One morning late last year, an RV pulled into a parking lot on the outskirts of Silicon Valley. Inside it was a blue Ikea bag with huge balloons, and metal tanks full of helium and sulfur dioxide gas.

The mission was led by Luke Iseman, a 41-year-old serial entrepreneur with a mohawk hairstyle and an orange t-shirt that read “Cool Earth.” Inspired by a science fiction novel, Iseman had founded a company in 2022 called Make Sunsets. In the novel, a billionaire undertakes a type of “solar geoengineering”: shooting vast amounts of reflective particles high into the atmosphere to reflect sunlight off Earth and counter global warming. Now, Iseman was trying to do it in the real world.

Iseman took a wrench and opened the tanks, releasing the sulfur dioxide first and then the helium. He siphoned the gasses into a long tube while his business partner, Andrew Song, stood just outside the RV using the tube to inflate a weather balloon. As the balloon grew to about 6 feet wide, some sulfur dioxide – which can be hazardous to human health in high concentrations – started to leak. I began to cough, and my eyes watered.

“Don’t take big whiffs of air,” Song said.

“Just don’t breathe,” Iseman said with a laugh.

The three balloons that Make Sunsets launched that day made it to the stratosphere – a layer of the atmosphere about six to 30 miles above Earth’s surface. When the balloons popped, the sulfur dioxide gas turned to particles– reflecting enough sunlight, the company says, to offset the warming of 175 gas-powered cars for a year. But Make Sunsets and its balloons signal something bigger: a growing number of startups, research projects, and billionaire-backed nonprofits hoping to ready this tech to potentially cool the earth on a wider scale.

In the past year, the conversation around solar geoengineering as a climate solution has become more serious, says David Keith, geophysics professor and head of a new University of Chicago initiative to study a broad array of climate geoengineering ideas. “Suddenly we’re getting conversations with senior political leaders and senior people in the environmental world who are starting to think about this and engage with it seriously in a way that just wasn’t happening five years ago,” Keith says.

But as money flows in – some from investors who hope to profit from this technology – regulations around outdoor experiments and possible broader deployments aren’t keeping up, experts say. Because of the way the stratosphere works, a large-scale release of particles in one part of the world could impact a large part of the planet. Questions persist about possible risks of solar geoengineering for everything from global crops to droughts. And there are risks of unintended consequences that scientists and investors haven’t yet imagined – the unknown unknowns of trying to engineer a cooler Earth.

Shuchi Talati, founder and executive director of the nonprofit The Alliance for Just Deliberation on Solar Geoengineering, says Make Sunsets’ actions crystallize the need for urgent regulation of this tech.

“At some point down the road, they’re going to do this at a big enough scale to trigger some sort of climate impact,” Talati says. “It can be done in an effective, globally governed way, or it can be done by two crazy people in California, and it can look horrible for a lot of people.”

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