Over the past few months, Europe has been hit by five heatwaves with devastating effects on the environment. Wildfires, droughts, and record-high temperatures are becoming an increasingly severe public health issue. Almost 3000 people have died because of heat-related causes in the UK alone between May and June. More than 10,000 people have died across Europe in June due to heatwaves, while healthcare systems are close to collapse and healthcare workers find themselves operating in unsafe conditions.

Some scientists and policymakers are quietly debating a radical, planetary-scale intervention as a potential solution for this climate crisis: solar radiation modification (SRM). The most widely discussed SRM technique, called stratospheric aerosol injection (SAI), involves spraying reflective nanoparticles into the stratosphere to mimic the cooling effect of major volcanic eruptions, reflecting sunlight back into space before it heats the planet.

On paper, the public health arguments for SAI appear compelling. Climate models suggest that just 1 °C of cooling from SAI could prevent more than 400,000 heat-related deaths annually by the year 2080. SAI is projected to reduce surface ozone levels by roughly 7%, averting approximately 67,000 respiratory-related deaths every year. Although ozone depletion elevates surface ultraviolet (UV) radiation, increasing skin cancer risks, research indicates that the mortality benefits of cleaner air would outweigh UV risks by a 60-to-1 ratio.

But there is a catch. Much of this research is being funded by proponents of solar geoengineering, including big tech, Aarti Gupta, PhD, professor of global environmental governance at Wageningen University in Wageningen, Netherlands, told Medscape News Europe. She is co-initiator and co-lead of an academic initiative calling for an International Non-Use Agreement on Solar Geoengineering. This call has now been signed by more than 600 scientists worldwide.

While the scientific literature increasingly includes optimistic modeling of SAI interventions, the broader scientific consensus is to stop solar geoengineering research, she said. “If you look at who is pushing this conversation, it’s Silicon Valley investors, a group of billionaires and their philanthropists, largely in the United States, funding these advocacy organizations and research groups who are proliferating now,” Gupta said. “It is still a relatively small core group of researchers who focus on solar geoengineering. The vast majority of scientists do not think about it or think it’s a bad idea.”

The potential consequences on public health remain largely unexplored. “Any technology that has the potential to alter the Earth’s climate system should not be considered solely a technological or climate issue. It is also, and perhaps above all, a public health issue,” Alessandro Miani, MD, PhD, president of the Italian Society of Environmental Medicine (SIMA) and professor at the University of Milan, Milan, Italy, told Medscape News Europe.

“At present, our knowledge remains limited. The scientific literature specifically addressing the health effects of solar radiation modification is still relatively scarce,” he said. “Much of what we know is inferred from climate modeling, atmospheric science, volcanic analogs, and existing evidence on how environmental changes affect health. I am not aware of research groups specifically and systematically investigating the human health implications of SRM. This represents an important scientific gap.”

A Medicine for the Planet?

From an environmental medicine perspective, climate remains one of the primary determinants of population health. “As environmental physicians, our responsibility is to ask a simple but fundamental question: Will this intervention ultimately improve people’s health, or could it introduce new and unforeseen health risks?” Miani said.

The projected benefits are far from evenly spread. Model projections show that reductions in heat-related deaths are heavily concentrated in hotter, climate-vulnerable regions, whereas cooler regions might experience a minor increase in winter mortality due to reduced seasonal warmth.

“Average global outcomes are never sufficient. We must understand who benefits, who bears the risks, and whether those risks are distributed fairly across populations and countries,” Miani said. He added that older adults, children, and outdoor workers stand to gain the most from reduced thermal stress but could face unpredictable risks if rainfall or food systems are disrupted. “The absence of evidence of harm should never be interpreted as evidence of safety.”

The Governance of Unknowable Risks

Beyond climate models, many social scientists and health policy experts view SAI as a governance impossibility that carries immense, unknowable risks.

A major concern is that small-scale trials or computer models can never reveal the full global health consequences of solar geoengineering. “One can never know the planetary scale risks and the impact of any use of SAI until we have actually used it for a sustained period of time,” Gupta said. “But planetary scale experimentation essentially is the same as deployment.”

Also, Gupta pointed out that while SAI is proposed as a temporary solution, “it’s not a few weeks; it might be decades or centuries of aerosol injection.”

“Climate systems are nonlinear, and there are many feedback loops. We don’t know what the impact would be on food systems, on the precipitation pattern, droughts, floods, extreme events,” she said.

A Skewed Research Landscape

A major point of contention within the medical and scientific community is the source of funding that is driving SRM research. Much of the literature promoting SRM as a quick and easy climate fix is funded by its proponents. And that funding landscape has expanded rapidly in recent years.

Private funding for solar geoengineering has grown twentyfold from less than $6 million in 2020 to roughly USD 120 million in 2025. Private sources (philanthropy and venture capital) account for more than 70% of the total funding and governments trail behind at less than 12%. The vast majority of private funders are in the Global North and maintain strong ties to the technology sector. The US and the UK are the primary sources of capital, and approximately two thirds of the recipients of this funding are based in the US or Europe. In 2025, the UK’s Advanced Research and Invention Agency entered the field with the largest-ever government-funded SRM program. This initiative includes five research projects that intend to conduct outdoor testing, following a previous USD 75 million commitment by the UK government to advance the field.

By contrast, research examining the potential harms of SAI — including toxicologic hazards, disease redistribution, and ecological disruption — is typically underfunded, carried out in independent researchers’ free time, or backed by government science foundations and international research initiatives.

The current reliance on private capital for funding SAI research raises concerns regarding vested interests. Private control of intellectual property could allow industries to influence future deployment choices for their own benefit rather than the global public good.

This risks creating a dangerous mitigation deterrence, where the false promise of a technological quick-fix weakens the political resolve to phase out fossil fuels, which is the ultimate driver of climate-driven health crises, Gupta said. “Keeping solar geoengineering on the table will ensure that it takes the urgency out of what we know we need to do to address climate change,” she said.

Regulatory Divides and the Need for Precaution

SRM techniques would require deployment across generations and over global scales. Current international law lacks a clear, equitable global governance framework to oversee deployment, handle transboundary liability, or manage compensation for regions harmed by unexpected climate shifts, Gupta explained.

“It’s very implausible that we could ever be able to govern such a planet-altering technology globally and over decades or centuries in an effective, stable, and equitable manner,” she said.

In fact, regulatory approaches to geoengineering are already diverging across continents. While the UK and US are exploring outdoor geoengineering experiments and deployment frameworks, the European Union has taken a firm, precautionary stance.

In late 2024, comprehensive reports from the EU Scientific Advice Mechanism, including evidence reviews by Science Advice for Policy by European Academies and independent opinions from the Group of Chief Scientific Advisors and the European Group on Ethics in Science and New Technologies, called for strict governance and a precautionary approach toward outdoor SRM deployment. Meanwhile, international bodies like the African Ministerial Conference on the Environment have explicitly stated that solar geoengineering should play no part in climate management.

For frontline physicians and public health leaders, rigorous, independent evaluation and precaution must come before any open-field testing, Miani said. “Environmental medicine has always embraced the precautionary principle. Any technology capable of influencing environmental systems on a large scale should undergo rigorous and independent health impact assessment before any deployment and, depending on the scale and objectives, even before field experiments.”

Miani said that solar geoengineering can never be treated as a substitute for decarbonization. “[SRM] does not address the underlying cause of climate change. I strongly support independent scientific research, but I believe it would be premature to consider large-scale deployment with the evidence currently available.”

Gupta and Miani reported having no relevant financial relationships.

Manuela Callari is an independent science journalist specializing in human and planetary health. Her work has been published in The Medical Republic, Rare Disease Advisor, New Scientist, The Guardian, MIT Technology Review, and others.