Solar radiation modification

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Schematic with five proposed methods for solar radiation modification technologies Illustration different solar climate intervention techniques.png
Schematic with five proposed methods for solar radiation modification technologies

Solar radiation modification (SRM) (or solar radiation management or sunlight reflection methods [1] 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 methods, stratospheric aerosol injection (SAI) is the most-studied, [2] : 350  followed by marine cloud brightening (MCB); others such as ground- and space-based methods show less potential or feasibility and receive less attention. SRM could be a supplement to climate change mitigation and adaptation measures, [3] but would not be a substitute for reducing greenhouse gas emissions. [4] SRM is a form of climate engineering or geoengineering, and might be able to prevent some kinds of tipping. [5]

Contents

Scientific studies, based on evidence from climate models, have consistently shown that SRM could reduce global warming and many effects of climate change. [6] [7] [8] 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. [3]

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. [9] Few existing governance instruments and institutions are applicable, and 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. [10] : 1494  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 is receiving increasing attention. [11] This increased attention is reflected in increasing research funding, discussions among policy makers, and media coverage.

Context

Potential complementary responses to climate change: greenhouse gas emissions abatement, carbon dioxide removal, SRM, and adaptation. Potential complementarity responses to climate change.jpg
Potential complementary responses to climate change: greenhouse gas emissions abatement, carbon dioxide removal, SRM, and adaptation.

The interest in solar radiation modification (SRM) arises from ongoing global warming, increasing risks to both human and natural systems. [13]

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 may not be feasible. [14] [15] The 2024 UN Environment Programme (UNEP) Emissions Gap Report said that current policies would likely lead to 3.1°C global warming country’s commitments and pledges to reduce emissions would likely lead to 1.9°C warming. [16] : xviii 

SRM aims to increase Earth's brightness (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. [6] : 625 

Unlike emissions reduction or CDR, SRM could reduce global temperatures within months of deployment. [17] : vii  [7] : 14  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." [18] : 1489 

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 use. To counteract global warming, the report mentioned "deliberately bringing about countervailing climatic changes," including "raising the albedo, or reflectivity, of the Earth". [19] [20]

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. [21] [22] : 38 

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. [23] The first modeled results of and review article on SRM were published in 2000. [24] [25] 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. [26]

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

In the late 2010s, SRM was increasingly distinguished from carbon dioxide removal, and "geoengineering" and similar terms were used less often. [17] [32] : 550 

Atmospheric methods

The atmospheric methods for SRM include stratospheric aerosol injection (SAI), marine cloud brightening (MCB), and cirrus cloud thinning (CCT). [6] : 624 

Stratospheric aerosol injection (SAI)

Pinatubo eruption cloud: This volcano released large quantities of stratospheric sulfur aerosols, improving understanding of stratospheric aerosol injection (SAI). VulcanoPinatuboJune1991.gif
Pinatubo eruption cloud: This volcano released large quantities of stratospheric sulfur aerosols, improving understanding of stratospheric aerosol injection (SAI).

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." [2] : 350  This technique would replicate natural cooling phenomena observed following large volcano eruptions. [6] : 627 

Sulfates are the most commonly proposed aerosol due to their natural occurrence in volcanic eruptions. Alternative substances, including calcium carbonate and titanium dioxide have also been suggested. [6] : 624 

Custom-designed aircraft are considered the most feasible delivery method, with artillery and balloons occasionally proposed. [30]

SAI could produce up to 8 W/m² of negative radiative forcing. [6] : 624 

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." [8] : 21 

A key concern with SAI is its potential to delay the recovery of the ozone layer, depending on which aerosols are used. [8] : 21 

Marine cloud brightening (MCB)

This section is an excerpt from Marine cloud brightening.[ edit ]
The exhaust from ships already causes more and brighter clouds above the oceans. ShipTracks MODIS 2005may11.jpg
The exhaust from ships already causes more and brighter clouds above the oceans.

Marine cloud brightening also known as marine cloud seeding and marine cloud engineering is a proposed solar radiation management technique that would make clouds brighter, reflecting a small fraction of incoming sunlight back into space in order to offset global warming. Along with stratospheric aerosol injection, it is one of the two solar radiation management methods that may most feasibly have a substantial climate impact. [33] The intention is that increasing the Earth's albedo, in combination with greenhouse gas emissions reduction, would reduce climate change and its risks to people and the environment. If implemented, the cooling effect is expected to be felt rapidly and to be reversible on fairly short time scales. However, technical barriers remain to large-scale marine cloud brightening, and it could not offset all the current warming. [34] [35] There are also risks with such modification of complex climate systems.

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. [36] : 628  MCB could be implemented using fleets of unmanned rotor ships to disperse seawater mist into the air, increasing cloud albedo and reflecting more radiation. [37] : 43 

Cirrus cloud thinning (CCT)

Cirrus clouds merging into cirrocumulus clouds Cirrus fibratus and Cirrocumulus.jpg
Cirrus clouds merging into cirrocumulus clouds

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. [6] : 628 

Cirrus clouds generally have a net warming effect. By dispersing them through targeted interventions, CCT could enhance Earth's ability to radiate heat away. However, the method remains highly uncertain, as some studies suggest CCT could cause net warming rather than cooling due to complex cloud-aerosol interactions. [38]

This method is often grouped with SRM despite working primarily by increasing outgoing radiation rather than reducing incoming shortwave radiation. [6] : 624 

Other methods

Reflective surfaces

The IPCC describes surface-based albedo modification as "increase ocean albedo by creating microbubbles;... paint the roof of buildings white...; increase albedo of agriculture land, add reflective material to increase sea ice albedo." [6] : 624 

Surface-based approaches could be considered localized and would have limited global impact. [6] : 624  While urban cooling could be achieved through reflective roofs and pavement, large-scale desert albedo modification could significantly alter regional precipitation patterns. [6] : 629  Covering glaciers with reflective materials has been proposed to slow melting, though feasibility and effectiveness at scale remains uncertain. [6] : 629 

Space-based methods

Znamya-2, an orbital mirror experiment conducted by Russia in the 1990s. Znamya-2.jpg
Znamya-2, an orbital mirror experiment conducted by Russia in the 1990s.

Space-based SRM involves deploying mirrors, reflective particles, or shading structures at lower Earth orbit, geosynchronous orbit, or near the L1 Lagrange point between Earth and the Sun. Unlike atmospheric methods, space-based approaches would not directly interfere with Earth's climate systems.

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 future, costs and deployment challenges make them infeasible for near-term climate intervention. [27] [39]

Assessments conclude that space-based SRM is not feasible at reasonable costs. [30] : 12  The most recent IPCC Assessment Report (in 2021) did not consider these methods. [6]

Cost

SRM could have relatively low direct financial costs of deployment compared to the projected economic damages of unmitigated climate change. [10] : 1492, 1494  These costs could be on the order of billions to tens of billions of US dollars per degree of cooling. [7] : 36 

Stratospheric aerosol injection (SAI) is the most studied and has the most cost estimates. UNEP reported a cost of $18 billion per degree, [7] : 32  although individual studies have estimated that SAI deployment could cost between $5 billion to $10 billion per year. [40]

MCB could cost, according to UNEP, $1 to 2 billion per W/m2 of negative radiative forcing, [7] : 32  which implies $1.5 to 3 billion per degree.

Cirrus cloud thinning (CCT) is even less studied, and no formal cost estimates exist. [7] : 32 

Climatic and environmental effects

Projected end-of-century temperature (T) and precipitation (P) changes relative to preindustrial conditions for (a) representative concentration pathway RCP4.5 ("no geo"), and (b) RCP4.5 with SRM ("with geo") to reduce mean global warming to 1.5 degC. Projections of temperature and precipitation under moderate SRM.jpg
Projected end-of-century temperature (T) and precipitation (P) changes relative to preindustrial conditions for (a) representative concentration pathway RCP4.5 ("no geo"), and (b) RCP4.5 with SRM ("with geo") to reduce mean global warming to 1.5 °C.

Modelling studies have consistently concluded that moderate SRM use would significantly reduce many of the impacts of global warming, including changes to average and extreme temperature, extreme precipitation, Arctic and terrestrial ice, cyclone intensity and frequency , and the Atlantic Meridional Overturning Circulation. [6] : 625  SRM would take effect rapidly, unlike mitigation or carbon dioxide removal, making it the only known method to lower global temperatures within months. [41] : 14 

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." [6] : 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] : 15 

SRM could partially offset agricultural losses arising from climate change. [30] : 66  The CO₂ fertilization effect, which enhances plant growth under high CO₂ levels, would continue under SRM. Some studies indicate that SRM might improve crop yields, while others suggest that reducing overall sunlight could slightly decrease agricultural productivity. [42] [43]

Ecosystem impacts are less well understood. An EU report concluded "The potential effects on societies and especially ecosystems of SAI and SD are identified as a critical knowledge gap, with studies emphasising that the impacts and risks would vary based on the implementation scenario, geographic region and specific characteristics of ecosystems. SAI implementation may prevent some of the consequences of climate change on societies and ecosystems but it could also have unintended, and potentially unexpected, impacts." [30] : 65  Some studies suggest that SRM could prevent coral decline and mass bleaching events by reducing sea surface temperatures. [30] : 67  Terrestrial ecosystems could experience uncertain shifts in composition and plant productivity. [30] : 62, 65 

Environmental and physical uncertainties and risks

While climate models indicate 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. [6] : 624–625  Furthermore, 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. [6] : 625  This is because 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. [6] : 627 

Precipitation

Models indicate that SRM would reverse warming-induced changes to precipitation more effectively than changes to temperature. [6] : 625–626  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, [44] [ 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. [6] : 627 

The intensity of tropical monsoons is increased by climate change and would generally be decreased by SRM and especially SAI. [6] : 624  [45] : 458–459  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 averted heat. [45] : 458–459  Ultimately the impact would depend on the particular implementation regime. [6] : 625 

Effect on sky and clouds

SRM would change the ratio between direct and indirect solar radiation, affecting 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. [46] As a result, deployment of atmospheric SRM would affect the growth rates of plants, with the expected impact differing between canopy and subcanopy plants. [10] : 1491  [30] : 62–63, 66 

Uniformly reduced net shortwave radiation would reduce solar power, [30] : 61, 66  but the real-world impact would be complex.

Stratospheric ozone

SAI would affect stratospheric ozone, which protects organisms from harmful ultraviolet radiation, with the effect depending on the characteristics of deployment. [6] : 624, 627–628  [8] Sulfates, the most commonly proposed aerosol, would delay the current recovery of stratospheric ozone.

Failure to reduce ocean acidification

Change in sea surface pH caused by anthropogenic CO2 between the 1700s and the 1990s. This ocean acidification will still be a major problem unless atmospheric CO2 is reduced, and subsurface acidification will persist even afterwards. Estimated change in annual mean sea surface pH from 1770s-1990s (GLODAP).png
Change in sea surface pH caused by anthropogenic CO2 between the 1700s and the 1990s. This ocean acidification will still be a major problem unless atmospheric CO2 is reduced, and subsurface acidification will persist even afterwards.

SRM does not directly influence atmospheric carbon dioxide concentration and thus does not reduce ocean acidification. [10] : 1492  While not a risk of SRM per se, this indicates a critical limitation of relying on it to the exclusion of emissions reduction.

Uncertainty

Much uncertainty remains about some of SRM's likely effects. [6] : 624–625  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. [47] Volcanoes are an imperfect analogue as they release the material in the stratosphere in a single pulse, as opposed to sustained injection. [7] : 11 

Governance and policy issues

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. [48] Because international law is generally consensual, this creates a challenge of widespread participation being required. Key issues include who, if anyone, would control the deployment of SRM and under what governance regime the deployment could 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. [49] Other researchers have suggested that building a global agreement on SRM deployment would be very difficult, and speculated whether power blocs might emerge. [50] However, there may be significant incentives for states to cooperate in choosing a specific SRM policy, which make unilateral deployment unlikely. [51] [ obsolete source ]

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. [52] Likewise, blame for actual or perceived local negative impacts from SRM could be a source of international tensions. [53]

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. [54] 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". [55] Others argue that states will insist on maintaining control of SRM. [56] As of 2024 the United Nations Environment Assembly has been unable to agree a policy. [57] It has been suggested that the UN should ask some parties to the Environmental Modification Convention to meet to discuss SRM. [58]

Lessened climate change mitigation and moral hazard

A common concern is that the use of SRM, or even the idea, might reduce the political and social impetus for climate change mitigation. [59] In other words: that consideration of SRM may reduce ambition for emissions reduction. [60] This hypothesis is often called a moral hazard. [60] Some scientists have argued that 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. [60] The same authors also stated that whether moral hazard plays out should depend on how SRM is framed, e.g. as a panacea or get-out-of-jail card vs. a complementary measure.

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. [61] [62] [63] [64]

Deployment length

A modeling study in 2023 assessed the possible SRM deployment length in scenarios that are consistent with current 2030 emission targets and that use SRM to cool down to 1.5°C of warming. [65] Despite the scenarios having similar starting conditions in 2030, the results show that the range of possible deployment timescales is vast. This is because the evolution of mitigation under SRM after 2030, the availability of carbon removal technologies in the future and the effects of climate reversibility (the response of the climate to negative emissions) 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. Possibilities range from decades to multiple centuries. This is a knowledge gap that must be considered before any SRM proposal is seriously considered. [65]

For all scenario simulations that follow 2022 NDC (nationally determined contributions) median 2100 warming projections (2.4°C), none deploy SRM for a shorter period than 100 years. [65]

The direct climatic effects of SRM are reversible within short timescales. [17] Models project that SRM interventions would take effect rapidly, but would also quickly fade out if not sustained. [66] In 2024, U.S. government agencies were trying to create an airborne early warning system for detecting small concentrations of aerosols to determine where other countries might be carrying out geoengineering attempts, however it was not yet operational. [67]

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. [68] 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. [69] [70]

Advocacy

For the last few decades, calls for further SRM research have been controversial. An article in MIT Technology Review stated in 2017: "Few serious scientists would argue that we should begin deploying geoengineering anytime soon." [71]

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. [72]

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 well-known climate scientists, some of whom have endorsed one or both public letters that support further SRM research. [73] [74] For example, in a publication of 2025 James Hansen and others said "Research on purposeful global cooling should be pursued, as recommended by the U.S. National Academy of Sciences". [60] In their publication, they expressed a preference for the term purposeful global cooling rather than geoengineering.

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

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." [85] Another called for "balance in research and assessment of solar radiation modification" and was endorsed by about 150 experts, mostly scientists. [86]

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. [87] It works toward "changing the global environment in which SRM is evaluated, ensuring informed and confident representation from developing countries." [87]

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. [88]

SilverLining is an American organization that advances SRM research as part of "climate interventions to reduce near-term climate risks and impacts." [89] It is funded by "philanthropic foundations and individual donors focused on climate change". [89] [90] 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). [91]

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. [92] The Carnegie Climate Governance Initiative catalyzed governance of SRM and carbon dioxide removal, [93] although it ended operations in 2023.

The Climate Overshoot Commission is a group of global, eminent, and independent figures. [94] 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". [95] : 15 

Campaigners have claimed that the fossil fuels lobby advocates for SRM research. [96] [97] However, researchers have pointed out the lack of evidence in support of this claim. [98] Some academics say that privately funded research is less transparent than publicly financed research would be and have called for more information to be disclosed, such as who is ultimately funding the reseach and its publication and by how much. [99]

Opposition to deployment and research

Opposition to SRM has come from various academics and NGOs. [100] 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. [101]

The ETC Group, an environmental justice organization, has been a pioneer in opposing SRM research. [102] It was later joined by the Heinrich Böll Foundation [103] (affiliated with the German Green Party) and the Center for International Environmental Law. [104]

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. [105] [106] 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. [100] 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. [107] The academic paper launched a campaign which, as of December 2024, has been supported by nearly 540 academics [108] and 60 advocacy organizations [109] have endorsed the proposal.

Research funding

As of 2018, total research funding worldwide remained modest, at less than 10 million US dollars annually. [110] [ needs update ]Almost all research into SRM has to date consisted of computer modeling or laboratory tests, [111] and there are calls for more research funding as the science is poorly understood. [112] [28] : 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". [113]

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)". [83]

Government funding

Few countries have an explicit governmental position on SRM. Those that do, such as the United Kingdom [114] and Germany, [115] : 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". [115] : 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. [116] NOAA in the United States spent $22 million USD from 2019 to 2022, with only a few outdoor tests carried out. [117] As of 2024, NOAA provides about $11 million USD a year through their solar geoengineering research program. [91] As of 2025 the federal US government does not have a policy on SRM. [118]

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". [119] 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." [120] Successful applicants will be announced in 2025. [121]

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." [122] 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)". [123] Funding comes from the LAD Climate Fund. [124] [125]

Another example is Reflective, which is "a philanthropically-funded initiative focused on sunlight reflection research and technology development". [126] 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. [126]

Deployment

Legality

In 2024 the Scientific Advice Mechanism to the EU advised banning deployment, but continuing research and reviewing the ban every five to ten years. [127] Although there is no United States federal law against deployment, it is explicitly banned in the state of Tennessee. [128]

Private sector

At least one startup in the private sector has tried to sell "cooling credits" for SRM activities. Make Sunsets [129] launches balloons containing helium and sulfur dioxide. The company sells cooling credits, making the contested[ citation needed ] claim that each US$10 credit would offset the warming effect of one ton of carbon dioxide warming for a year. [130] 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. [131] Even people who advocate for more research into SRM have criticized Make Sunsets' undertaking. [72]

Public awareness and opinions

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. [132] [133] 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. [134] [135] [136]

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. [137]

There is disinformation and unfounded conspiracy theries, such as chemtrails. [138] A publication by climate scientist James Hansen and others in 2025 has called for the public to have better understanding of SRM related options. The authors also called for better understanding of the risks and rewards of SRM, versus scenarios where no such purposeful cooling occurred. [60]

See also

References

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