Solar geoengineering is the deliberate interference with the environment in order to cool the climate, typically by reflecting sunlight back into space to prevent energy being trapped in the atmosphere as heat. Proponents argue the methods could cheaply and substantially reduce global temperatures. Critics highlight the risk of altering weather systems and disrupting atmospheric chemistry—with global and ungovernable consequences—while distracting from the work of cutting carbon emissions.
Paul Crutzen, a Nobel-prize-winning atmospheric chemist, proposed in 2006 that a comparatively small amount of sulphur injected high into the stratosphere could provide as much cooling as the vast quantities of sulphur pollution in the lower atmosphere, and with far less harm to human life. The idea grew partly from observations that sulphate aerosols from coal burning reflect sunlight and thicken clouds; China's clean-air improvements since 2013, which cut sulphur emissions by 20m tonnes a year, have contributed to accelerated warming as these cooling aerosols diminished.
Almost all previous proposals for outdoor geoengineering experiments have been cancelled or put off owing to public outcry, leaving a great deal of critical basic science undone.
In April 2025 Britain's Advanced Research and Invention Agency (ARIA) announced £56.8m ($75.4m) in funding for geoengineering projects, to be disbursed over five years to 21 projects. The commitment made Britain the largest state funder of solar-geoengineering research, representing nearly 40% of all solar-geoengineering funding that SRM360, an educational non-profit, estimates was awarded up to the end of 2024.
ARIA's programme, directed by Mark Symes, an electrochemist at the University of Glasgow, earmarked half the cash for five outdoor experiments:
All experiments are subject to strict limits on geographic scale, environmental assessments and the agreement of nearby communities. An oversight committee provides ARIA with independent expert advice.
Marine cloud brightening (MCB) adds tiny particles of sea salt to the troposphere—from ships or barges—to form reflective hazes and make cloud droplets smaller and more numerous, rendering clouds longer-lived and more reflective and thus reducing sunlight reaching the sea below. No one has yet built hardware capable of delivering the ultrafine saltiness needed.
Matthew Henry of Exeter University and colleagues modelled whether MCB could preserve Arctic sea ice, spraying virtual sea-salt aerosols above all open Arctic water while greenhouse-gas levels rose realistically; in all three models, ice cover persisted and side-effects outside the area were minimal.
Stardust Solutions is an Israeli startup developing tiny particles for stratospheric injection. On May 14th 2026 it published preprints describing its particles, which are spheres less than a thousandth of a millimetre across made of amorphous silica (the same as opals) with a specially treated surface; a second version has a silica shell and calcium-carbonate core. Stardust pitches itself as an "end-to-end" supplier of geoengineering technology to governments. Its initial $15m of funding came through Awz Ventures, a security-linked technology investor based in America, Canada and Israel whose advisory board is led by Stephen Harper, a former Canadian PM. In October 2025 a second round brought $60m more (including from the venture-capital arm of Exor, a shareholder in The Economist's parent company), making Stardust the most-funded solar-geoengineering research outfit in the field according to SRM360. Most other geoengineering research is grant-funded; critics note Stardust's commercial interest in deployment.
Spraying sulphur dioxide into the stratosphere at high latitudes is technically more feasible than doing so in the tropics, because the tropopause—the boundary between troposphere and stratosphere—drops from around 20km at the equator to heights reachable by ordinary airliners at 60°N and 60°S.
However, polar-only SAI damages the ozone layer, deposits sulphate on ecosystems at high and mid latitudes, and cools the northern hemisphere enough to shift the intertropical convergence zone (ITCZ), altering rainfall and drought patterns throughout the tropics. It also appears illegal: all countries in and near the Arctic have signed the Convention on Long-Range Transboundary Air Pollution (CLRTAP), whose Gothenburg protocol limits sulphur emissions to levels far below what SAI would require.
Research led by Alistair Duffey, formerly of University College London and now at Reflective, an NGO focused on solar geoengineering, suggests that SAI using jets flying at 13km to inject 12m tonnes of sulphur dioxide a year—half at 60°N, half at 60°S—could lower the global average temperature by 0.6°C. But the same cooling could be achieved with half the sulphur if injected at 30°N and 30°S, which would also cool the tropics more—home to the people most vulnerable to warming.
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