We can safely experiment on reflecting sunlight away from Earth. Here's how | Dakota Gruener and Daniele Visioni

"The world is warming fast and our options to avoid catastrophic harm are narrowing. 2024 was the first full year more than 1.5C hotter than the 19th-century average. Emissions are still rising, with fossil fuel use expected to hit a new high in 2025. Permanent carbon removal technologies often cited as a fix are removing just tens of thousands of tonnes annually, almost nothing relative to the 5-10bn tonnes needed."

"But how? In 1991, Mount Pinatubo erupted and sent about 15m tonnes of sulfur dioxide into the stratosphere, cooling the planet by about 0.5C. That eruption became a natural experiment, and inspired the idea of stratospheric aerosol injection (SAI). Models suggest SAI could offset 1C of warming with about 12m tonnes of SO per year far less than we emit now unintentionally from industrial processes, but with far greater cooling effect."

"Let's be clear: SAI is no substitute for cutting emissions. If deployed and then suddenly halted, the planet will experience rapid rebound warming. Poorly designed or uncoordinated interventions could shift precipitation patterns in catastrophic ways. But that's exactly why research is needed not to green-light deployment, but to understand whether SAI could ever be used safely, effectively and in the public interest. Some argue that the risks of misuse mean it shouldn't even be studied."

Global temperatures have risen rapidly, with 2024 the first full year more than 1.5C above the 19th-century average while emissions and fossil-fuel use continue rising. Existing permanent carbon removal technologies remove only tens of thousands of tonnes annually versus the 5–10 billion tonnes needed. Reflecting a small fraction of incoming sunlight could strengthen Earth's natural heat shield; the 1991 Mount Pinatubo eruption injected roughly 15 million tonnes of sulfur dioxide, cooling the planet about 0.5C. Models indicate stratospheric aerosol injection could offset roughly 1C with about 12 million tonnes of SO annually. SAI is not a substitute for emissions cuts; sudden halt would cause rapid rebound warming, and poorly designed deployment could alter precipitation catastrophically. Careful, open research is necessary to determine whether a well-governed approach could reduce warming safely and effectively.

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