David Keith was a graduate student in 1991 when a volcano erupted in the Philippines, sending a cloud of ash toward the edge of space.

Seventeen million tons of sulfur dioxide released from Mount Pinatubo spread across the stratosphere, reflecting some of the sun’s energy away from Earth. The result was a drop in average temperatures in the Northern Hemisphere by roughly one degree Fahrenheit in the year that followed.

Today, Dr. Keith cites that event as validation of an idea that has become his life’s work: He believes that by intentionally releasing sulfur dioxide into the stratosphere, it would be possible to lower temperatures worldwide, blunting global warming.

Such radical interventions are increasingly being taken seriously as the effects of climate change grow more intense. Global temperatures have hit record highs for 13 months in a row, unleashing violent weather, deadly heat waves and raising sea levels. Scientists expect the heat to keep climbing for decades. The main driver of the warming, the burning of fossil fuels, continues more or less unabated.

Against this backdrop, there is growing interest in efforts to intentionally alter the Earth’s climate, a field known as geoengineering.

Already, major corporations are operating enormous facilities to vacuum up the carbon dioxide that’s heating up the atmosphere and bury it underground. Some scientists are performing experiments designed to brighten clouds, another way to bounce some solar radiation back to space. Others are working on efforts to make oceans and plants absorb more carbon dioxide.

But of all these ideas, it is stratospheric solar geoengineering that elicits the greatest hope and the greatest fear.

Proponents see it as a relatively cheap and fast way to reduce temperatures well before the world has stopped burning fossil fuels. Harvard University has a solar geoengineering program that has received grants from the Microsoft co-founder Bill Gates, the Alfred P. Sloan Foundation and the William and Flora Hewlett Foundation. It’s being studied by the Environmental Defense Fund along with the World Climate Research Program, an international scientific effort. The European Union last year called for a thorough analysis of the risks of geoengineering and said countries should discuss how to regulate an eventual deployment of the technology.

But many scientists and environmentalists fear that it could result in unpredictable calamities.

Because it would be used in the stratosphere and not limited to a particular area, solar geoengineering could affect the whole world, possibly scrambling natural systems, like creating rain in one arid region while drying out the monsoon season elsewhere. Opponents worry it would distract from the urgent work of transitioning away from fossil fuels. They object to intentionally releasing sulfur dioxide, a pollutant that would eventually move from the stratosphere to ground level, where it can irritate the skin, eyes, nose and throat and can cause respiratory problems. And they fear that once begun, a solar geoengineering program would be difficult to stop.

“The whole notion of spraying sulfur compounds to reflect sunlight is arrogant and simplistic,” the Canadian environmentalist David Suzuki said. “There are unintended consequences of powerful technologies like these, and we have no idea what they will be.”

Raymond Pierrehumbert, an atmospheric physicist at the University of Oxford, said he considered solar geoengineering a grave threat to human civilization.

“It’s not only a bad idea in terms of something that would never be safe to deploy,” he said. “But even doing research on it is not just a waste of money, but actively dangerous.”

Shuchi Talati, the founder of a nonprofit organization called the Alliance for Just Deliberation on Solar Geoengineering, called the technology “a double-edged sword.”

“It could be a way to limit human suffering,” she said. “At the same time, I think it can also exacerbate suffering if used in a bad way.”

In a series of interviews, Dr. Keith, a professor in the University of Chicago’s department of geophysical sciences, countered that the risks posed by solar geoengineering are well understood, not as severe as portrayed by critics and dwarfed by the potential benefits.

If the technique slowed the warming of the planet by even just one degree Celsius, or 1.8 degrees Fahrenheit, over the next century, Dr. Keith said, it could help prevent millions of heat-related deaths each decade.

A planet transformed by solar geoengineering would not be noticeably dimmer during the daytime, according to his calculations. But it could produce a different kind of twilight, one with an orange hue.

He agrees that nations should stop burning coal, oil and gas, period. But Dr. Keith believes in going further.

Lean and athletic at 60, with glacier-blue eyes, Dr. Keith has spent his life outside the lab rock climbing, sea kayaking and skiing in the Arctic. He is deeply troubled by the myriad ways climate change is disrupting the natural world.

By lowering global temperatures, solar geoengineering could help restore the planet to its preindustrial state, recreating conditions that existed before enormous amounts of carbon dioxide were pumped into the atmosphere and began to cook the Earth, he said.

If there were a global referendum tomorrow on whether to begin solar geoengineering, he said he would vote in favor.

“There certainly are risks, and there certainly are uncertainties,” he said. “But there’s really a lot of evidence that the risks are quantitatively small compared to the benefits, and the uncertainties just aren’t that big.”

The only thing more dangerous than his solution, he suggested, may be not using it at all.

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To understand just how contentious Dr. Keith’s work can be, consider what happened when he tried to perform an initial test in preparation for a solar geoengineering experiment known as Scopex.

Then a professor at Harvard, Dr. Keith wanted to release a few pounds of mineral dust at an altitude of roughly 20 kilometers and track how the dust behaved as it floated across the sky.

A test was planned in 2018, possibly over Arizona, but Dr. Keith couldn’t find a partner to launch a high-altitude balloon. When details of that plan became public, a group of Indigenous people objected and issued a manifesto against geoengineering.

Three years later, Harvard hired the Swedish space corporation to launch a balloon that would carry the equipment for the test. But before it took place, local groups once again rose up in protest.

The Saami Council, an organization representing Indigenous peoples, said it viewed solar geoengineering “to be the direct opposite of the respect we as Indigenous Peoples are taught to treat nature with.”

Greta Thunberg, the Swedish climate activist, joined the chorus. “Nature is doing everything it can,” she said. “It’s screaming at us to back off, to stop — and we are doing the exact opposite.”

Within months, the experiment was called off.

“A lesson I’ve learned from this is that if we do this again, we won’t be open in the same way,” Dr. Keith said.

Behind the scenes, the Harvard team and its advisory committee became mired in finger pointing over who was to blame for the collapse of the project. Dr. Talati, a member of the Scopex advisory board, said it was “the moment of peak chaos.”

It didn’t help that there were personality conflicts. Several committee members said Dr. Keith could be ornery and headstrong, correcting colleagues in casual conversation and belittling those with whom he disagreed.

“I can be abrasive and difficult,” Dr. Keith acknowledged. “I am sometimes inappropriately forceful in making my point. I’m intense.”

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Opponents of solar geoengineering cite several main risks.

They say it could create a “moral hazard,” mistakenly giving people the impression that it is not necessary to rapidly reduce fossil fuel emissions.

“The fundamental problem is that we think we’re so smart that we don’t have to pay attention to nature’s boundaries,” Dr. Suzuki said. “But we haven’t dealt with the root cause of the problem, which is us.”

The second main concern has to do with unintended consequences.

“This is a really dangerous path to go down,” said Beatrice Rindevall, the chairwoman of the Swedish Society for Nature Conservation, which opposed the experiment. “It could shock the climate system, could alter hydrological cycles and could exacerbate extreme weather and climate instability.”

And once solar geoengineering began to cool the planet, stopping the effort abruptly could result in a sudden rise in temperatures, a phenomenon known as “termination shock.” The planet could experience “potentially massive temperature rise in an unprepared world over a matter of five to 10 years, hitting the Earth’s climate with something that it probably hasn’t seen since the dinosaur-killing impactor,” Dr. Pierrehumbert said.

On top of all this, there are fears about rogue actors using solar geoengineering and concerns that the technology could be weaponized. Not to mention the fact that sulfur dioxide can harm human health.

Dr. Keith is adamant that those fears are overblown. And while there would be some additional air pollution, he claims the risk is negligible compared to the benefits.

“There’s plenty of uncertainty about climate responses,” he said. “But it’s pretty hard to imagine if you do a limited amount of hemispherically balanced solar geo that you don’t reduce temperatures everywhere.”

Last year, after the failure to launch the Scopex experiment in Sweden, Dr. Keith made a move that stunned his colleagues. He announced he was closing the door on 13 years at Harvard and taking his ambitions to the University of Chicago, where he would build a new program around climate interventions, including solar geoengineering.

“I don’t know whether that stuff will ever get used,” said Mr. Gates, a major investor in climate technology. “I do believe that doing the research and understanding it makes sense.”

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Dr. Keith’s career can be traced to his father, Tony Keith, a wildlife biologist who attended the first global gathering to address threats to nature, the 1972 United Nations Conference on the Human Environment in Stockholm.

Dyslexia prevented him from learning to read until late in 4th grade, but when he was finally able to make sense of written words, he became a voracious reader. He also loved camping and, at 17, hiked a stretch of the Appalachian Trail solo.

After graduating from the University of Toronto, he spent months rock climbing. Looking for a way to get paid to live in the wilderness, he got a job studying walruses in the Canadian Arctic.

Dr. Keith eventually enrolled in a doctoral program at the Massachusetts Institute of Technology to study experimental physics.

In 1992, he published an academic paper, “A Serious Look at Geoengineering,” that raised the questions that would shape his career: Who should authorize the use of these technologies? Who is liable if something goes wrong?

His academic career took him from Carnegie Mellon University to the University of Calgary, where he began investigating ways to capture and store carbon dioxide. The next stop was Harvard, where he got serious about solar geoengineering.

In 2006, a mutual acquaintance introduced him to Mr. Gates, who wanted to learn more about technologies that might help fight global warming. The two men discussed climate and technology in a series of meetings over the next 10 years.

Then in 2009, Dr. Keith founded Carbon Engineering, a company that developed a process for pulling carbon dioxide from the atmosphere. Investors included Mr. Gates, Chevron and N. Murray Edwards, who made billions pumping oil from the Canadian oil sands.

Last year Carbon Engineering was acquired by Occidental Petroleum, a major oil and gas producer based in Texas, for $1.1 billion. Dr. Keith owned about 4 percent of the company at the time of the sale, delivering him a personal windfall of about $72 million.

Occidental is now building a series of enormous carbon capture plants. It plans to sell carbon credits to big companies like Amazon and AT&T that want to offset their emissions. Critics say that will only delay the phaseout of fossil fuels while allowing an oil company to profit.

“Of course I’m uncomfortable about it being sold to an oil company, no question,” Dr. Keith said, adding that he plans to give away most of his profits from the sale of Carbon Engineering, perhaps to a conservation group.

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On a summer Monday in Cambridge, Mass., the Harvard campus was mostly quiet. But inside one classroom, a standing-room-only crowd listened as experts discussed the merits and risks of solar geoengineering.

Among those featured was Frank Keutsch, Dr. Keith’s former collaborator on the Scopex experiment.

Dr. Keutsch is less sanguine than Dr. Keith when considering its potential risks.

“I compare stratospheric solar geoengineering with opiates,” he said on the panel. “They only treat the symptom and not the actual cause. You can get addicted to it if you don’t actually address the cause. In addition, like any painkiller, you’re going to have side effects. And then there are withdrawal symptoms, and that’s termination shock.”

Dr. Keutsch is now investigating whether calcium carbonate or diamond dust might be a better material than sulfur, and pondering issues around how a deployment might one day be governed. There are no current plans for a field experiment.

Academic energy in the field has followed Dr. Keith to the University of Chicago, which is allowing him to hire 10 full-time faculty members and build a new program focused on various types of geoengineering. The total cost could reach as much as $100 million.

The move has puzzled some. Dr. Pierrehumbert, who recently departed the University of Chicago for Oxford, said he was “flabbergasted” and contended that those research dollars could be better spent investigating ways to reduce the use of fossil fuels.

To celebrate his 60th birthday in October, Dr. Keith went hiking in the Canadian Rockies and came across a glacier that had shrunk dramatically in recent years. It was a visual reminder that global warming is upending the natural world, and it confirmed his central, controversial belief: Humans have already altered the planet, heating the climate with greenhouse gases. To repair the climate and return it to a more pristine state, we may need to take even more drastic action and engineer the stratosphere.

“I’m more motivated even now to push on solar geo because the rationalist case for it is looking stronger,” Dr. Keith said. “While there are still lots of strong individual voices of opposition, there are a lot of people in serious policy positions that are taking it seriously, and that’s really exciting.”