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SARATOGA, Calif. — A silver Winnebago pulled up to a self storage warehouse on the outskirts of a Silicon Valley suburb and three renegade climate entrepreneurs piled out, all mohawks, mustaches and camouflage shorts.

Working swiftly, the men unlocked a storage unit crammed with drones and canisters of pressurized gas. Using a dolly, they wheeled out four tanks containing sulfur dioxide and helium, and stacked them on the floor of the camper van. Then, almost as quickly as they arrived, they were on the road, headed for the golden hills near the Pacific Ocean.

With their jury-rigged equipment and the confidence that comes with having raised more than $1 million in venture capital, they were executing a plan to release pollutants into the sky, all in the name of combating global warming.

“We’re stealth,” said Luke Iseman, one of the co-founders of Make Sunsets, delighting in their anonymity as he rode in the back. “This looks like just another R.V.”

Make Sunsets is one of the most unusual start-ups in a region brimming with wild ideas. Iseman, 41, and his co-founder, Andrew Song, 38, claim that by releasing sulfur dioxide into the stratosphere, they can reflect some of the sun’s energy back into space, thereby cooling the planet.

It’s a gutsy undertaking, yet it has at least a partial grounding in science. For 50 years, climate scientists have suggested that releasing aerosols into the stratosphere could act as a buffer and reduce the heat from the sun. Volcanic eruptions have temporarily cooled the planet this way in the past, but no one has attempted to intentionally replicate the effect at scale.

As the perils of climate change become more extreme, interest in the idea, known as stratospheric solar geoengineering, is growing. Scientists at Harvard, Cornell, Colorado State and Princeton are studying it and the University of Chicago recently launched an ambitious research program.

But all geoengineering is not created equal. While universities are pouring millions of dollars into research, others, avowing concern about global warming and seeing a business opportunity, are barreling ahead without any scientific study. Mr. Iseman got the idea for Make Sunsets from a sci-fi novel.

So far, the company is releasing sulfur dioxide on a tiny scale. But some experts say that broader efforts to disrupt the delicate interactions between the Earth’s atmosphere, ocean, land and sea ice could result in catastrophic unintended consequences. For example, blocking sunlight could interfere with the monsoon season, which is critical for agriculture, income and food supply in India.

Animated by the “move fast and break things” credo that permeates Silicon Valley, the founders of Make Sunsets have no such concerns. They are selling “cooling credits” to customers who want to offset their personal carbon emissions. And a few times each month, after selling enough credits, they head for the hills and release balloons full of sulfur dioxide into the California sky.

“This is the one tool realistically that can cool Earth in our lifetime,” said Mr. Iseman. “Every day we’re not doing this leads to needless harm.”

Sikina Jinnah, a professor of environmental studies at the University of California Santa Cruz who has studied geoengineering, is also concerned about harm. “They are a couple of tech bros who have no expertise in doing what they’re claiming to do,” she said. “They’re not scientists and they’re making claims about cooling credits that nobody has validated.”

∴

A 30 minute drive along winding roads led the men to a steep unpaved road, and then to a patch of dirt on an undeveloped hillside. Mr. Iseman bought the land during an online foreclosure auction a few years ago, thinking he might camp on it someday and commute to work. That never happened, but when he started sending up balloons, he realized he already owned an ideal launch site.

After bringing the Winnebago to a rest, Mr. Iseman and Mr. Song, along with another Make Sunsets employee, Kiran Kling, got to work. They spread out a dirty canvas tarp and rolled out the tanks of sulfur dioxide and helium. Then Mr. Iseman produced a large, cream-colored weather balloon from a blue Ikea shopping bag.

The men donned respirators, then ran a hose from one of the tanks to the balloon. As Mr. Iseman held the balloon off the ground, Mr. Song opened the tank’s valve, releasing a steady stream of sulfur dioxide.

After a few minutes, Mr. Kling, who stands well over six feet tall, used a hand-held scale to weigh the balloon. It contained about 1.7 kilograms, or almost four pounds, of sulfur dioxide.

The men then switched tanks and continued to inflate the balloon, this time with helium. When the balloon was full and Mr. Iseman was holding it above his head, he sealed it with rubber bands and some electrical tape, then affixed a GPS device that would track its movements.

The technology is rudimentary and little changed from Mr. Iseman’s first efforts to alter the stratosphere, which he attempted after a peripatetic career on the fringes of the tech industry.

Mr. Iseman grew up near Wilkes-Barre, Pa., and studied business at the University of Pennsylvania. After backpacking around the world, he moved to Austin, Tex., where he started a bicycle taxi company. It was an entrepreneurial lark for Mr. Iseman, and an expression of his belief that societal change could follow individual action.

He then co-founded Edyn, a start-up that made an automated garden monitoring system. That company was nurtured by Y Combinator, the Silicon Valley incubator previously run by Open A.I. founder Sam Altman.

The gardening system worked, and was even on the shelves of Home Depot. But sales were lackluster, Edyn went bust and Mr. Iseman became the head of hardware for Y Combinator. That’s where he met Mr. Song, who was working at Indiegogo, the crowdfunding platform.

After bouncing around a bit more, he was drawn to the kite surfing and spearfishing in Baja California, Mexico, and decided to set up shop there. Then, in early 2022, as Mr. Iseman installed solar panels on the roof of his R.V., he listened to the audiobook of “Termination Shock,” a science fiction novel.

The book, by Neal Stephenson, plays out what happens when a billionaire in Texas takes it upon himself to start a massive solar geoengineering program, spraying huge quantities of sulfur dioxide into the air with a giant cannon. Mr. Stephenson declined to discuss Make Sunsets.

Mr. Iseman began researching the technology and was immediately convinced it could work. “I was like, what am I missing here?” he said. “Like, why is no one doing this?”

In April 2022, he ordered a few small weather balloons and some yellow powdered sulfur from Amazon. Attempting to turn the sulfur into gas by lighting it aflame in a pot, he started choking as the fumes burned his lungs, and stopped.

After that botched attempt, Mr. Iseman found a way to cover the pot and pump the sulfur dioxide into a balloon. He then added some helium, secured it with a zip tie, and let it go.

“As crazy as it sounds, that was likely the first time someone intentionally put sulfur dioxide into the stratosphere,” he said.

∴

Back in California, the balloon was fully inflated and the midday sun was beating down. Turkey vultures circled and a light wind rustled the palm trees. With Mr. Kling holding the payload aloft, Mr. Iseman counted down.

“Three, two, one, launching,” he said.

The balloon leaped away, soaring into the blue sky. The sulfur dioxide inside would fulfill the orders for 1,700 cooling credits, which Make Sunsets had sold for a total of $2,200. Each credit, they claim, offsets the warming produced by one ton of carbon dioxide emissions for one year.

It is an equivalency they arrived at after studying academic literature on stratospheric geoengineering, calculating the projected cooling effects of large amounts sulfur dioxide in the stratosphere and contrasting that with the amount of warming caused by carbon dioxide emissions. But there have been no experiments done to validate their claims, nor any detailed analysis of whether such small deployments can achieve a cooling effect.

“There doesn’t seem to be any transparency behind their calculations,” said Michael Gerrard, the founder of the Sabin Center for Climate Change Law at Columbia University, who has studied solar geoengineering. “I don’t want to overplay my scientific knowledge, but what little I have makes me deeply suspicious.”

There is a growing regulated market for so-called carbon credits, which companies and individuals buy to offset their emissions. Carbon credits are typically created by planting trees, preserving nature or sucking carbon dioxide out of the air and storing it underground.

But there are no standards for “cooling credits,” an invention of the Make Sunsets team. Mr. Iseman said that the largest marketplaces that facilitate the buying and selling of carbon credits declined to work with them.

Scientists say the amounts of sulfur dioxide they are releasing have essentially no impact. “It’s too small-scale to have any effect on temperatures,” said Holly Jean Buck, a professor at the University at Buffalo who has written a book on geoengineering.

Many tons of sulfur dioxide would need to be injected into the stratosphere over a sustained period of time to have a noticeable effect on global temperatures, according to researchers.

Make Scientists has neither staff scientists nor a scientific advisory board. Nevertheless, the company’s pitch is appealing to at least a few hundred paying customers, among them Sam Caspersen, a lawyer based in Connecticut.

A few years ago, Mr. Caspersen met David Keith, a scientist who is the leading proponent of stratospheric solar geoengineering. After becoming convinced that the technology could help cool the planet, he donated a six-figure sum to Dr. Keith’s program at Harvard.

But when that program failed to complete even a basic engineering test in preparation for a solar geoengineering experiment, Mr. Caspersen said he grew jaded by the plodding pace of academia. “I have to say it left a very bad taste in my mouth,” he said.

(Dr. Keith has since moved to the University of Chicago to create its new geoengineering center.)

Mr. Caspersen acknowledged that Make Sunsets is not operating a scale that would allow them to measure the impact of their work. “Make Sunsets has not deployed anywhere near enough sulfur oxides into the upper atmosphere to measure the efficacy and cooling,” he said.

At the same, he defended the approach and said it could be making a small impact. “The lack of measurable data from their current program does not mean there is not a very tiny amount of current cooling,” Mr. Caspersen said. “If you plant one tree, doesn’t that mitigate climate change, even if one cannot measure it?”

The same logic also appeals to the venture capitalists who are backing Make Sunsets.

When Adam Draper, a partner at Boost VC, was introduced to Mr. Iseman in late 2022, he was impressed and became its first investor, writing a $500,000 check. “You can research all you want, but you don’t know until you try something,” Mr. Draper said. “And I’d say the fear of trying is the worst fear ever, because that means that no one’s ever going to do anything.”

Despite the concerns of many academics, and the absence of data, Mr. Draper said he is convinced that the men are making a difference. “There is real science that backs up exactly what they’re doing,” he said.

After raising his initial round of financing, Mr. Iseman convinced Mr. Song to come onboard. The men set up an online store to sell cooling credits in November 2022. Within weeks the first orders started coming in.

Mr. Iseman and Mr. Song raised another $1 million, including from the firm founded by Mr. Draper’s father, the famous venture capitalist Tim Draper.

Since then, they have launched more than 80 balloons into the stratosphere. After media reports that they had sent some up from Baja California, the Mexican government said it would consider banning solar geoengineering experiments in the country.

Ms. Jinnah worries that in acting on their own, without independent verification of their work, the Make Sunsets team is setting back the whole field of solar geoengineering. “They’re making it much more difficult for people who are doing legitimate research,” she said. “They’re being quite irresponsible with the way they’re rolling out their project.”

Mr. Iseman doesn’t care. With planet-warming emissions still on the rise and deadly heat waves, floods and storms claiming lives around the world, he believes it is imperative to do everything possible to blunt global warming.

There are no laws prohibiting the dispersal of small amounts of sulfur dioxide in California. And if the government did try to curtail their work, Mr. Iseman said Make Sunsets would simply go do it somewhere else, possibly offshore from boats on the ocean.

By proceeding with a small-scale attempt at solar geoengineering, Mr. Iseman said he is moving the Overton window, expanding what is possible in the future. “The fact that we’re doing it makes it more likely that others will do it,” said he.

∴

An hour after releasing the balloon, the Make Sunsets team was tucking into lunch at a Mexican restaurant. Over tacos, the trio reflected on the day’s work and kept tabs on the balloon via the GPS device.

It had gained altitude and was drifting south.

Mr. Iseman remained unbothered by the criticism that he and his colleagues had gone rogue, believing the moment calls for immediate, drastic action. “It’s intellectually dishonest for people to just pretend that we’re going to plant a couple trees and fix this,” Mr. Iseman said. “The numbers don’t even vaguely work out. It’s comically out of balance with what we need.”

After lunch, the men piled back into the Winnebago and headed for home.

A few hours later, the results were in. The balloon had drifted about 50 miles, flying more or less directly above Ms. Jinnah’s office in Santa Cruz.

When it reached an altitude of 35 kilometers, the pressure became too great and the balloon popped, just as it was supposed to, dispersing a tiny bit of sulfur dioxide into the stratosphere, effects unknown.

This article originally appeared in The New York Times."