Solar radiation modification
The interest in solar radiation modification (SRM) arises from ongoing global warming, increasing risks to both human and natural systems.
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."
To counteract global warming, the report mentioned "deliberately bringing about countervailing climatic changes," including "raising the albedo, or reflectivity, of the Earth".
[25][26] 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 climate engineering should not be dismissed.
[29] MCB could be implemented using fleets of unmanned rotor ships, such as Flettner vessels, to disperse seawater mist into the air, increasing cloud albedo and reflecting more radiation.
[29] While urban cooling could be achieved through reflective roofs and pavement, large-scale desert albedo modification could significantly alter regional precipitation patterns.
[31] Space-based solar radiation modification involves deploying mirrors, reflective particles, or shading structures at or near the L1 Lagrange point between Earth and the Sun.
[34] Unlike atmospheric methods, space-based approaches would not directly interfere with Earth's climate systems, but they require significant financial investment and decades of technological development.
The Royal Society (2009) and later assessments concluded that while space-based methods may be viable in the distant future, costs and deployment challenges make them infeasible for near-term climate intervention.
[36] Costs are expected to increase over time due to reduced efficiency from larger aerosol particles,[clarification needed] requiring greater mass injections to maintain cooling levels.
[29] The method remains highly uncertain, as some studies suggest CCT could cause net warming rather than cooling due to complex cloud-aerosol interactions.
[39] Modelling studies have consistently concluded that moderate SRM use would significantly reduce many of the impacts of global warming, including average and extreme temperature, water availability, and cyclone intensity.
[40] SRM would take effect rapidly, unlike mitigation or carbon dioxide removal, making it the only known method to lower global temperatures within months.
[41] The IPCC Sixth 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.
"[29]: 69 A 2023 UNEP report similarly concluded that an operational SRM deployment could reduce some climate hazards but would also introduce new risks to ecosystems and human societies.
Some studies suggest that SRM could prevent mass coral bleaching events by reducing sea surface temperatures, but it would not address ocean acidification.
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.
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.
[60] The potential use of SRM poses several governance challenges because of its high leverage, low apparent direct costs, and technical feasibility as well as issues of power and jurisdiction.
[73] Some scientists have argued that this is unlikely, and even if true not a compelling reason to avoid SRM if it could spare future generations the considerable suffering likely to follow from unchecked global warming.
Some engagement work has suggested that SRM may in fact increase the likelihood of emissions reduction because the pursuit of such a risky approach underlines the seriousness of global warming.
Since these effects will be mostly uncertain at the time of SRM initialization, a precedent prediction of deployment length seems unlikely, with possibilities ranging from decades to multiple centuries.
[79] In 2024, U.S. government agencies were trying to create an airborne early warning system for detecting small concentrations of aerosols to determine where other countries might be carrying out geoengineering attempts, however it was not yet operational.
"[99] 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.
[116] 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.
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."
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.
[124][16]: 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".
"[133] The article listed the following billionaires as being notable geoengineering research supporters: Mike Schroepfer, Sam Altman, Matt Cohler, Rachel Pritzker, Bill Gates, Dustin Moskovitz.
In response to these activities, which were conducted without prior notification or consent, the Mexican government announced measures to prohibit SRM experiments within its borders, although it is unclear whether this became actual policy.
