journal article describing SRM as 'Purposeful Global Cooling'. They stated it is false to describe SRM as "geoengineering", because purposeful cooling would in fact be the opposite - action to reverse the unprecedented warming artificially caused by humans. While not advocating for the immediate deployment of SRM , the article calls for the public to have better understanding of SRM related options. The paper addressed the moral hazard argument sometimes used against SRM - the view that consideration of SRM may reduce ambition for emissions reduction. In the authors opinion, this is unlikely, and even if true not a compelling reason to avoid SRM if it could spare future generations the considerable suffering likely to follow from unchecked global warming. They called for better understanding of the risks and rewards of SRM, versus scenarios where no such purposeful cooling occurred. ^[81]

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.^[83][84]

Scientific and other large organizations that have called for further research on SRM include:

• In the UK in 2009: the Royal Society,^[14] the Institution of Mechanical Engineers (UK)^[85]
• In Australia in 2012: Australia's Office of the Chief Scientist^[86]
• In the Netherlands in 2013: Netherlands' scientific assessment institute^[87]
• In the United States from 2015 to 2022: the US National Academies,^[15][16] the American Geophysical Union,^[88] the American Meteorological Society, the U.S. Global Change Research Program,^[89] the Council on Foreign Relations^[90]
• Global organizations from 2023 to 2024: the World Climate Research Programme^[91] and reports from the UN Environment Programme^[4] and the UN Educational, Scientific and Cultural Organization^[92]
• In the European Union: the Group of Chief Scientific Advisors^[93] (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."^[94] Another called for "balance in research and assessment of solar radiation modification" and was endorsed by about 150 experts, mostly scientists.^[95]

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.^[96] It works toward "changing the global environment in which SRM is evaluated, ensuring informed and confident representation from developing countries."^[96]

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.^[97]

SilverLining is an American organization that advances SRM research as part of "climate interventions to reduce near-term climate risks and impacts."^[98] It is funded by "philanthropic foundations and individual donors focused on climate change".^[98][99] 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).^[100]

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.^[101] The Carnegie Climate Governance Initiative catalyzed governance of SRM and carbon dioxide removal,^[102] although it ended operations in 2023.

The Climate Overshoot Commission is a group of global, eminent, and independent figures.^[103] 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".^[104]: 15

Campaigners have claimed that the fossil fuels lobby advocates for SRM research.^[105][106] However, researchers have pointed out the lack of evidence in support of this claim.^[107]

Opposition to SRM has come from various academics and NGOs.^[108] 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.^[109]

The ETC Group, an environmental justice organization, has been a pioneer in opposing SRM research.^[110] It was later joined by the Heinrich Böll Foundation^[111] (affiliated with the German Green Party) and the Center for International Environmental Law.^[112]

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.^[113][114] 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.^[108] 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.^[115] The academic paper launched a campaign which, as of December 2024, has been supported by nearly 540 academics^[116] and 60 advocacy organizations^[117] 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.^[118]

As of 2018, total research funding worldwide remained modest, at less than 10 million US dollars annually.^[119] Almost all research into SRM has to date consisted of computer modeling or laboratory tests,^[120] and there are calls for more research funding as the science is poorly understood.^{[121][16]: 17}

A study from 2022 investigated where the funding for SRM research came from globally concluded there are "close ties to mostly US financial and technological capital as well as a number of billionaire philanthropists".^[122]

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)".^[91]

Few countries have an explicit governmental position on SRM. Those that do, such as the United Kingdom^[123] and Germany,^[124]: 58 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".^[124]: 58

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.^[125] NOAA in the United States has spent $22 million USD from 2019 to 2022, with only a few outdoor tests carried out to date.^[126] As of 2024, NOAA provides about $11 million USD a year through their solar geoengineering research program.^[100]

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.^[16]: 17

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".^[127] 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."^[128] Successful applicants will be announced in 2025.^[129]

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."^[130] 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)".^[131] Funding comes from the LAD Climate Fund.^[132][133]

Another example is Reflective, which is "a philanthropically-funded initiative focused on sunlight reflection research and technology development".^[134] 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.^[134]

At least one startup in the private sector has tried to sell "cooling credits" for SRM activities. Make Sunsets^[135] 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.^[136] 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.^[137] Even people who advocate for more research into SRM have criticized Make Sunsets' undertaking.^[82]

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.^[138][139] 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.^{[140][141][142]}

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.^[143]

• 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