In April 2022, in the Baja California region of Mexico near the Sea of Cortez, a man named Luke Iseman took a few grams of sulfur, lit it on fire and pumped the resulting gas into a six-foot helium balloon he bought on Amazon. Then he released the balloon into the bright sky, letting it sail. In the high atmosphere, he hoped, the balloon would burst and release sulfur dioxide particles — reflecting the sun’s rays and microscopically cooling the Earth.

To some scientists, this move, first reported by MIT Technology Review, was a pointless stunt. To others, it marked the first-ever-recorded act of stratospheric solar geoengineering — a controversial technology that could blunt the Earth’s rising temperatures.

Iseman is the founder and CEO of Make Sunsets — a two-person firm that plans several more test flights this month. His start-up has triggered the worst fears of researchers who have struggled for decades to establish ground rules for solar geoengineering. The technology has almost always been seen as a last resort to counter runaway warming. Make Sunsets is not only promising to deploy this break-the-glass approach now — but to sell it for profit.

Iseman, 39, acknowledges that he is, in many ways, a geoengineering novice. A former director of hardware for the start-up incubator Y Combinator, he got interested in the subject by reading Neal Stephenson’s novel “Termination Shock.” (The book features a rogue Texas oil billionaire who uses a gigantic gun to fire sulfur into the air.)

The idea of reflecting sunlight to curb climate change has been around almost as long as humanity has been worried about an overheated planet. The very first climate report given to a U.S. president — Lyndon B. Johnson, in 1965 — suggested brightening the oceans’ surface, rather than curbing fossil fuel use.

Researchers have largely focused on the idea of injecting sulfur aerosols into the stratosphere, 12 miles into the air, to reflect sunlight and cool down the Earth. Nature does this already: After Mount Pinatubo in the Philippines erupted in 1991, sending 20 million tons of sulfur dioxide spewing into the atmosphere, global temperatures fell by about 1 degree Fahrenheit the following year.

Now, as temperatures around the world continue to rise, “stratospheric solar geoengineering,” as it’s called, is inching ahead. In 2021, the National Academies of Science released a report recommending that the United States “cautiously pursue” solar geoengineering research given the urgency of climate change. The White House is coordinating a five-year research plan. A major project at Harvard University to use balloons to test releasing sulfur particles in the atmosphere, SCoPEx, has been in the works for years.

Trying it out in the real world, however, remains contentious. In 2021, SCoPEx researchers planned to launch a balloon and gondola in Kiruna, Sweden — not to release any particles, but to test their instruments. They ultimately canceled the experiment in the face of public opposition from Indigenous and environmental groups.

Most scientists agree that even research into geoengineering — let alone actually releasing sulfur particles — should involve consulting local communities and governments. While the risks of small-scale releases are low, critics fear they could pave a path to larger ones that would affect agriculture and temperatures around the world in unpredictable ways.

Iseman, by his own account, conducted the balloon flight alone, without consulting any members of the public, a scientific team or local authorities in Mexico.

From a climate perspective, the project was probably harmless. The quantity of aerosols the company released was negligible: Iseman estimated that his balloon only dispersed “a few grams” of SO2. The United States alone releases approximately 1.8 million tons of SO2 every year, from factories, power plants, cars and other sources. (The balloon also didn’t include instruments to collect data; Iseman said he didn’t have any available, and it’s possible that the payload actually never made it to the stratosphere.)

But it also speaks to the allure and the strangeness of some types of geoengineering: Almost anyone can do it. More than a decade ago, Russ George, an American entrepreneur, dumped 100 tons of iron sulfate off a rented fishing boat and into the Pacific Ocean off the coast of Canada, trying to create an algal bloom that would absorb CO2 from the atmosphere. Many policy experts and scientists criticized the move, and the Canadian government initially investigated him for illegal dumping. (George still defends his actions, saying that he had notified Canadian authorities in advance.)

Iseman said in a video interview with The Washington Post that he was frustrated by delays to other projects, and wanted to know if the process was as simple as he imagined. “It was mainly to see that I could do it.”

“This field is not moving forward,” he added. “We’re the guys willing to go out on a limb.”

Some experts worry that the new company could set a dangerous precedent. Jesse Reynolds, an expert in solar geoengineering governance and law, pointed to the Oxford Principles for Geoengineering, a set of guidelines for research into the field that include public engagement, independent assessment of possible impacts and more.

“I looked at the five Oxford principles and they’re acting in a way that’s consistent with none of them,” Reynolds said. While it’s unlikely that the company is currently breaking any laws — due largely, he said, to the minuscule scale that it’s currently operating on — he worries that it could erode the careful norms around a new technology that have taken decades to establish.

“This is something where decisions should be made by governments, informed by the broader public,” Reynolds said.

Iseman argued that his work is critical, given the current state of the planet. Before starting the company, he said, he was depressed by what’s happening with the climate and how slow humanity has been to act to cut emissions. “Every day that we don’t inject sulfur dioxide into the stratosphere as responsibly as the state of the science will let us and as much as we can economically, species are needlessly going extinct and people are dying,” he said.

To pay for the company’s operations, according to Iseman and his co-founder, Andrew Song, Make Sunsets will sell “cooling credits” on the theory that a single gram of sulfur dioxide, released into the stratosphere, lowers global temperatures equal to keeping a ton of carbon dioxide out of the atmosphere for a year. Their website already provides an option to spend $10 to buy a single “cooling credit” worth a gram of SO2.

Under this business model, Iseman said, Make Sunsets has raised $750,000 from investors including Boost VC and Pioneer Fund. Boost VC confirmed in an email that it has supported the venture with $500,000; Pioneer Fund did not respond to a request for comment but lists the company in its online portfolio.

Several experts, including David Keith, a Harvard University professor and a lead researcher for the SCoPEx project, question the idea of selling such offsets. As a rough estimate, Keith said, one gram of SO2 offsets the warming effect of one ton of CO2. But sulfur dioxide decays in the atmosphere, only remaining for a year or two. Carbon dioxide, on the other hand, remains in the atmosphere for hundreds of years. If a person emitted a ton of carbon dioxide in 2023, therefore, they would have to continue paying Make Sunsets $10 a year to offset the warming of that ton for centuries.

The offsets’ low price, Keith said, could inspire people to buy them rather than cut emissions or remove CO2 from the atmosphere. If buyers rushed in, he added, so much sulfur dioxide could be released that it could lead global temperatures to plummet, or devalue a market that now funds projects protecting rainforests or storing CO2 deep underground.

“Solar geoengineering appears to be so cheap that if you ever connected those markets, you would either freeze the planet — or destroy the legitimate market for cutting emissions,” he said.

And reflecting sunlight, of course, does not actually remove carbon dioxide from the atmosphere. It can help curb temperatures, but it doesn’t address other effects of high carbon dioxide emissions, such as ocean acidification.

Iseman said he supports removing carbon dioxide from the atmosphere, and his company’s credits could be “bundled” with other forms of carbon offsets. “What I want people to do is pay $20 per ton, and I want that to consist of carbon dioxide removal and us,” he said.

Right now, not many people seem tempted. Apart from their early investors, Song said, Make Sunsets had 17 or 18 orders for cooling credits as of early January.

Geoengineering experts are generally not concerned that the company will do serious harm to the atmosphere. If the operation got much larger, they said, governments and local authorities would likely intervene. But they worry that such operations could set back the pace of other research, or spark copycats who might try their own ventures.

Reynolds says it’s a question of which values should rule in an era of rising temperatures and frustration: the start-up mentality or that of researchers and nations.

“This is the ‘move fast and break things’ worldview,” he said.