Solar radiation modification (SRM), also called solar geoengineering, is a set of large-scale methods used to reduce global warming by reflecting more sunlight back into space. It is not meant to replace efforts to cut greenhouse gas emissions but instead to support them as a possible way to slow global warming. SRM is a type of geoengineering.
The most studied SRM method is stratospheric aerosol injection (SAI), which involves adding tiny reflective particles to the upper atmosphere to reflect sunlight. Other methods include marine cloud brightening (MCB), which would make ocean clouds more reflective, or building a space sunshade or mirror to block some sunlight from reaching Earth.
Climate models show that SRM could help reduce global warming and some effects of climate change, including certain potential climate tipping points. However, its effects would differ by region and season, and the climate after using SRM would not be the same as a climate that never had warming. Scientists still have limited understanding of these regional effects, including possible environmental risks and side effects.
SRM also raises complex political, social, and ethical questions. Some people are worried that developing SRM might reduce the urgency of cutting emissions. Its low cost and technical feasibility suggest it could, in theory, be used by one country alone, leading to concerns about how it should be governed internationally. No complete global system exists to manage SRM research or use.
Interest in SRM has increased in recent years because of ongoing global warming and slow progress in reducing emissions. This has led to more scientific studies, policy discussions, and public debates, even though SRM remains controversial.
SRM is also called sunlight reflection methods, solar climate engineering, albedo modification, and solar radiation management.
Context
Interest in solar radiation modification (SRM) comes from the ongoing rise in Earth's temperature and the growing dangers it poses to people and nature.
In theory, reducing emissions and removing carbon dioxide from the atmosphere could stop global warming. However, efforts to cut emissions have not met goals, and large-scale carbon dioxide removal might not be possible. The 2024 UN Environment Programme (UNEP) Emissions Gap Report stated that current policies could lead to 3.1 °C of warming. If countries follow their current plans to reduce emissions, warming could reach 1.9 °C.
SRM works by making Earth brighter (increasing its reflectiveness) to reflect more sunlight. A 1% increase in Earth's reflectiveness could reduce heat trapped by the planet by 2.35 W/m², which would mostly counteract warming caused by current greenhouse gas levels. A 2% increase could balance the warming effect of doubling carbon dioxide in the atmosphere.
Unlike reducing emissions or removing carbon dioxide, SRM could lower global temperatures quickly, within months of being used. This fast action might help avoid the worst effects of climate change while emissions reductions and carbon dioxide removal are expanded. However, SRM would not remove carbon dioxide from the air, so problems like ocean acidification would continue.
The IPCC Sixth Assessment Report says SRM is not a replacement for reducing emissions or removing carbon dioxide. It states: "Most scientists agree that SRM cannot be the main way to address climate change risks. It can only help as a supplement to achieving long-term net zero or net negative carbon emissions globally."
Global dimming shows both the possible benefits of SRM and the urgency of human-caused climate change. Industrial activities have increased the number of tiny particles (aerosols) in the lower atmosphere, which has cooled Earth by reflecting sunlight. This cooling has partially offset warming from these particles and from increased cloud reflectiveness. As regulations have reduced these aerosols, global dimming has decreased, and Earth has warmed faster.
History
In 1965, during the time when U.S. President Lyndon B. Johnson was in office, the President's Science Advisory Committee released a report titled Restoring the Quality of Our Environment. This was the first report to warn about the dangers of carbon dioxide emissions from burning fossil fuels. To help reduce the effects of global warming, the report suggested "intentionally causing changes that would balance out the warming," such as increasing the Earth's albedo, or reflectivity.
In 1974, a Russian scientist named Mikhail Budyko proposed that if global warming became a major problem, it could be addressed by spreading aerosols into the stratosphere. He suggested that airplanes burning sulfur could create aerosols that would reflect sunlight away from Earth, helping to cool the planet.
In 1992, the U.S. National Academies included solar radiation management (SRM) in a broader discussion about geoengineering in a climate change report. The first studies and reviews about SRM were published in 2000. In 2006, Nobel Prize winner Paul Crutzen wrote a paper stating that, because efforts to reduce greenhouse gas emissions had not been enough, research on SRM's feasibility and effects should not be ignored.
Important reports that examined the possible advantages and risks of SRM were published by various groups.
In the late 2010s, SRM was treated as a separate idea from methods that remove carbon dioxide from the atmosphere. Terms like "geoengineering" were used less often during this time.
Methods
Stratospheric aerosol injection (SAI) involves introducing tiny particles into the upper atmosphere to reflect sunlight and reduce global warming. Among solar radiation management (SRM) methods, SAI has received the most continued interest. The Intergovernmental Panel on Climate Change (IPCC) stated in 2021 that SAI is the most-researched SRM method, with strong agreement that it could help limit warming to below 1.5 °C. This technique mimics natural cooling effects seen after major volcanic eruptions.
Sulfates are the most commonly suggested aerosol for SAI because they occur naturally during volcanic eruptions. Other substances, such as calcium carbonate and titanium dioxide, have also been proposed.
Custom-designed aircraft are considered the most practical way to deliver these particles, though artillery and balloons have occasionally been suggested.
SAI could create up to 8 W/m² of negative radiative forcing.
The World Meteorological Organization's 2022 Scientific Assessment of Ozone Depletion noted that SAI might help limit rising global temperatures by increasing particle levels in the stratosphere. However, it also warned that SAI carries significant risks and could lead to unintended consequences.
A major concern with SAI is that it might slow the recovery of the ozone layer, depending on the type of aerosols used.
Marine cloud brightening (MCB), also called marine cloud seeding or marine cloud engineering, involves making stratocumulus clouds over the ocean brighter to reflect more sunlight back into space, reducing global warming. It is one of two SRM methods that could have a meaningful climate impact, though it operates in the lower atmosphere compared to SAI. MCB might help cool specific areas and, if used on a large scale, could increase Earth's albedo (reflectivity), helping to limit climate change when combined with efforts to reduce greenhouse gas emissions. The cooling effect of MCB would likely occur quickly and be reversible in a short time. However, technical challenges remain for large-scale use, and MCB cannot fully counteract current warming. Since clouds are complex and not fully understood, the risks of MCB are still unclear as of 2025.
Tiny seawater droplets are sprayed into the air to make clouds more reflective. Sea salt particles help form more cloud droplets, increasing cloud brightness. MCB could be carried out using fleets of unmanned ships to release seawater mist. Small tests were conducted on the Great Barrier Reef in 2024.
Cirrus cloud thinning (CCT) involves seeding cirrus clouds to make them less thick and shorter-lived, allowing more heat to escape into space. Cirrus clouds generally trap heat, so reducing their thickness might help Earth release more heat. However, this method is uncertain because some studies suggest CCT could actually increase warming due to complex interactions between clouds and aerosols.
CCT is often grouped with SRM even though it primarily works by increasing outgoing heat rather than reducing incoming sunlight.
The IPCC describes surface-based albedo modification as methods to increase reflectivity, such as creating microbubbles in oceans, painting building roofs white, and using reflective materials on farmland or sea ice.
Surface-based approaches might have limited global effects but could help cool cities through reflective surfaces. Large-scale efforts to increase desert reflectivity might change regional rainfall patterns. Covering glaciers with reflective materials has been proposed to slow melting, though its effectiveness at a large scale is uncertain.
Space-based SRM involves placing mirrors, reflective particles, or shading structures in space, such as in 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, ideas have included orbiting mirrors, space dust clouds, and electromagnetic reflectors. The Royal Society (2009) and later studies concluded that while space-based methods might be possible in the future, their high costs and technical challenges make them impractical for near-term climate action.
Recent assessments, including the IPCC's 2021 report, have not considered space-based SRM as a viable option due to cost and feasibility issues.
Cost
SRM may have much lower direct financial costs than the expected economic harm from uncontrolled climate change. These costs could range from billions to tens of billions of US dollars for each degree of cooling.
Stratospheric aerosol injection (SAI) is the most researched method and has the most cost estimates. UNEP said the cost is $18 billion per degree, although other studies suggest SAI could cost between $5 billion and $10 billion each year.
Marine cloud brightening (MCB) may cost, according to UNEP, $1 to $2 billion per W/m² of negative radiative forcing. This would mean costs of $1.5 to $3 billion per degree of cooling.
Cirrus cloud thinning (CCT) is studied less than other methods, and no official cost estimates are available.
Effects
Scientific studies have shown that using SRM moderately could greatly reduce many effects of global warming, such as changes in average and extreme temperatures, extreme rainfall, ice loss in the Arctic and on land, stronger or more frequent cyclones, and changes to ocean currents. Unlike methods to reduce carbon emissions or remove carbon dioxide from the air, SRM would act quickly, possibly lowering global temperatures within months.
The IPCC Sixth Assessment Report explains that SRM might help reduce some climate changes caused by rising greenhouse gases, including effects on the carbon and water cycles. However, some areas might still experience significant climate changes, and scientists are still unsure about how aerosols, clouds, and sunlight interact. SRM would increase the ability of land and oceans to absorb carbon dioxide, but it would not stop carbon dioxide levels from rising or prevent ocean acidification if human emissions continue.
SRM might help reduce some losses to agriculture caused by climate change. The increased plant growth from higher carbon dioxide levels, known as the CO2 fertilization effect, would still occur under SRM. Some research suggests SRM could improve crop yields, while other studies say reducing sunlight might slightly lower agricultural productivity.
Some studies suggest SRM might help protect coral reefs by lowering sea surface temperatures and reducing the risk of coral bleaching.
SRM would not completely reverse all effects of climate change. Regional rainfall patterns, cloud cover, and air movement might still change, with some areas getting too cold or too hot. This happens because greenhouse gases warm the planet evenly, while SRM reflects sunlight more in low-latitude regions and during summer, and only during the day. Adjusting how much SRM is used by location and season might help reduce these uneven effects.
Models show SRM could reverse changes to rainfall caused by warming more effectively than changes to temperature. If SRM were used to return global temperatures to preindustrial levels, it might cause too much change in rainfall patterns, possibly leading to droughts. However, the exact effects would depend on how strongly SRM is used. Soil moisture is more important for plants than average yearly rainfall, so SRM might better balance soil moisture changes than rainfall changes.
Climate change increases the strength of tropical monsoons, but SRM, especially SAI, might reduce their strength. At moderate SRM use, this could slightly lower monsoon intensity, though the impact on people and ecosystems might be less severe due to reduced heat. The exact effects would depend on how SRM is implemented.
SRM would change the balance between direct and indirect sunlight, which could affect plant growth and solar energy production. Visible light, needed for photosynthesis, would decrease more than infrared light because of a process called Mie scattering. This might slow plant growth, with different effects on plants in the top layers of forests compared to those in the shade.
Reducing sunlight overall would lower the amount of solar energy available, but the real-world effects would depend on many factors.
SAI could affect stratospheric ozone, which protects life from harmful ultraviolet light. The impact would depend on how SAI is deployed. Sulfates, the most common type of aerosol used in SRM, might slow the recovery of ozone in the stratosphere.
SRM does not directly lower carbon dioxide levels, so it would not reduce ocean acidification. While this is not a direct risk of SRM itself, it shows a major drawback of relying only on SRM instead of reducing emissions.
Scientific models suggest SRM could reduce some dangers of global warming, but uncertainties remain about how accurately models predict SRM effects, how aerosols and clouds interact, and how regional climates respond. Most information about SRM’s effects comes from climate models and volcanic eruptions. Some model uncertainties, such as how aerosols behave or how air moves in the stratosphere, are especially important for SRM and need more research. Volcanoes are not a perfect example because they release materials in the stratosphere in one burst, not over time.
A 2023 UNEP report stated that using SRM could reduce some climate risks but might also create new dangers for ecosystems and human societies.
The effects of SRM on ecosystems are not fully understood. An EU report noted that the impacts of SAI and SD on societies and ecosystems are a major unknown, with risks depending on how SRM is used, where it is used, and the types of ecosystems involved. SAI might help avoid some climate change effects on societies and ecosystems but could also cause unexpected harm. Terrestrial ecosystems might experience uncertain changes in plant life and productivity.
Governance
Solar Radiation Management (SRM) raises several important questions about how it should be managed. The Intergovernmental Panel on Climate Change (IPCC) lists these key goals for SRM governance: (i) protect people and the environment from possible dangers; (ii) support scientific research and learning about SRM; (iii) involve the public and experts in decisions about SRM to make them fair and informed; and (iv) ensure SRM is only used as part of a larger plan that includes reducing greenhouse gas emissions.
A common worry is that studying or using SRM might reduce efforts to cut greenhouse gas emissions, a concern called "moral hazard." Experts disagree about how likely this is. Some say it is not a strong reason to stop studying SRM if it could help reduce global warming, while others believe it should not be used at all. Studies using models, surveys, and experiments have not provided clear answers. A recent review says evidence about this issue is weak, but points out that these methods may not fully consider how real-world political decisions might affect emissions.
Another concern is that SRM could be used without international agreement because it is technically possible, costs little (for example, with a method called stratospheric aerosol injection), and may be controlled by a single country or a small group. This raises questions about how to manage SRM globally. Countries and other groups may not agree on how much or if SRM should be used, leading to poor decisions or conflicts, especially if some people are harmed. Experts also disagree about whether a single country or group could use SRM effectively and whether global rules could work.
This is made more complicated by two issues. First, SRM is still a new idea, so rules made now might be too strict or too loose, failing to adapt to future changes. Second, international laws usually require agreement among countries, so any rules about SRM would need to involve countries that might use it.
If SRM were used to hide warming and then stopped suddenly, the world could quickly heat up again, a risk called "termination shock." This could cause rapid temperature rises, more extreme weather, rising seas, and faster CO₂ buildup. The IPCC says stopping SRM slowly while also reducing emissions would help avoid these problems. Some experts think this risk could be managed because countries would likely restart SRM if needed, and keeping backup systems ready could help.
Large-scale use of SRM would likely require long-term efforts, possibly lasting decades or even centuries, to keep its cooling effects. This is needed because greenhouse gas levels continue to rise due to ongoing emissions and the long time CO₂ stays in the atmosphere.
There is no single law specifically for SRM, but some international agreements, national laws, and nonbinding documents may apply to certain SRM activities. For example, the UN Framework Convention on Climate Change (UNFCCC) and the Paris Agreement aim to limit global warming but do not directly address SRM. The UN Convention on the Law of the Sea (UNCLOS) allows scientific research that helps protect the ocean, which might support SRM studies, but could also limit large-scale outdoor SRM if it harms marine life.
The Environmental Modification Convention (ENMOD) is the only treaty that directly bans using SRM for military purposes but allows peaceful uses. It also encourages sharing information and working together on peaceful environmental projects.
The Vienna Convention for the Protection of the Ozone Layer and its Montreal Protocol require countries to prevent activities that harm the ozone layer, which some SRM methods might do.
Under international law, countries must avoid causing harm that crosses borders, such as through large-scale SRM. This includes planning carefully, consulting other countries, and working together to reduce risks. Some experts debate whether SRM should follow the same rules as other activities.
The International Law Commission suggests that any large-scale changes to the atmosphere should be done carefully and in line with international laws, including rules about environmental impact assessments.
Support for research
Support for research on solar radiation management (SRM) comes from scientists, international groups, governments, and non-governmental organizations (NGOs). One reason people support SRM research is that climate change poses serious and immediate risks, and SRM is the only known way to quickly stop or reverse warming.
An article in MIT Technology Review from 2017 said, "Few serious scientists would argue that we should begin deploying geoengineering anytime soon."
Some groups have claimed that companies in the fossil fuel industry support SRM research. However, research by SRM360 and others has not found evidence that private fossil fuel companies are funding or promoting SRM. Many researchers who receive SRM funding say they will not accept money from fossil fuel sources.
In 2023, two letters signed by scientists and experts called for more "responsible SRM research." One letter asked for an objective study of how SRM might reduce climate risks, how to minimize its dangers, and what information is needed for governance. It was supported by more than 110 scientists who study climate and its effects. Another letter asked for a balanced approach to SRM research and was supported by about 150 experts, mostly scientists.
In 2025, James Hansen and others said, "Research on purposeful global cooling should be pursued, as recommended by the U.S. National Academy of Sciences."
Organizations that support more research on SRM include scientific groups and large institutions.
Few countries have an official position on SRM. The United Kingdom, Canada, and Germany support some SRM research but do not consider it a current climate policy option. For example, the German government stated in 2023 that it is not currently considering SRM as a climate solution. However, it said it will continue to study SRM’s risks and benefits as part of basic research. As of 2025, the U.S. government does not have a policy on SRM.
Under the World Climate Research Programme, a project called "Research on Climate Intervention" began in 2024. This project includes studies on large-scale carbon dioxide removal and SRM.
Some NGOs support SRM research and discussions about how to govern it.
The Degrees Initiative is a UK charity that helps developing countries evaluate SRM. It aims to ensure that developing countries have a strong voice in SRM discussions. A researcher from the German NGO Geoengineering Monitor said this charity might be pushing its own research goals onto the Global South and is mostly funded by groups in the Global North.
Operaatio Arktis is a Finnish youth group that supports SRM research as a way to protect polar ice and avoid dangerous climate changes.
SilverLining is an American group that promotes SRM research as part of efforts to reduce near-term climate risks. It is funded by groups focused on climate change, including the Quadrature Climate Foundation, which plans to spend $40 million on SRM research over the next three years.
The Alliance for Just Deliberation on Solar Geoengineering works to include more voices in SRM discussions, especially from the Global South. The Carnegie Climate Governance Initiative helped shape SRM governance but stopped operating in 2023.
The Climate Overshoot Commission is a group of global experts who studied ways to reduce climate risks. They recommended more SRM research, a pause on large-scale experiments, and expanding how SRM research is governed.
SRM research projects and non-profit groups include SRM360, which helps people understand SRM. It is funded by the LAD Climate Fund.
Another group, Reflective, focuses on SRM research and technology. It is funded by groups like Outlier Projects, Navigation Fund, and Open Philanthropy.
As of 2024, about $200 million had been spent on SRM research, with spending increasing to more than $30 million each year. As of May 2025, $164 million more was committed for research from 2025 to 2029.
In 2025, 42% of SRM research funding came from governments. Countries that have funded SRM research include the U.S., U.K., Australia, Argentina, Germany, China, Finland, Norway, Japan, and the European Union.
NOAA in the U.S. spent $22 million from 2019 to 2022, with only a few outdoor tests. As of 2024, NOAA spends about $11 million a year on SRM research.
In 2025, the U.K. government invested over 60 million pounds on SRM research, including outdoor experiments. In late 2024, the UK’s Advanced Research and Invention Agency announced $75 million (about 57 million pounds) for projects studying "Climate Cooling," including outdoor tests.
As of 2025, 48% of SRM research funding came from philanthropy. The largest donors include the Simons Foundation, Quadrature Climate Foundation, and Open Philanthropy. According to Bloomberg News, several American billionaires, including Mike Schroepfer, Sam Altman, Matt Cohler, Rachel Pritzker, Bill Gates, and Dustin Moskovitz, support SRM research.
Opposition to deployment and research
Opposition to research and use of solar radiation management (SRM) has come from activist groups, scientists, and U.S. Republican lawmakers. Common concerns include that SRM might make it harder to reduce greenhouse gas emissions, be difficult to manage globally, or cause conflicts between countries. Critics often point out that strong efforts to reduce emissions also help improve public health and the environment, such as reducing air pollution, which could be ignored if SRM is used.
The ETC Group, an organization that studies the effects of new technologies on society and the environment, was one of the first to oppose SRM research. Later, the Heinrich Böll Foundation, a German group linked to the Green Party, and the Center for International Environmental Law joined this effort. The Climate Action Network, a global group that supports climate action, also opposes outdoor experiments and the use of SRM.
In 2021, Harvard University stopped plans for a small SRM experiment in Sweden after the Saami Council, an Indigenous group, raised concerns. The Council opposed a test flight over their traditional lands, even though the flight would not have released any materials. They criticized the lack of discussion with local communities and expressed worries about the risks and ethics of SRM.
A group of scholars and advocates proposed an "International Non-Use Agreement on Solar Geoengineering," urging governments to stop funding, testing, patenting, or using SRM. They argue that SRM is too risky, hard to control politically, and could weaken efforts to reduce emissions. As of December 2024, nearly 540 academics and 60 organizations supported this agreement. Most of the supporters are social scientists, not natural scientists. The campaign began with an article in the academic journal Wiley Interdisciplinary Reviews (WIREs): Climate Change. The group did not reveal who funded the effort.
Later, the journal published two responses. First, Wiley added a note stating that there was a conflict of interest in the peer review process. Mike Hulme, the journal’s editor-in-chief, had previously co-authored a version of the article that was rejected by another journal. Wiley concluded this was a conflict of interest, and Hulme resigned during the investigation.
Second, a group of scholars argued that the "Non-Use Agreement" campaign incorrectly describes the current state of research and overstates the risks of testing SRM. They say such an agreement could stop scientific research, ignore voices from developing countries, and make it harder to govern new technologies responsibly.
Since 2024, especially after Donald Trump was re-elected as U.S. president, lawmakers in at least 28 U.S. states have introduced or supported bills to ban SRM or related practices. These laws often focus on SRM and weather modification. They are influenced by beliefs about chemtrails. In 2024, Tennessee passed a law banning SRM, supported by members of one political party and signed by Governor Bill Lee. The next year, Florida passed a similar law, signed by Governor Ron DeSantis. U.S. Congresswoman Marjorie Taylor Greene announced plans to introduce a federal law that would make outdoor SRM or weather modification a crime.
Members of the Trump administration supported these efforts. Robert F. Kennedy Jr., the U.S. Secretary of Health and Human Services during the Trump administration, wrote 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." When the U.S. Environmental Protection Agency took action against the startup Make Sunsets, EPA Administrator Lee Zeldin said in a press release: "The idea that individuals, supported by venture capitalists, are putting harmful air pollutants into the air to sell 'cooling' credits shows how climate extremism has overtaken common sense."
Society and culture
Make Sunsets is a private company that sells "cooling credits" for its small-scale solar radiation management (SRM) activities. The company claims that each $10 credit cancels the warming effect of one ton of carbon dioxide for one year. Make Sunsets releases balloons filled with helium and sulfur dioxide. The company conducted some of its first experiments in Mexico, which led the Mexican government to announce plans to ban SRM experiments within its borders. Even scientists who support more research on SRM criticize Make Sunsets' actions. In April 2025, the U.S. Environmental Protection Agency asked the company to provide information about its sulfur dioxide releases.
Public opinion about SRM is still developing and varies depending on the situation. Most people globally know little or nothing about SRM, with 75–80% of people in recent surveys saying they are unfamiliar with it. However, research on public attitudes toward SRM is increasing and becoming more diverse, though studies from the UK, U.S., and Germany still dominate. Research in the Global South is less common, but some studies show people in these regions may be more open to SRM due to their immediate climate challenges. Studies now often use large surveys, but scientists worry that people's opinions may change if they know less about SRM.
Public views on SRM depend on values, how much people believe climate change will harm them, and how SRM is described. Common worries include fear that SRM might reduce efforts to cut carbon emissions, concerns about changing natural systems, fairness, and the need to involve the public before using SRM. Most people prefer reducing greenhouse gases or removing carbon dioxide from the air over using SRM. Europeans, especially in Germany, Austria, and Switzerland, are more against SRM, while people in southern Europe and the Global South are more accepting, especially when facing serious climate risks. Some studies also note that SRM is sometimes linked to conspiracy theories, such as "chemtrails," which can confuse people further.
The "chemtrails" conspiracy theory is the wrong belief that the long-lasting clouds left by airplanes are not normal contrails but contain harmful chemicals sprayed for secret purposes. Supporters claim that normal contrails disappear quickly, but those that linger must have added substances. They suggest these substances might be used for solar radiation management, weather control, or other harmful goals, and that they cause health problems.
The chemtrails theory began in the late 1990s after a 1996 U.S. Air Force report on weather modification. The theory spread online and was popularized by radio host Art Bell in 1999. Scientists have repeatedly explained that chemtrails are not real and are just normal contrails made of water vapor. There is no evidence that chemtrails exist or that they differ from normal contrails. Studies claiming to prove chemical spraying have been incorrect or based on misunderstandings. Scientists and governments worldwide continue to clarify that chemtrails are not real.