As the planet warms faster and greenhouse gas concentrations continue to rise, interest in solar geoengineering — deliberate interventions to cool the Earth — is increasing, including from private companies and investors. With the world off track to keep warming below 2 °C above pre‑industrial levels and 2025 recorded as the third‑hottest year, some are looking to alternatives to help limit temperature rise.
The leading concept under study is stratospheric aerosol injection (SAI): releasing highly reflective particles into the stratosphere (roughly 6–50 km above the surface) so they scatter sunlight back to space. The idea draws on the cooling that followed large volcanic eruptions; for example, the 1991 Mount Pinatubo eruption lofted sulfur gases into the stratosphere and temporarily cooled the planet by about 0.5 °C for several years.
Until recently, most geoengineering research was driven by public institutions, nonprofits, governments and philanthropic funders. Over the last three years, however, commercial startups have entered the field. Two companies that have attracted attention are Israeli‑US Stardust Solutions and California‑based Make Sunsets. Stardust reports raising $60 million in venture capital. Make Sunsets began launching balloons filled with sulfur dioxide over Mexico and the United States in 2022 and says it has released 213 balloons, marketing the releases as “cooling credits.” Mexico later banned solar geoengineering deployments over its territory. The volumes released to date are far too small to have any measurable impact on global climate.
Empirical SAI work remains limited. Since 2008 only two outdoor experiments involving small aerosol releases have taken place (in Russia and the UK), and several proposed university‑led field tests in the UK and Sweden were canceled after public opposition. Much research so far has been laboratory‑based or modeling studies.
If deployed at scale, SAI would require continuous injections for decades because stratospheric aerosols persist for months to a few years. Once injected, particles would be transported by stratospheric winds and jet streams, spreading globally but unevenly. Models project a modest global average cooling — perhaps a few tenths of a degree up to around half a degree Celsius — but effects would be highly uneven regionally. That spatial patchiness could change precipitation patterns and increase the risk of weather extremes in some places.
Health and environmental risks are central concerns. Many studies focus on sulfate aerosols similar to volcanic sulfate, which can damage the ozone layer and, when they descend into the lower atmosphere, worsen air pollution and potentially increase respiratory illnesses. Scientists also caution that particles thought to be chemically inert in the stratosphere may transform through atmospheric processes and become harmful by the time they reach the surface.
Private‑sector involvement adds further governance challenges. Researchers who support careful, transparent study of geoengineering warn that privately funded projects might proceed with insufficient public oversight. Observers — including climate policy experts — have warned that private investment, which is not directly accountable to the public, could quickly outpace earlier public funding and complicate democratic oversight. Some scientists stress that technologies with uncertain global impacts require public trust and multilateral governance rather than being driven primarily by market incentives.
Some companies claim to be developing safer particles. Stardust Solutions says it has designed a particle composed of abundant, chemically inert materials and plans to publish supporting research; however, it has not disclosed the particle’s composition and has said it will not conduct outdoor testing for now. The company is seeking a patent and has engaged in outreach to policymakers, reportedly working with a law firm to advise US officials. Stardust says it intends to operate only with governments that have robust regulatory frameworks.
Currently there is no dedicated international treaty that governs SAI research or deployment, and most national governments lack specific regulations. Some experts urge an international non‑use agreement or strong multilateral oversight. There is precedent for restricting commercial geoengineering: in 2013 ocean iron fertilization was effectively limited after environmental groups and some governments objected to commercialization because of risks to marine ecosystems — research remains permitted but commercial activity was constrained.
Scientists emphasize the need for transparency, broad public engagement, clear rules and international agreement before any demonstrations or deployments proceed. They also stress that SAI would not address the root cause of climate change — rising greenhouse gas concentrations — and could create long‑term dependencies and unforeseen consequences. Given the scale of potential impacts and many “unknown unknowns,” researchers urge that decisions about geoengineering be taken through democratic, multilateral processes rather than being driven solely by private interests.
Edited by Tamsin Walker