Solar radiation modification

Solar radiation modification is a group of large-scale approaches to reduce global warming by increasing the amount of sunlight that is reflected away from Earth and back to space. It is not intended to replace efforts to reduce greenhouse gas emissions, but rather to complement them as a potential way to limit global warming. SRM is a form of geoengineering.
The most-researched SRM method is stratospheric aerosol injection, in which small reflective particles would be introduced into the upper atmosphere to reflect sunlight. Other approaches include marine cloud brightening, which would increase the reflectivity of clouds over the oceans, or constructing a space sunshade or a space mirror, to reduce the amount of sunlight reaching earth.
Climate models have consistently shown that SRM could reduce global warming and many effects of climate change, including some potential climate tipping points. However, its effects would vary by region and season, and the resulting climate would differ from one that had not experienced warming. Scientific understanding of these regional effects, including potential environmental risks and side effects, remains limited.
SRM also raises complex political, social, and ethical issues. Some worry that its development could reduce the urgency of cutting emissions. Its relatively low direct costs and technical feasibility suggest that it could, in theory, be deployed unilaterally, prompting concerns about international governance. Currently, no comprehensive global framework exists to regulate SRM research or deployment.
Interest in SRM has grown in recent years, driven by continued global warming and slow progress in emissions reductions. This has led to increased scientific research, policy debate, and public discussion, although SRM remains controversial.
SRM is also known as sunlight reflection methods, solar climate engineering, albedo modification, and solar radiation management.

Context

The interest in solar radiation modification arises from ongoing global warming, increasing risks to both human and natural systems.
In principle, achieving net-zero emissions through emissions reductions and carbon dioxide removal could halt global warming. However, emissions reductions have consistently fallen short of targets, and large-scale CDR may not be feasible. The 2024 UN Environment Programme Emissions Gap Report said that current policies would likely lead to 3.1 °C global warming country's commitments and pledges to reduce emissions would likely lead to 1.9 °C warming.
SRM aims to increase Earth's brightness 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.
Unlike emissions reduction or CDR, SRM could reduce global temperatures within months of deployment. This rapid effect means SRM could help limit the worst climate impacts while emissions reductions and CDR are scaled up. However, SRM would not reduce atmospheric carbon dioxide concentrations, meaning that ocean acidification and other climate change effects would persist.
The IPCC Sixth Assessment Report emphasizes that SRM is not a substitute for emissions reductions or CDR, stating: "There is high agreement in the literature that for addressing climate change risks, SRM cannot be the main policy response to climate change and is, at best, a supplement to achieving sustained net zero or net negative CO₂ emission levels globally."
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, caused by the aerosol's reflectivity and by increasing' clouds' reflectivity. As regulation has reduced tropospheric aerosols, global dimming has decreased and the planet has warmed at a faster rate.

History

In 1965, during the administration of U.S. President Lyndon B. Johnson, the President's Science Advisory Committee delivered Restoring the Quality of Our Environment, the first report which warned of the harmful effects of carbon dioxide emissions from fossil fuel use. To counteract global warming, the report mentioned "deliberately bringing about countervailing climatic changes," including "raising the albedo, or reflectivity, of the Earth".
In 1974, Russian climatologist Mikhail Budyko suggested that if global warming ever became a serious threat, it could be countered by releasing aerosols into the stratosphere. He proposed that aircraft burning sulfur could generate aerosols that would reflect sunlight away from the Earth, cooling the planet.
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. The first modeled results of and review article on SRM were published in 2000. In 2006, Nobel Laureate Paul Crutzen published an influential paper arguing that, given the lack of adequate greenhouse gas emissions reductions, research on the feasibility and environmental consequences of SRM should not be dismissed.
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.

Methods

Stratospheric aerosol injection (SAI)

For stratospheric aerosol injection, 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." This technique would replicate natural cooling phenomena observed following large volcano eruptions.
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.
Custom-designed aircraft are considered the most feasible delivery method, with artillery and balloons occasionally proposed.
SAI could produce up to 8 W/m² of negative radiative forcing.
The World Meteorological Organization's 2022 Scientific Assessment of Ozone Depletion stated that "Stratospheric Aerosol Injection 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."
A key concern with SAI is its potential to delay the recovery of the ozone layer, depending on which aerosols are used.

Marine cloud brightening (MCB)

Cirrus cloud thinning (CCT)

involves seeding cirrus clouds to reduce their optical thickness and decrease cloud lifetime, allowing more outgoing longwave radiation to escape into space.
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.
This method is often grouped with SRM despite working primarily by increasing outgoing radiation rather than reducing incoming shortwave radiation.

Reflective surfaces

The IPCC describes surface-based albedo modification as "increase ocean albedo by creating microbubbles;... paint the roof of buildings white...; increase albedo of agriculture land, add reflective material to increase sea ice albedo."
Surface-based approaches could be considered localized and would have limited global impact. While urban cooling could be achieved through reflective roofs and pavement, large-scale desert albedo modification could significantly alter regional precipitation patterns. Covering glaciers with reflective materials has been proposed to slow melting, though feasibility and effectiveness at scale remains uncertain.

Space-based methods

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 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.
Assessments conclude that space-based SRM is not feasible at reasonable costs. The most recent IPCC Assessment Report did not consider these methods.

Cost

SRM could have relatively low direct financial costs of deployment compared to the projected economic damages of unmitigated climate change. These costs could be on the order of billions to tens of billions of US dollars per degree of cooling.
Stratospheric aerosol injection is the most studied and has the most cost estimates. UNEP reported a cost of $18 billion per degree, although individual studies have estimated that SAI deployment could cost between $5 billion to $10 billion per year.
MCB could cost, according to UNEP, $1 to 2 billion per W/m2 of negative radiative forcing, which implies $1.5 to 3 billion per degree.
Cirrus cloud thinning is even less studied, and no formal cost estimates exist.