Solar radiation modification (SRM), also called solar geoengineering, is a set of large methods used to help reduce global warming by reflecting more sunlight back into space instead of letting it reach Earth. It is not meant to replace efforts to lower greenhouse gas emissions but is instead used to support those efforts 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 putting tiny reflective particles into the upper atmosphere to reflect sunlight. Other methods include marine cloud brightening (MCB), which would make ocean clouds more reflective, or building a large shade or mirror in space to block some sunlight from reaching Earth.

Climate models show that SRM could help reduce global warming and some effects of climate change, including possible tipping points. However, its effects would differ depending on the region and time of year, and the climate after using SRM would not be the same as a climate that never warmed. Scientists still have limited knowledge about how SRM might affect different areas and what risks or side effects could happen.

SRM also creates difficult political, social, and ethical questions. Some people are worried that using SRM might make people less focused on reducing emissions. Because SRM could be done with relatively low cost and is technically possible, it might be used by one country alone, leading to concerns about how countries should work together to manage it. Right now, there is no complete global system to control research or use of SRM.

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 is still a topic of disagreement.

SRM is also called sunlight reflection methods, solar climate engineering, albedo modification, and solar radiation management.

Context

The interest in solar radiation modification (SRM) comes from ongoing global warming and growing risks to people and natural systems.

In theory, reaching net-zero emissions through reducing emissions and removing carbon dioxide (CDR) could stop global warming. However, efforts to cut emissions have not met goals, and large-scale CDR may not be possible. The 2024 UN Environment Programme (UNEP) Emissions Gap Report stated that current policies would likely lead to 3.1°C of global warming. If countries follow their current commitments to reduce emissions, warming could reach 1.9°C.

SRM aims to make Earth brighter (albedo) by changing the atmosphere or surface to reflect more sunlight. A 1% increase in Earth's brightness could reduce warming by 2.35 W/m², which would counter most of the warming caused by current greenhouse gas levels. A 2% increase could cancel the warming effect of doubling carbon dioxide in the atmosphere.

Unlike emissions reductions or CDR, SRM could lower global temperatures quickly after being used. This fast action might help reduce the worst effects of climate change while emissions cuts and CDR efforts grow. However, SRM would not remove carbon dioxide from the air, so ocean acidification and other climate impacts would continue.

The IPCC Sixth Assessment Report says SRM cannot replace emissions reductions or CDR. It states: "There is strong agreement in scientific studies that SRM cannot be the main way to address climate risks and is, at best, a supplement to achieving long-term net zero or net negative carbon emissions globally."

Global dimming shows both how SRM might work and highlights the urgency of human-caused climate change. Industrial activities have increased the number of tiny particles (aerosols) in the lower atmosphere. These particles have cooled the planet by reflecting sunlight (a method similar to stratospheric aerosol injection) and by making clouds more reflective (a method similar to marine cloud brightening). As regulations have reduced these aerosols, global dimming has decreased, and the planet 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 from burning fossil fuels. To reduce global warming, the report suggested "intentionally causing changes that would balance out the warming," such as "increasing the Earth's ability to reflect sunlight."

In 1974, Russian climatologist Mikhail Budyko proposed that if global warming became a major problem, it could be addressed by putting tiny particles into the upper atmosphere. He suggested that planes burning sulfur could create these particles, which would reflect sunlight away from Earth, helping to cool the planet.

Along with methods to remove carbon dioxide from the air, solar radiation management (SRM) was discussed as part of a larger idea called geoengineering in a 1992 climate change report from the U.S. National Academies. The first computer models and review articles about SRM were published in 2000. In 2006, Paul Crutzen, a Nobel Prize winner, wrote a key paper stating that research on SRM should not be ignored, given the lack of progress in reducing greenhouse gas emissions.

Major reports that studied the possible benefits and risks of SRM include those by:

• The Royal Society (2009)
• The U.S. National Academies of Sciences, Engineering, and Medicine (2015, 2021)
• The United Nations Environment Programme (2023)
• The United Nations Educational, Scientific and Cultural Organization (UNESCO) (2023)
• The European Union Scientific Advice Mechanism (2024).

In the late 2010s, SRM became more clearly separated from methods that remove carbon dioxide, and terms like "geoengineering" were used less frequently.

Methods

Stratospheric Aerosol Injection (SAI) involves adding tiny particles to the upper atmosphere. These particles help reflect sunlight and reduce global warming. Among solar radiation management (SRM) methods, SAI has been studied the most. The Intergovernmental Panel on Climate Change (IPCC) reported in 2021 that SAI "is the most-researched SRM method, with high agreement that it could limit warming to below 1.5 °C." This technique mimics natural cooling events after large volcanic eruptions.

Sulfates are the most common type of particle proposed for SAI because they naturally occur during volcanic eruptions. Other materials, such as calcium carbonate and titanium dioxide, have also been suggested.

Custom-built aircraft are considered the best way to deliver these particles. Artillery and balloons are other, less common options.

SAI could create a cooling effect of up to 8 W/m². The World Meteorological Organization’s 2022 report stated that SAI "has the potential to limit the rise in global surface temperatures by increasing particle concentrations in the stratosphere." However, it also warned that SAI could harm the ozone layer’s recovery, depending on the aerosols used.

Marine Cloud Brightening (MCB), also called marine cloud seeding, aims to make sea clouds brighter. Brighter clouds reflect more sunlight, which could slow global warming. MCB is one of two SRM methods that might significantly affect the climate. It works closer to Earth than SAI and could cool specific areas. If used widely, MCB might increase Earth’s reflectiveness, helping reduce climate change when combined with efforts to cut greenhouse gases. Cooling effects from MCB would be quick and could be reversed. However, technical challenges remain, and MCB might not stop all warming. Scientists are still learning about clouds, so risks are 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 brightness. MCB could use unmanned ships to spray seawater mist. Small tests were done near the Great Barrier Reef in 2024.

Cirrus Cloud Thinning (CCT) involves adding materials to cirrus clouds to make them thinner. Thinner clouds allow more heat to escape into space. Cirrus clouds usually trap heat, so CCT could help Earth release more heat. However, some studies suggest CCT might actually increase warming due to complex interactions between clouds and particles. This method is often grouped with SRM even though it works by increasing outgoing heat, not reducing incoming sunlight.

Surface-based albedo modification includes methods like painting buildings white, adding reflective materials to farmland, and creating microbubbles in oceans to increase reflectiveness. These approaches might have limited global effects. Cooling cities with reflective roofs or pavement could work, but changing large deserts or covering glaciers with reflective materials might alter weather patterns. The effectiveness of these methods at scale is uncertain.

Space-based SRM involves placing mirrors, reflective particles, or shading structures in space. These could be placed in low Earth orbit, geosynchronous orbit, or near the L1 Lagrange point between Earth and the Sun. Unlike atmospheric methods, space-based approaches would not directly affect Earth’s climate systems. Ideas like orbiting mirrors and space dust clouds have been proposed. However, the Royal Society (2009) and later studies concluded that these methods are too costly and difficult to implement in the near future. The most recent IPCC report (2021) did not consider space-based SRM methods.

Cost

SRM may have lower direct money costs for use compared to the expected economic harm caused by uncontrolled climate change. These costs might range between billions and 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 information. UNEP reported a cost of $18 billion per degree, though other studies suggest SAI use could cost between $5 billion and $10 billion each year.

Marine cloud brightening (MCB) might cost, according to UNEP, $1 to $2 billion for each unit of negative radiative forcing, which equals $1.5 to $3 billion per degree.

Cirrus cloud thinning (CCT) is studied less than other methods, and no official cost information is available.

Effects

Scientific studies show that using SRM moderately can reduce many effects of global warming, such as changes in temperature, ice, and weather patterns. Unlike other methods, SRM works quickly, lowering global temperatures within months.

The IPCC Sixth Assessment Report states that SRM might reduce some climate changes caused by greenhouse gases, such as effects on water and carbon cycles. However, some areas might still experience changes in weather or temperature, and scientists are unsure how tiny particles in the air affect clouds and sunlight. Cooling from SRM would increase how much CO₂ is absorbed by land and oceans, but it would not stop CO₂ levels in the atmosphere from rising or prevent ocean acidification if humans continue to emit CO₂.

SRM might help reduce some losses in farming caused by climate change. Plants grow more with more CO₂, and this effect would continue under SRM. Some research suggests SRM could increase crop growth, while other studies say less sunlight might slightly reduce farming productivity.

Some research shows that SRM might help protect coral reefs by cooling ocean temperatures.

SRM cannot fully reverse all effects of climate change. Some areas might still experience changes in rainfall, cloud cover, or wind patterns, with some regions getting too cold or too hot. This happens because greenhouse gases warm the planet evenly, but SRM reflects sunlight more at lower latitudes and during summer, and only during the day. Adjusting how much SRM is used by region and season might help reduce these differences.

Models suggest SRM would reverse changes in rainfall better than changes in temperature. If SRM is used to bring global temperatures back to preindustrial levels, it might cause too much dryness in some areas, but the exact effect depends on how strongly SRM is used. Soil moisture is more important for plants than yearly rainfall totals. Because SRM reduces evaporation, it might better balance changes in soil moisture than changes in yearly rainfall.

Climate change increases the strength of tropical monsoons, but SRM might reduce their strength, especially if sulfate aerosols are used. Using SRM moderately might slightly weaken monsoons, but this could be less harmful to people and ecosystems if heat-related dangers are avoided. The exact impact depends on how SRM is implemented.

SRM changes how much sunlight reaches Earth directly or indirectly, which affects plants and solar energy. Visible light, which helps plants grow, is reduced more than other types of sunlight. This might slow plant growth, with different effects on plants in the top of forests compared to those below.

Using SRM would lower the amount of sunlight reaching Earth, which could reduce solar power production. However, real-world effects would depend on many complex factors.

Using sulfate aerosols in the stratosphere might delay the recovery of ozone, which protects life from harmful sunlight. This depends on how SRM is deployed.

SRM does not lower the amount of CO₂ in the atmosphere, so it does not reduce ocean acidification. While this is not a direct risk of SRM, it shows a major problem with relying only on SRM instead of reducing emissions.

Scientific models suggest SRM could reduce some dangers from global warming, but there are still many unknowns. Scientists are unsure about how accurately models predict SRM’s effects, how particles in the air affect clouds, and how different regions might respond. Most information about SRM comes from computer models and volcanic eruptions. However, volcanoes are not perfect examples because they release particles in one burst, not over time.

A 2023 UNEP report said that using SRM might reduce some climate risks but could also create new dangers for ecosystems and people.

Effects on ecosystems are not fully understood. A European report said that the impacts of SRM on societies and ecosystems are unclear, and these effects might vary depending on how SRM is used, where it is used, and the types of ecosystems involved. While SRM might help avoid some climate-related problems, it could also cause unexpected or harmful effects. Changes in land ecosystems might lead to uncertain shifts in plant life and productivity.

Governance

Solar Radiation Management (SRM) brings up many questions about how it should be managed and controlled. The Intergovernmental Panel on Climate Change (IPCC) lists several possible goals for SRM governance.

A common concern is that using SRM might reduce efforts to cut greenhouse gas emissions, a situation sometimes called "moral hazard." Experts disagree about how likely this is and whether it should stop SRM research. Studies so far have not given clear answers. Some research suggests that even if SRM reduces emissions, it might not be enough to prevent harm from climate change. Others argue that SRM could be risky if it leads to less action on emissions.

Another concern is that SRM could be used by a single country or a few countries without international agreement. This might lead to problems if countries disagree on how SRM should be used. It could cause conflicts or poor decisions if SRM harms people or the environment. Experts also disagree about whether it is likely that countries would act alone and whether rules for managing SRM could work.

These issues are more complicated because SRM is still being studied. Rules made too early might not fit future needs. Also, international laws usually require agreement among countries, so creating rules for SRM would need cooperation from countries that might want to use it.

If SRM were stopped suddenly, the climate might warm quickly, a situation called "termination shock." This could cause faster temperature rises, more extreme weather, and harm to ecosystems. The IPCC says that ending SRM slowly while also cutting emissions could reduce these risks. Some experts believe that countries would have strong reasons to restart SRM if needed, and keeping backup systems ready could help.

Using SRM on a large scale would likely require long-term efforts to keep its cooling effects. This is because greenhouse gas levels are still rising, and carbon dioxide stays in the atmosphere for a long time.

There is no single law that specifically governs SRM research or use. However, some international agreements, laws, and guidelines may apply to certain SRM activities.

The UN Framework Convention on Climate Change (UNFCCC) and its related treaties do not directly address SRM. However, SRM could be related to the Paris Agreement's goal of limiting global warming. The UNFCCC is based on the idea of taking action to prevent harm even when there is uncertainty, but how this applies to SRM is unclear.

The UN Convention on the Law of the Sea (UNCLOS) allows scientific research that protects the ocean, which could include studying SRM's effects on marine life. However, it might also limit large-scale outdoor activities if they risk harming the ocean. Since SRM does not directly fix ocean acidification, its connection to UNCLOS goals is unclear.

The Environmental Modification Convention (ENMOD) is the only treaty that directly limits activities that change the environment in harmful ways. SRM falls under ENMOD's rules and is banned from being used for military purposes. However, it allows peaceful uses of environmental modification. ENMOD also encourages sharing information and working together on peaceful uses.

The Vienna Convention for the Protection of the Ozone Layer and its Montreal Protocol require countries to avoid activities that harm the ozone layer, which some SRM methods might do. These agreements also require cooperation to protect human health and the environment.

Customary international law says countries must avoid causing harm across borders. This rule would apply to large-scale SRM if it risks harming other countries. Countries would need to study the effects, warn other nations, and work together to reduce risks. Some experts debate whether SRM should follow different legal rules.

The International Law Commission created draft guidelines to protect the atmosphere. These suggest that activities affecting the atmosphere should be justified by scientific research, studied for environmental effects, and managed carefully.

The Conference of Parties to the Convention on Biological Diversity has made rules about "climate-related geoengineering," which includes SRM. In 2010, it created a framework for studying SRM's effects on nature. In 2016, it encouraged more research and sharing of knowledge about SRM.

Support for research

Support for SRM research comes from scientists, international organizations, governments, and non-governmental organizations (NGOs). A key reason people support SRM research is that climate change poses serious and urgent risks, and SRM is the only known method 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 people claim that the fossil fuel industry supports SRM research. However, research by SRM360 and others has not found evidence that private fossil fuel interests are funding or promoting SRM. Many organizations receiving SRM funding have stated they will not accept money from fossil fuel sources.

In 2023, two sign-on letters from scientists and experts called for more "responsible SRM research." One letter asked 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 supported by more than 110 scientists studying climate and its effects. Another letter called for "balance in research and assessment of solar radiation modification" 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."

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

• In the UK: the Royal Society, the Institution of Mechanical Engineers, and the editorial board of Nature
• In Australia: the Office of the Chief Scientist
• In the Netherlands: the Netherlands' scientific assessment institute
• In the United States: the U.S. National Academies, the American Geophysical Union, the American Meteorological Society, the U.S. Global Change Research Program, and the Council on Foreign Relations
• International organizations: the World Climate Research Programme and reports from the UN Environment Programme and the UN Educational, Scientific and Cultural Organization
• In the European Union: the Group of Chief Scientific Advisors

Few countries have an official position on SRM. Those that do, such as the United Kingdom, Canada, and Germany, support some SRM research even if they do not see it as a current climate policy option. For example, the German government stated in 2023 that it is not currently considering SRM as a climate policy option. However, it plans to continue studying SRM's risks and implications as part of basic research. As of 2025, the U.S. federal 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 includes research on large-scale carbon dioxide removal and SRM.

Some non-governmental organizations support SRM research and discussions about its governance.

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 discussions about SRM. A researcher from the German NGO Geoengineering Monitor has said this charity may be "imposing its research agenda on the Global South" and is "funded mainly by foundations run by billionaires from the Global North."

Operaatio Arktis is a Finnish youth climate group that supports SRM research alongside efforts to reduce carbon emissions and preserve polar ice caps.

SilverLining is an American organization that promotes SRM research as part of "climate interventions to reduce near-term climate risks." It is funded by "philanthropic foundations and individual donors focused on climate change." One of its funders, the Quadrature Climate Foundation, plans to spend $40 million on SRM research over the next three years.

The Alliance for Just Deliberation on Solar Geoengineering works to ensure fair and inclusive discussions about SRM, especially by involving civil society groups in the Global South. The Carnegie Climate Governance Initiative helped shape governance for SRM and carbon dioxide removal, though it stopped operating in 2023.

The Climate Overshoot Commission is a group of global experts who studied ways to reduce climate risks. It recommended more SRM research alongside a pause on large-scale experiments and deployment. It also said "governance of SRM research should be expanded."

SRM research initiatives and non-profit knowledge hubs include SRM360, which supports discussions about sunlight reflection methods (SRM). Funding for SRM360 comes from the LAD Climate Fund.

Another example is Reflective, a project focused on sunlight reflection research and technology development. Reflective is funded entirely by grants or donations from philanthropies that address climate change, including Outlier Projects, Navigation Fund, Astera Institute, Open Philanthropy, Crankstart, Matt Cohler, and Richard and Sabine Wood.

By 2024, about $200 million had been spent on SRM research, with spending increasing to more than $30 million per year in recent years. As of May 2025, $164 million more had been committed for 2025–2029.

As of 2025, 42% of SRM research funding comes 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.

In the U.S., NOAA spent $22 million from 2019 to 2022, with only a few outdoor tests conducted. As of 2024, NOAA provides about $11 million per year for solar geoengineering research.

In 2025

Opposition to deployment and research

Opposition to research and use of solar radiation management (SRM) has come from groups that work to raise awareness, scientists, and U.S. Republican lawmakers. Common concerns include that SRM might reduce efforts to lower greenhouse gas emissions, be hard to manage globally, or cause international conflicts. Opponents often say that strong efforts to reduce emissions also help public health and the environment, such as by reducing air pollution, which might be ignored if SRM becomes popular.

The ETC Group, an organization focused on the effects of new technologies, 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 the effort. The Climate Action Network, a global group that supports climate action, also opposes outdoor experiments and use of SRM.

In 2021, researchers at Harvard University stopped plans for a small SRM experiment in Sweden after the Saami Council, a group that represents Indigenous people, raised concerns. The Council objected to a test flight over their ancestral land. Although the flight would not have released any materials, the Council criticized the lack of discussion with local communities and expressed worries about the ethics and risks of SRM.

A group of scholars and advocates proposed an "International Non-Use Agreement on Solar Geoengineering," asking governments to stop funding, testing, patenting, using, or supporting SRM. They argue that SRM is too risky, hard to govern, and might reduce efforts to cut emissions. As of December 2024, nearly 540 academics and 60 organizations supported the campaign. The campaign calls its supporters "scientists," but most are social scientists. The idea was first shared in an article published in the journal Wiley Interdisciplinary Reviews (WIREs): Climate Change. The group did not share details about who funded the campaign.

Later, the journal added a note saying there was a conflict of interest in the review process. Mike Hulme, the journal’s editor-in-chief, had co-written an earlier version of the article, which was rejected by another journal. Wiley, the publisher, said this was a conflict of interest, and Hulme stepped down as editor-in-chief during an investigation.

In response, another group of scholars said the "Non-Use Agreement" campaign misunderstood the current research and exaggerated the risks of testing SRM. They argued that such an agreement could stop scientific research, ignore voices from developing countries, and make it harder to manage new technologies responsibly.

Since 2024, especially after Donald Trump was re-elected as U.S. President, lawmakers in at least 28 U.S. states introduced or supported bills to ban SRM or related practices. These laws often target SRM and weather modification. They are influenced by the chemtrails conspiracy theory. In 2024, Tennessee passed such a law, approved by party members 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 bill that would make outdoor SRM or weather modification a crime.

Members of the Trump administration supported these efforts. Robert F. Kennedy Jr., who was 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 (see below), 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. It claims that each $10 credit reduces the warming effect of one ton of carbon dioxide for one year. The company releases balloons filled with helium and sulfur dioxide. Make Sunsets first tested its activities in Mexico, which led the Mexican government to say it plans to stop SRM experiments within its borders. Even people 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 support research into SRM but are unsure about using it. Many people globally are not familiar with SRM, with between 75 and 80 percent of people in recent surveys saying they know little or nothing about it. However, more studies about public attitudes toward SRM are being done, though most research comes from the UK, U.S., and Germany. Studies in the Global South are fewer, but those done so far show that people there are more open to SRM because they feel climate effects are more immediate. Research methods now focus on large surveys, but questions remain about whether people’s opinions will change if they learn more.

Public views on SRM depend on values, how serious people think climate change is, and how SRM is described. Common concerns include fear that SRM might reduce efforts to cut carbon emissions, worries about changing natural climate 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 central and northern countries like Germany, Austria, and Switzerland, are more opposed to SRM, while people in southern Europe and the Global South are more accepting, especially if they face severe climate risks. Some studies also link SRM to conspiracy theories, such as "chemtrails," which can confuse people further.

The chemtrail conspiracy theory is the wrong belief that long-lasting airplane trails in the sky, called contrails, are actually "chemtrails" that contain harmful chemicals sprayed for secret purposes. People who believe this say that normal contrails disappear quickly, but long-lasting ones must have extra substances. They guess the purpose might be to manage solar radiation, change weather, control populations, or test harmful agents, and they think these trails cause health problems.

This theory began in the late 1990s after a 1996 U.S. Air Force report about weather modification. The Air Force was accused of secretly spraying substances from planes. The idea spread online and was promoted by radio host Art Bell starting in 1999. Scientists say chemtrails are not real and are the same as normal water-based contrails from planes under certain weather conditions. People who believe in chemtrails have tried to prove chemical spraying happens, but their evidence is incorrect or based on misunderstandings. Because the theory is still popular and raises questions about government actions, scientists and governments worldwide have repeatedly explained that chemtrails are just normal contrails.