Last weekend, an underwater volcanic eruption covered the island nation of Tonga in ash. It sent tsunami waves surging across the entire West Coast. And it also released a cloud of sulfur dioxide, a chemical that, in large enough quantities, reflects the sun’s rays and cools the planet.

Scientists quickly determined that, unlike Mount Pinatubo’s eruption in 1991 — which cooled the planet by around 1 degree Fahrenheit for several years — the Tonga volcano hadn’t released enough sulfur dioxide to alter global temperatures. But the eruption illustrated a question that has been dogging scientific and climate experts for decades: If the world got unbearably hot, should scientists and governments opt to put sulfur dioxide or similar chemicals into the atmosphere to slow the rate of global warming? Is it ethical to even research such technologies?

In an open letter published Monday in the journal WIREs Climate Change, more than 60 researchers from around the globe offered a resounding “no” to both questions. They called for an “international non-use agreement” on so-called solar geoengineering technologies, which would cool the planet by releasing sun-reflecting chemicals into the atmosphere. The authors want governments to ban outdoor experiments and deployment of solar geoengineering, prohibit national funding agencies from providing financial support, and refuse patents for such technologies. The signatories included many prominent climate scientists, as well as the writer Amitav Ghosh and Sheila Jasanoff, an expert on science policy at Harvard’s Kennedy School of Government. 

Solar geoengineering technology, they say, poses an “unacceptable risk” to the planet’s environment, climate, and most vulnerable people. “Governments and the United Nations need to take effective political control and restrict the development of solar geoengineering technologies before it is too late,” they wrote.

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The prospect of dimming the sun to combat global warming has been in discussion for almost as long as climate change itself. The first report on global warming that was handed to a U.S. president — Lyndon Johnson in 1965 — suggested it as a way to halt rising temperatures without stopping the use of fossil fuels. And in the past few years, attention to the concept has grown. Last year, the U.S. National Academies of Science created a plan for a research program that would investigate the idea, and a Harvard project planned to test a solar geoengineering balloon in Kiruna, Sweden. (The test flight was halted after backlash from Swedish indigenous communities.)

Critics of the technology argue that it could create a moral hazard: that is, if solar geoengineering becomes an option, the world might not try so hard to reduce greenhouse gas emissions, the actual underlying cause of global warming. Companies heavily invested in fossil fuels could also use it as an excuse to avoid reducing the use of oil and gas. 

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The writers of the open letter argue that solar geoengineering could cause uneven impacts around the globe — potentially affecting local weather patterns or food supply. What’s more, they suggest that the new technology is effectively “ungovernable.” If one country decides to spray aerosols into the atmosphere, the repercussions will affect the entire globe, whether the residents of poor countries have agreed to it or not. The deployment of solar geoengineering, they write, would require creating international organizations with “unprecedented enforcement powers” that don’t yet exist.

But other researchers have argued that solar geoengineering may be necessary to research — even if it is never deployed. Some have critiqued the open letter as an attempt to stifle scientific progress, or have argued that further research could eventually be useful to countries who face the worst impacts of climate change — heat, extreme weather, and drought. 

Holly Jean Buck, a professor at the University of Buffalo and an expert in geoengineering, wrote on Twitter: “Can you not imagine someone in, say, 2050, who is suffering from extreme heat, wondering why their parents’ generation decided to forbid research on something that might be able to cool the climate and save them from a dangerous heat wave?”