As global heating accelerates and greenhouse gas concentrations keep rising, interest in solar geoengineering is growing — including from private companies and investors. The world remains off track to limit warming to 2 °C above pre-industrial levels, and 2025 was the third hottest year on record, sharpening calls for alternative approaches to limit temperature rise.
Solar geoengineering refers to deliberate interventions to alter Earth’s climate system to cool the planet. The most widely studied method is stratospheric aerosol injection (SAI), which would place highly reflective particles into the stratosphere (roughly 6–50 km above the surface) to reflect a fraction of sunlight back into space. This mimics volcanic eruptions: the 1991 Mount Pinatubo eruption injected millions of tons of sulfur gases into the stratosphere and temporarily cooled the globe by about 0.5 °C for several years.
Until recently, most research into solar geoengineering was led by nonprofits and public institutions, supported by governments and philanthropic funding. Over the past three years, though, commercial startups have entered the field. Two companies drawing attention are Israeli-US Stardust Solutions and California-based Make Sunsets. Stardust recently reported raising $60 million in venture capital. Make Sunsets began launching balloons filled with sulfur dioxide over Mexico and the US in 2022 and says it has released 213 balloons, selling them as “cooling credits.” Mexico subsequently issued a ban on solar geoengineering deployments over its territory. The amounts released so far are too small to affect global climate measurably.
SAI remains largely experimental. Since 2008, only two outdoor experiments involving small aerosol releases have been completed (in Russia and the UK), and several planned university-led field tests in the UK and Sweden were canceled after opposition from civil society groups. Most work has been limited to laboratory studies and computer modeling.
If deployed at scale, SAI would require continuous releases of reflective particles for decades, because aerosols persist in the stratosphere for only months to a few years. Once released, particles would be carried by stratospheric winds and jet streams, dispersing globally but unevenly. Models suggest a modest global average cooling — perhaps a few tenths of a degree Celsius to around half a degree — but with highly uneven regional outcomes. That patchiness could alter precipitation patterns and increase the risk of weather extremes in some places.
Health and environmental risks are a major concern. Much research has focused on sulfate aerosols similar to volcanic sulfate, but these can damage the ozone layer and increase air pollution when they descend into the lower atmosphere, potentially raising respiratory illness rates. Scientists emphasize that even particles chemically inert in the stratosphere could transform during atmospheric processing and become harmful by the time they reach the surface.
Private-sector involvement raises additional concerns about accountability, transparency, and governance. Researchers who support careful study of geoengineering warn that privately funded projects might operate without sufficient public oversight. Cynthia Scharf, a senior climate fellow at the Center for Future Generations, has warned that private money not accountable to governments could dwarf earlier public funding and complicate oversight. David Keith, a geophysical scientist, has highlighted the importance of public trust for technologies where safety and global effects are uncertain, and expressed skepticism about relying primarily on free-market funding for such high-trust research.
Some private companies claim to be developing safer alternatives. Stardust Solutions says it has created a particle made of abundant, chemically inert components and plans to publish research, but it has not disclosed particle composition and says it will not conduct outdoor testing for now. The company is seeking a patent on its particle and has engaged in outreach to policymakers; reporting indicates it has worked with a law firm to inform and lobby US officials. Stardust says it intends to work only with governments that have adequate regulatory frameworks and high global standards.
There is currently no dedicated international treaty regulating SAI research or deployment, and most national governments lack specific rules. Some experts call for an international non-use agreement or robust multilateral oversight. Precedent exists: ocean iron fertilization, another geoengineering approach, was effectively restricted in 2013 after environmental groups and some governments objected to commercialization because of risks to marine ecosystems; research remains permitted but commercial activity was banned.
Scientists stress the need for transparency in research, public engagement, and internationally agreed rules before any demonstrations or deployments proceed. They also warn that SAI does not address the root cause of climate change — rising greenhouse gas concentrations — and could carry long-term dependencies and unforeseen consequences. Given the scale of potential impacts and many “unknown unknowns,” researchers urge that decisions about geoengineering be made through democratic, multilateral processes rather than driven solely by private interests.
Edited by: Tamsin Walker