Solar radiation modification (SRM) (or solar geoengineering or solar radiation management), is a group of large-scale approaches to reduce 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,[1]: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,[2]:1489 but would not be a substitute for reducing greenhouse gas emissions.[3] SRM is a form of climate engineering or geoengineering, and might be able to prevent some kinds of tipping.[4]
Scientific studies, based on evidence from climate models, have consistently shown that SRM could reduce global warming and many effects of climate change.[5][6][7] However, because warming from greenhouse gases and cooling from SRM would operate differently across latitudes and seasons, a world where global warming would be reduced 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]:1491–1492
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.[8] 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.[2]: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.[9] This increased attention is reflected in increasing research funding, discussions among policy makers, and media coverage.
Other names for SRM include sunlight reflection methods, solar climate engineering, and albedo modification.
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.[5]:625
Unlike emissions reduction or CDR, SRM could reduce global temperatures within months of deployment.[15]:vii[6]: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 IPCCSixth 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]:1489
Global dimming provides both evidence of SRM's potential efficacy and further urgency of human-caused climate change. Industrial processes have increased the quantity of aerosols in the troposphere, or lower atmosphere. This has cooled the planet, offsetting some global warming,[5]:855–857 caused by the aerosol's reflectivity (the basis for stratospheric aerosol injection) and by increasing' clouds' reflectivity (the basis for marine cloud brightening).[5]:860–861 As regulation has reduced tropospheric aerosols, global dimming has decreased and the planet has warmed at a faster rate.[5]:851–853
In 1974, Russian climatologistMikhail 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.[18][19]: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.[20] The first modeled results of and review article on SRM were published in 2000.[21][22] In 2006, Nobel LaureatePaul 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.[23]
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.[25][1]:550
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."[1]:350 This technique would replicate natural cooling phenomena observed following large volcano eruptions.[5]: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.[5]:624
Custom-designed aircraft are considered the most feasible delivery method, with artillery and balloons occasionally proposed.[27]
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."[7]:21
A key concern with SAI is its potential to delay the recovery of the ozone layer, depending on which aerosols are used.[7]:21
Exhaust from ships causing more and brighter clouds above the ocean in 2006.
Marine cloud brightening (MCB), also known as marine cloud seeding or marine cloud engineering, may be a way to make stratocumulus clouds over the sea brighter, thus reflecting more sunlight back into space in order to limit global warming. It is one of two such methods that might feasibly have a substantial climate impact, but is lower in the atmosphere than stratospheric aerosol injection.[29] It may be able to keep local areas from overheating. If used on a large scale it might reduce the Earth's albedo; and so, in combination with greenhouse gas emissions reduction, limit climate change and its risks to people and the environment. If implemented, the cooling effect would be 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.[30][31] As clouds are complicated and poorly understood, the risks of marine cloud brightening are unclear as of 2025.
Very small droplets of sea water are sprayed into the air to increase cloud reflectivity. The fine particles of sea salt enhance cloud condensation nuclei, making more cloud droplets so making the clouds more reflective.[32][33]:628 MCB could be implemented using fleets of unmanned rotor ships to disperse seawater mist into the air.[34]:43 Data from small scale field tests on the Great Barrier Reef in 2024 is being analysed.[35]
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.[36]
This method is often grouped with SRM despite working primarily by increasing outgoing radiation rather than reducing incoming shortwave radiation.[5]:624
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."[5]:624
Surface-based approaches could be considered localized and would have limited global impact.[5]:624 While urban cooling could be achieved through reflective roofs and pavement, large-scale desert albedo modification could significantly alter regional precipitation patterns.[5]:629 Covering glaciers with reflective materials has been proposed to slow melting, though feasibility and effectiveness at scale remains uncertain.[5]:629
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.[24][27]
Assessments conclude that space-based SRM is not feasible at reasonable costs.[27]:12 The most recent IPCC Assessment Report (in 2021) did not consider these methods.[5]
Cost
SRM could have relatively low direct financial costs of deployment compared to the projected economic damages of unmitigated climate change.[2]:1492,1494 These costs could be on the order of billions to tens of billions of US dollars per degree of cooling.[6]:36
Stratospheric aerosol injection (SAI) is the most studied and has the most cost estimates. UNEP reported a cost of $18 billion per degree,[6]:32 although individual studies have estimated that SAI deployment could cost between $5 billion to $10 billion per year.[37]
MCB could cost, according to UNEP, $1 to 2 billion per W/m2 of negative radiative forcing,[6]:32 which implies $1.5 to 3 billion per degree.
Cirrus cloud thinning (CCT) is even less studied, and no formal cost estimates exist.[6]:32
Potential for reducing climate change
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.[5]:625 SRM would take effect rapidly, unlike mitigation or carbon dioxide removal, making it the only known method to lower global temperatures within months.[6]:14
The IPCCSixth 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."[5]:69
SRM could partially offset agricultural losses arising from climate change.[27]: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.[38][39]
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.[5]: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.[5]:627
Precipitation
Models indicate that SRM would reverse warming-induced changes to precipitation more effectively than changes to temperature.[5]: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,[40][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.[5]:627
The intensity of tropical monsoons is increased by climate change and would generally be decreased by SRM and especially SAI.[5]:624[41]: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.[41]:458–459 Ultimately the impact would depend on the particular implementation regime.[5]: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.[42] As a result, deployment of atmospheric SRM would affect the growth rates of plants, with the expected impact differing between canopy and subcanopy plants.[2]:1491[27]:62–63,66
Uniformly reduced net shortwave radiation would reduce solar power,[27]: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.[5]:624,627–628[7] 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 anthropogenicCO2 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.[2]:1492 While not a risk of SRM per se, this indicates a critical limitation of relying on it to the exclusion of emissions reduction.
Climate model uncertainties
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.[5]:624–625 Therefore, much uncertainty remains about some of SRM's likely effects.[5]: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.[43] Volcanoes are an imperfect analogue as they release the material in the stratosphere in a single pulse, as opposed to sustained injection.[6]:11
Risks to ecosystems
A 2023 UNEP report concluded that while an operational SRM deployment could reduce some climate hazards it would also introduce new risks to ecosystems and human societies.[6]:15
Ecosystem impacts are not yet 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."[27]:65 Terrestrial ecosystems could experience uncertain shifts in composition and plant productivity.[27]:62,65
Potential governance challenges
SRM raises a variety of governance issues. The IPCC lists these potential objectives of SRM governance:
(i) Guard against potential risks and harm; (ii) Enable appropriate research and development of scientific knowledge; (iii) Legitimise any future research or policymaking through active and informed public and expert community engagement; (iv) Ensure that SRM is considered only as a part of a broader, mitigation-centred portfolio of responses to climate change.[2]:1494
Displacement of mitigation
A common concern regarding SRM research and potential deployment is that it might reduce political and social momentum for climate change mitigation, especially the reduction of greenhouse gas emissions.[2]:1493 This hypothesis is often called "moral hazard." The likelihood and significance of moral hazard effects remain uncertain and contested among experts. Some have argued that this is unlikely and--even if true--is not a compelling reason to forgo researching and evaluating SRM if it could greatly reduce global warming and its impacts,[44] while others see the prospect as a reason to not pursue SRM.[45] Empirical evidence from game-theoretic modeling, opinion surveys, and behavioral experiments inconclusive.[27]:99 A recent review article calls evidence for mitigation displacement "weak" but notes that these research methods fail to account for "the precise concern that real political decisions under interest-group mobilization will cut emissions too little in the presence of SRM."[46]:355
Decisions whether to use
Another common concern with SRM is that, because its high leverage, low apparent direct costs (at least of SAI), and technical feasibility as well as issues of power and jurisdiction suggest that uni- or minilateral use is possible, without international agreement or sufficient understanding of its expected effects.[2]:1494–1495 A key issue is under what governance regime(s) the use could be controlled, monitored, and supervised. Yet 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 create international tensions, especially if local harms were perceived.[2]:1494 Experts diverge on whether uni- or minilateral use is likely and whether effective governance would be feasible[47][48][49] and on whether nonstate actors could deploy SRM at a significant scale.[50][51]
This is further complicated in two important ways. First, since SRM technologies are still emerging, there is a concern that premature regulations might be either "too restrictive or too permissive," failing to adapt adequately to future political, technological, or geophysical developments.[2]:1494 Second, because international law is generally consensual, any governance regime would need to particularly engage and secure cooperation from countries that perceive themselves as potential users of SRM.[27]:153
Termination
If SRM were masking significant warming and abruptly ceased without resumption within a short period (roughly a year), the climate would rapidly warm toward levels that would have existed without SRM, a phenomenon sometimes call "termination shock."[2]:1493 A sudden and sustained termination of SRM in a world of atmospheric high greenhouse-gas concentrations would trigger rapid glo{{bal temperature rise, intensified precipitation changes, sea level rise, land drying, weakened carbon sinks, and accelerated CO₂ accumulation.[5]:629 The IPCC notes that a gradual phase-out of SRM combined with mitigation would reduce the impacts of SRM's termination.[5]:629 Furthermore, some scholars argue that this risk might be manageable, as states would have strong incentives to resume deployment if necessary, and maintaining backup SRM infrastructure could enhance system resilience and provide a buffer against abrupt cessation.[52][53]
Deployment length
A large-scale deployment of SRM would likely require a multi-decade to century-long commitment to maintain its intended climate effects.[6]:8–10[27]:14 This may be necessary to achieve sustained cooling, particularly as greenhouse-gas concentrations continue to rise due to continued net emissions and carbon dioxide's long atmospheric lifetime.
Existing governance
There is currently no dedicated, formal law specifically governing SRM research, development, or deployment, though certain multilateral agreements, rules of customary international law, national and European laws, and nonbinding legal documents contain provisions that may be applicable to some SRM activities.[2]:1493,1495
Binding international law
The UN Framework Convention on Climate Change and its related treaties do not address SRM, though it could be considered within the framework of the Paris Agreement’s goal to limit global warming to well below 2°C, with efforts to stay within 1.5°C.[27]:163 While the UNFCCC is founded on the precautionary principle,[6]:137 its specific implications for SRM remain uncertain.[27]:1636–167
The UN Convention on the Law of the Sea could support SRM research by permitting legitimate scientific activities and encouraging studies that assess SRM’s effects on the marine environment. Its provisions to protect the marine environment may justify SRM research aimed at mitigating climate impacts on oceans, such as efforts to reduce warming or protect coral reefs. However, UNCLOS could also impose constraints on large-scale outdoor activities, particularly if activities under a state’s jurisdiction risk polluting or harming marine ecosystems. Additionally, because SRM does not directly address ocean acidification, its alignment with UNCLOS' environmental protection objectives remains uncertain.[15]:101–102
The Environmental Modification Convention is the only international treaty that directly regulates deliberate manipulation of natural processes with "widespread, long-lasting or severe effects" of a transboundary nature. SRM falls within ENMOD’s definition of environmental modification techniques and is therefore subject to its prohibition on military or hostile use. At the same time, the treaty states that it "shall not hinder the use of environmental modification techniques for peaceful purposes." ENMOD also encourages the exchange of information and international cooperation on peaceful environmental modification, with parties "in a position to do so" expected to support scientific and economic collaboration.[27]:162
The Vienna Convention for the Protection of the Ozone Layer and its Montreal Protocol obligate parties to take measures to reduce or prevent human activities that could have harmful effects from modifying the ozone layer, which some forms of SAI might have. Article 2 specifically requires states to cooperate to "protect human health and the environment against adverse effects resulting or likely to result from human activities which modify or are likely to modify the ozone layer."[27]:162
The rule of prevention of transboundary harm under customary international law obligates states to prevent significant transboundary environmental harm and to reduce the risks thereof. This rule would be relevant to large-scale outdoor SRM activities, if they were to present risk of causing significant transboundary harm on human health, ecosystems, or the climate system. Under this rule, states must exercise due diligence to prevent significant transboundary environmental harm by conducting environmental impact assessments, notifying and consulting affected states, and cooperating in good faith to mitigate risks. Failure to meet these obligations could result in state responsibility for harm caused by activities within their jurisdiction. Scholars have debated whether SRM research and deployment should be held to different legal standards. Furthermore, international cooperation obligations may require states to collaborate on impact assessments, data sharing, and governance mechanisms.[27]:156–161
Nonbinding international law
The International Law Commission developed draft guidelines for the protection of the atmosphere. One guidelines state, in its entirety:
Activities aimed at intentional large-scale modification of the atmosphere should only be conducted with prudence and caution, and subject to any applicable rules of international law, including those relating to environmental impact assessment.[54]
The Conference of Parties to the Convention on Biological Diversity have made several decisions regarding "climate related geoengineering," which would include SRM. That of 2010 established "a comprehensive non-binding normative framework"[55]:106 for "climate-related geoengineering activities that may affect biodiversity," requesting that such activities be justified by the need to gather specific scientific data, undergo prior environmental assessment, be subject to effective regulatory oversight.[15]:96–97[27]:161–162 The Parties' 2016 decision called for "more transdisciplinary research and sharing of knowledge... in order to better understand the impacts of climate-related geoengineering."[27]:161-162[56]
National and subnational law
As with international law, existing areas of national and subnational law—such as environmental regulation, tort liability, and intellectual property—would govern certain aspects of SRM. For example, in the US,[15]:91–96 under the National Environmental Protection Act and similar state laws, federally sponsored or authorized outdoor SRM research may require environmental review if it poses risk of significant physical impacts, though small-scale experiments are often exempt. Several federal regulatory statutes, including the Clean Air Act, Clean Water Act, Ocean Dumping Act, and Federal Aviation Administration rules, may apply to SRM field experiments depending on their design, particularly regarding emissions into air or water and the use of aircraft. Outdoor experiments could also expose researchers to tort liability under state common law theories such as negligence, strict liability, or nuisance, though plaintiffs may face challenges in proving causation and demonstrating that potential harms outweigh societal benefits. Intellectual property law, particularly patent rights, may influence the development of SRM technologies by incentivizing innovation while potentially limiting access, although current patent activity in the field remains limited.
The Ministry of Environment and Natural Resources of Mexico announced in 2023 that it would prohibit SRM experiments in that country.[57]
In 2025, several US states implemented or are considering prohibitions on "geoengineering." However, these are aimed not at SRM per se but at purported chemtrails or weather modification.[58]
Guidelines and principles
Groups of academics, research networks, and the broader SRM research community have developed multiple sets of principles or guidelines to help govern SRM activities.[15]:106[27]:134 For example, the Oxford Principles (which address SRM and carbon dioxide removal as "geoengineering") are the most prominent:[26]:21
Geoengineering to be regulated as a public good;
Public participation in geoengineering decision making;
Disclosure of geoengineering research and open publication of results;
More recently, the American Geophysical Union issued an ethical framework for researching "climate intervention" (again, SRM and carbon dioxide removal).[60][61]
Advocacy for research
An article in MIT Technology Review stated in 2017: "Few serious scientists would argue that we should begin deploying geoengineering anytime soon."[62]
Support for SRM research has come from scientists, NGOs, international organizations, 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.[63][64] 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".[44]
Scientific and other large organizations that have called for further research on SRM include:
In the European Union: the Group of Chief Scientific Advisors[28] (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."[74] Another called for "balance in research and assessment of solar radiation modification" and was endorsed by about 150 experts, mostly scientists.[75]
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.[78] It works toward "changing the global environment in which SRM is evaluated, ensuring informed and confident representation from developing countries."[78]
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.[79]
SilverLining is an American organization that advances SRM research as part of "climate interventions to reduce near-term climate risks and impacts."[80] It is funded by "philanthropic foundations and individual donors focused on climate change".[80][81] 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).[82]
The Alliance for Just Deliberation on Solar Geoengineering advances "just and inclusive deliberation" regarding SRM, in particular by engaging civil society organizations in the Global South and supporting a broader conversation on SRM governance.[83] The Carnegie Climate Governance Initiative catalyzed governance of SRM and carbon dioxide removal,[84] although it ended operations in 2023.
The Climate Overshoot Commission is a group of global, eminent, and independent figures.[85] 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".[86]:15
Campaigners have claimed that the fossil fuels lobby advocates for SRM research.[87][88] However, researchers have pointed out the lack of evidence in support of this claim.[89]
Opposition to deployment and research
Opposition to SRM has come from various academics, NGOs.[49] and--since 2024--U.S. Republicans.[90][91][92][93] 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.[94]
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.[98][99] 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.[49] 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.[100] The academic paper launched a campaign to call for an International Non-Use Agreement on Solar Geoengineering which, as of December 2024, has been supported by nearly 540 academics[101] and 60 advocacy organizations[102] have endorsed the proposal.
Opposition in U.S. state legislatures
Since 2024, and especially following the re-election of Donald Trump as U.S. President in November of that year, Republican lawmakers have introduced or supported bills in multiple U.S. states to prohibit “geoengineering” or related practices.[90][91][92][93] While many of these efforts are or appear to be motivated by the chemtrails conspiracy theory, the legislation often targets SRM or weather modification.
In January 2023, Robert F. Kennedy Jr. stated that “geoengineering, and particularly as it's related to climate… is probably as dangerous to us as climate change itself.”[103] After being appointed Secretary of Health and Human Services in the Trump administration, Kennedy posted on X: “24 States move to ban geoengineering our climate by dousing our citizens, our waterways and landscapes with toxins. This is a movement every MAHA [Make America Healthy Again] needs to support. HHS will do its part.”[92] In April 2025, Governor Ron DeSantis endorsed a bill to ban SRM and weather modification in Florida.[104]
Research funding
As of 2018, total research funding worldwide remained modest, at less than 10 million US dollars annually.[105][needs update]Almost all research into SRM has to date consisted of computer modeling or laboratory tests,[106] and there are calls for more research funding as the science is poorly understood.[107][15]: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".[108]
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)".[73]
Government funding
Few countries have an explicit governmental position on SRM. Those that do, such as the United Kingdom[109] and Germany,[110]: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".[110]: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.[105]NOAA in the United States spent $22 million USD from 2019 to 2022, with only a few outdoor tests carried out.[111] As of 2024, NOAA provides about $11 million USD a year through their solar geoengineering research program.[82] As of 2025 the federal US government does not have a policy on SRM.[112]
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".[113] 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."[114] Successful applicants will be announced in 2025.[115]
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."[116] The article listed the following billionaires as being notable geoengineering research supporters: Mike Schroepfer, Sam Altman, Matt Cohler, Rachel Pritzker, Bill Gates, Dustin Moskovitz.[116]
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)".[117] Funding comes from the LAD Climate Fund.[118][119]
Another example is Reflective, which is "a philanthropically-funded initiative focused on sunlight reflection research and technology development".[120] 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.[120]
Society and culture
Commercial actors
Make Sunsets[121] is a private startup that sells "cooling credits" for its small-scale SRM activities, claiming that each US$10 credit offsets the warming effect of one ton of carbon dioxide for a year.[122] The firm releases balloons containing helium and sulfur dioxide. Make Sunsets conducted some of its first activities in Mexico, causing the Mexican government announced its intention to prohibit SRM experiments within its borders.[123] Even those who advocate for more research into SRM criticize Make Sunsets' undertaking.[124]
Public awareness and opinions
Overall, public opinion on SRM is nascent, ambivalent, and context-dependent, with greater support for research than for deployment.[27]:100 Public awareness of SRM remains low globally, with 75–80% of respondents in recent multi-country surveys reporting little to no familiarity.[27]:96 Despite this, social science research on public attitudes toward SRM is growing and diversifying, although the UK, US, and Germany still dominate the existing academic literature.[27]:92 Public opinion in the Global South remains less well examined, though several studies thus far consistently find greater openness to SRM there, where climate impacts are perceived as more immediate.[27]:100-101 Methodologically, research has shifted toward large-scale surveys, but concerns remain about the durability of preferences given low baseline knowledge.[27]:98
Across studies, public views are shaped by values, perceived climate risk, and how SRM is framed. Common concerns include the fear of displacing mitigation, the unnaturalness of intervening in climate systems, justice and equity, and a desire to inform and consult with the public prior to use.[27]:99-100 SRM is generally viewed less favorably than greenhouse-gas emissions reduction and carbon dioxide removal.[27]:99Europeans tend to be more averse, especially in central and northern countries (e.g. Germany, Austria, Switzerland), while southern European and Global South populations are more accepting, particularly when facing high climate vulnerability.[27]:98 Some studies also highlight links between SRM and conspiracy theories, such as chemtrails, which can further complicate public understanding.[27]:100
The claim has been dismissed by the scientific community.[130] There is no evidence that purported chemtrails differ from normal water-based contrails routinely left by high-flying aircraft under certain atmospheric conditions.[131] Proponents have tried to prove that chemical spraying occurs, but their analyses have been flawed or based on misconceptions.[132][133] Because of the conspiracy theory's persistence and questions about government involvement, scientists and government agencies around the world have repeatedly explained that the supposed chemtrails are in fact normal contrails.[127][134][135]
The term 'chemtrail' is a portmanteau of the words 'chemical' and 'trail', just as 'contrail' blends 'condensation' and 'trail'.[136]
↑ APRI (11 November 2024). "The justice and governance of solar geoengineering: Charting the path at COP29 and beyond". APRI. Archived from the original on 16 January 2025. Retrieved 13 February 2025. Across several major powers and international forums, the growing momentum in solar geoengineering technologies, assessments, and research and development is raising urgent ethical, justice and human and environmental rights issues that need to be addressed.
↑ Emissions Gap Report 2024 (Report). UN environment programme. 17 October 2024. p.32. As table 4.3 shows, a continuation of the current NDC scenarios would result in an increase in the emissions gap in 2035 of 4 GtCO2e for a 2°C warming limit, and 7 GtCO2e for a 1.5°C limit, whereas a continuation of the mitigation effort implied by current policies would lead to an even wider gap in 2035.
↑ President’s Science Advisory Committee, Environmental Pollution Panel (1 November 1965). Restoring the Quality of Our Environment. Washington: U.S. Government Printing Office.{{cite book}}: CS1 maint: date and year (link)
1 2 3 Royal Society of London, ed. (2009). Geoengineering the climate: Science, governance and uncertainty. London. ISBN978-0-85403-773-5.{{cite book}}: CS1 maint: location missing publisher (link)
1 2 European Commission: Directorate-General for Research and Innovation and Group of Chief Scientific Advisors (2024). Solar radiation modification. Publications Office of the European Union.
↑ Committee on Geoengineering Climate: Technical Evaluation and Discussion of Impacts; Board on Atmospheric Sciences and Climate; Ocean Studies Board; Division on Earth and Life Studies; National Research Council (2015). Climate Intervention: Reflecting Sunlight to Cool Earth. National Academies Press. ISBN978-0-309-31482-4. Archived from the original on 14 December 2019. Retrieved 21 October 2016.
↑ Storelvmo, T.; Herger, N. (2014). "Cirrus cloud seeding has potential to cool climate". Geophysical Research Letters. 41 (20): 2014GL061652. doi:10.1002/2014GL061652 (inactive 9 February 2025).{{cite journal}}: CS1 maint: DOI inactive as of February 2025 (link)
↑ Mann, Michael E. (2021). The new climate war: the fight to take back our planet (Firsted.). Melbourne London: Scribe Publications. ISBN978-1-5417-5823-0.
↑ Geoengineering in relation to the Convention on Biological Diversity. CBD technical series. Montreal: Secretariat of the Convention on Biological Diversity. 2012. ISBN978-92-9225-429-2.
↑ "Responsible Climate Interventions (RCI)". Global Systems Institute. Retrieved 17 February 2025. Solar Radiation Modification (SRM) … Examples … include brightening clouds over the ocean and injecting reflective aerosols, or their gaseous precursors, into the stratosphere. Researchers at the University of Exeter use state-of-the-art climate models to evaluate these methods and their impact on the climate system… GeoEngineering Model Intercomparison Project (GeoMIP), more than 35 UoE peer-reviewed papers and lead author contributions to recent Montreal Protocol, UN Environment Program and World Climate Research Programme reports..
↑ de la Garza, Alejandro (21 February 2023). "Exclusive: Inside a Controversial Startup's Risky Attempt to Control Our Climate". "'Skeptics of solar geoengineering experimentation as well as proponents are rarely unified,' says Kevin Surprise, a lecturer on environmental studies at Mount Holyoke College. "I have not seen a single person in the field say this is a good idea.'"
1 2 Fraser, Stephen (2009). "Phantom menace? Are conspirators using aircraft to pollute the sky?". Current Science. 94 (14): 8–9. ProQuest195877531. Some theorists speculate that the goal is population control; some say it's climate modification; others say it's military weapons testing.
1 2 3 Watson, Traci (7 March 2001). "Conspiracy theories find menace in contrails". USA Today. p.A.04. Archived from the original on 17 December 2012. Retrieved 11 August 2021. Exasperated by persistent questions, the Environmental Protection Agency, NASA, the Federal Aviation Administration, and the National Oceanic and Atmospheric Administration joined forces last fall to publish a fact sheet explaining the science of contrail formation. A few months earlier, the Air Force had put out its own fact sheet, which tries to refute its opponents' arguments point by point. 'If you try to pin these people down and refute things, it's, Well, you're just part of the conspiracy, says atmospheric scientist Patrick Minnis of NASA's Langley Research Center in Hampton, Va. 'Logic is not exactly a real selling point for most of them.'
↑ James, Nigel (2003). "Contrails". In Knight, Peter (ed.). Conspiracy Theories in American History: An Encyclopedia. ABC-CLIO. pp.197–199. ISBN978-1-57607-812-9. there are no books on the subject to date. Reports on contrails are carried by dedicated websites...Mainstream news agencies rarely report on concerns over contrails, and when they do it is in terms of anti-government "paranoia". When USA Today ran a contrail story it likened the story to something out of The X-Files, arguing that it was only those who are suspicious of the government who believe that lines in the sky are evidence of malfeasance. Some suggested that they are trying to slow down global warming with compounds that reflect sunlight into the sky.
↑ Cama, Timothy (13 March 2015). "EPA confronts 'chemtrails' conspiracy talk". The Hill. Retrieved 10 December 2016. Conspiracy theorists say that government officials or others are using jets to spray harmful chemicals into the atmosphere. They cite the contrails left by jets as evidence of the chemicals. The EPA has added a new notice to its website, which links to a fact sheet explaining that the trails left by jets in the atmosphere are only ice particles and contain no harmful chemicals. "Contrails are line-shaped clouds or 'Condensation trails' composed of ice particles that are visible behind jet aircraft engines under certain atmospheric conditions and at times can persist", says the notice, posted to the EPA's website Friday. "EPA is not aware of any deliberate actions to release chemical or biological agents into the atmosphere". Theorists have posited that the chemicals are meant to control the climate, harm humans, or kill them. The fact sheet from the EPA and other federal agencies like the Federal Aviation Administration and the National Oceanic and Atmospheric Administration was first published in 2000 when the chemtrails conspiracy became popular on the Web. An EPA spokeswoman said the agency frequently receives questions about chemtrails.
↑ Smith, Oliver (24 September 2013). "'Chemtrails' and other aviation conspiracy theories". The Telegraph. Archived from the original on 11 January 2022. Retrieved 11 December 2016. So persistent is the chemtrail theory that US government agencies regularly receive calls from irate citizens demanding an explanation...The conspiracy theory took root in the Nineties, with the publication of a US Air Force research paper about weather modification ... Governments and scientific institutions have of course dismissed the theories, and claim those vapor trails which persist for longer than usual or disperse to cover a wide area, are just normal contrails.
↑ "chemtrail". Oxford English Dictionary (Thirded.). Oxford University Press. December 2011.
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