Solar geoengineering—also called solar radiation modification (SRM)—is a set of proposed interventions to cool Earth by reflecting more sunlight back to space or allowing more heat to escape. It could lower average temperatures without lowering the greenhouse-gas concentrations that cause long-term warming, so it would treat one symptom rather than remove the cause.
The main proposals work in different parts of the atmosphere and would not produce identical results. Models and official assessments indicate that global-average cooling could coexist with uneven regional temperatures, rainfall changes, atmospheric effects and ecological risks. As of the United Nations Environment Programme’s November 28, 2025 assessment, SRM remained largely confined to models, simulations and theory, not an established climate solution.
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What solar geoengineering is—and what it is not
Earth’s climate is governed partly by an energy balance: sunlight arrives, some is reflected, and the rest warms the surface and atmosphere before leaving as infrared radiation. Solar geoengineering proposes changing that balance deliberately. Some approaches would reflect a larger share of incoming sunlight; another would alter high clouds so more outgoing thermal energy can escape.
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These ideas are often discussed because reducing greenhouse-gas emissions does not immediately return atmospheric carbon dioxide to its earlier level. SRM could, in theory, change temperatures more quickly than the slow removal of accumulated carbon dioxide. But it would not remove carbon dioxide, methane or other greenhouse gases, and it would not stop their effects on ocean acidification or other parts of the climate system. The United Nations Environment Programme describes it as no substitute for emissions cuts and adaptation, which remain the measures that address the underlying cause and unavoidable impacts of warming (UNEP, November 28, 2025).
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How the principal proposals would work
Stratospheric aerosol injection (SAI)
SAI would place tiny reflective particles in the stratosphere, roughly 10–15 miles (16–25 kilometers) above Earth’s surface. The particles could scatter a portion of incoming sunlight back to space, reducing the solar energy absorbed by the climate system. The National Academies compares the basic physical mechanism with the cooling that follows a large volcanic eruption, when sulfur-containing aerosols reach the stratosphere (National Academies, updated June 15, 2023).
The altitude matters. Stratospheric material can spread over wide areas and persist longer than particles released near the ground, creating a potentially global intervention rather than a local weather project. That same reach would make monitoring, attribution and international consent difficult. SAI is a proposal, not a report of deliberate full-scale deployment.
Marine cloud brightening (MCB)
MCB would add particles to low clouds over ocean areas. The particles could increase the clouds’ reflectivity or otherwise change their optical properties so that more sunlight is scattered back to space. Its intended footprint could be more regional than SAI, but cloud responses depend on existing atmospheric conditions, aerosol chemistry and local meteorology. The National Academies identifies MCB as a proposed method; it does not establish a demonstrated, controllable deployment.
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CCT would modify high ice clouds to make the atmosphere more transparent to outgoing longwave (thermal) radiation. More heat could then escape to space. Because it acts on outgoing infrared energy rather than reflected incoming sunlight, the National Academies says CCT is not technically a solar-geoengineering method, even though it is often discussed alongside SRM (National Academies, 2023).
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How the methods differ
| Proposal | Radiation target | Where it would act | Intended reach | Evidence described in official sources |
|---|---|---|---|---|
| Stratospheric aerosol injection | Reflects some incoming sunlight | Stratosphere, about 10–15 miles (16–25 km) high | Potentially broad or global because particles can spread around the planet | Physical analogy from volcanic aerosols; deliberate deployment remains proposed and largely modeled |
| Marine cloud brightening | Increases reflection from low clouds | Low marine clouds over selected ocean regions | Could be more regional, depending on cloud and wind conditions | Cloud and climate responses are uncertain and primarily studied through models and experiments |
| Cirrus cloud thinning | Increases outgoing longwave heat loss | High ice clouds | Depends on where suitable cirrus clouds occur and how they respond | Classified by the National Academies as a non-solar radiation-modification concept because it changes outgoing heat |
No official assessment supplies a complete quantitative ranking that identifies one approach as safest or most controllable. The meaningful comparisons concern the radiation pathway, altitude or location, geographic reach, observational evidence, possible water-cycle effects and the governance needed to monitor consequences.
What evidence shows about possible cooling
A natural event demonstrates that changing atmospheric reflectivity can cool the surface. After the 1991 Mount Pinatubo eruption, aerosols injected into the stratosphere reflected enough sunlight to cool Earth’s surface for the following two years, according to the National Academies. That observation supports the underlying physical mechanism; it does not prove that an intentional intervention would have the same magnitude, geographic pattern, controllability or risks.
The United Nations Secretary-General’s Scientific Advisory Board summarizes the potential and the uncertainty this way: “The technologies for altering the reflective capacities of the Earth’s surface and atmosphere could relatively quickly reduce temperature increase and cool the planet.” The statement is an institutional assessment, not a prediction that a particular project would deliver a guaranteed amount of cooling (UN Scientific Advisory Board, May 28, 2025).
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Most knowledge about deliberate SRM comes from climate models, simulations, laboratory or small-scale studies and theory. Models can examine scenarios, but they cannot fully reproduce the coupled atmospheric, ecological and political conditions of a planet-scale intervention. Results therefore depend on the method, injection or release strategy, atmospheric state, latitude, season and other assumptions.
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Why global cooling would not mean equal effects everywhere
“Global temperature” is an average. Lowering that average does not require every region to cool by the same amount, and it says nothing by itself about rainfall. A method could reduce one climate hazard while changing circulation or precipitation in ways that create losses elsewhere.
- Uneven temperatures: high-latitude, tropical, land and ocean regions could respond differently to the same global radiative change.
- Water-cycle changes: modeled outcomes include regional shifts in rainfall and other hydrological effects. A cooler average could still bring drought risk in one area and heavier precipitation in another.
- Climate variability: SRM could interact with natural variability, making it difficult to distinguish an intervention’s signal from ordinary year-to-year fluctuations.
- Circulation and seasonal effects: changing the distribution of heating can alter atmospheric patterns rather than simply turning down a planetary thermostat.
For that reason, a global-average temperature target cannot serve as the sole measure of success. Any serious assessment would need regional temperature, precipitation, water availability, extreme events, ecosystem indicators and impacts on vulnerable communities.
Potential atmospheric and ecological risks
Official summaries identify risks as possibilities and uncertainties, not as outcomes that must occur in every scenario. The UN Scientific Advisory Board flags uneven cooling, additional climate variability, disruption in the lower and middle stratosphere and acid rain. UNEP’s 2025 note also highlights possible disruption of climate patterns, biodiversity, the ozone layer and regional hydrology (UN Scientific Advisory Board; UNEP).
These risks arise because particles and clouds participate in chemistry and circulation as well as reflection. Stratospheric aerosols, for example, could affect ozone-related chemistry; changes in rainfall could propagate through agriculture, freshwater supplies and ecosystems; and altered light levels could affect biological productivity. The scale and direction of each effect would depend on implementation details and background conditions, which are not established by the available evidence.
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Governance, ethics and cross-border consequences
SRM would raise governance questions before any technical system could be considered ready. Atmospheric effects would not respect national borders, so a decision by one country or a small coalition could expose others to altered temperatures or rainfall. The people receiving benefits might not be the people bearing the risks, creating questions of consent, liability, compensation and climate justice.
The National Academies’ report, Reflecting Sunlight: Recommendations for Solar Geoengineering Research and Research Governance, focuses on how research should be conducted and overseen rather than granting permission for deployment (National Academies, 2021). Its relevance is that better observations, open data, public engagement and independent oversight would be needed to evaluate claims and detect harm. The UN advisory brief likewise identifies geopolitical and ethical risks. There is no basis in these sources for claiming that a settled, comprehensive global authorization framework already exists.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What “largely in models” means in practice
UNEP’s note published November 28, 2025 describes SRM as still largely confined to models, simulations and theory. That wording distinguishes scenario analysis from an operating climate-control system. It also means that estimates of cooling, rainfall changes or side effects should be read as conditional results: they describe what a model produces under specified assumptions, not a measured performance record.
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Where SRM fits in climate policy
Climate policy has three distinct tasks: reduce greenhouse-gas emissions, prepare for impacts that cannot be avoided, and investigate whether additional interventions could reduce near-term temperature risks. SRM belongs, at most, in the third category as a research subject. Cutting emissions limits the driver of warming; adaptation protects people and ecosystems from hazards already occurring or expected; SRM would attempt to alter the energy balance while greenhouse gases remain.
That division matters for decision-making. Treating a modeled cooling option as a substitute for emissions reductions could prolong dependence on fossil fuels and leave carbon-dioxide-driven harms untouched. Treating it as a guaranteed fix could also understate regional risks. A responsible approach keeps mitigation and adaptation central while subjecting SRM research to transparent governance, independent review, international discussion and measurements that include regional as well as global outcomes.
Bottom line for readers
Solar geoengineering is a proposed way to put a partial shade on Earth’s energy balance, either by reflecting sunlight or by increasing the escape of heat to space. Volcanic history shows that atmospheric particles can cool the surface, and models indicate that SRM might reduce average warming relatively quickly. But the methods are not interchangeable, regional effects could diverge sharply from the global average, and possible atmospheric, hydrological, ecological, ethical and geopolitical consequences remain uncertain. As of late 2025, it was still primarily a modeled and theoretical field—not a proven, deployed replacement for cutting greenhouse-gas emissions and adapting to climate change.
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