This scientist has a risky plan to cool Earth. There’s growing interest.

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.
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 1 degree Fahrenheit in the year that followed.
Today, 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.
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. 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 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. The European Union last year 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,” 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.”
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 1 degree Celsius, or 1.8 degrees Fahrenheit, over the next century, Keith said, it could help prevent millions of heat-related deaths each decade.

To understand just how contentious 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, 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 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 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.
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 second main concern has to do with unintended consequences.
“This is a really dangerous path to go down,” said Beatrice Rindevall, the chair 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,” 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.
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.
In 2006, a mutual acquaintance introduced Keith to Gates, who wanted to learn more about technologies that might help fight global warming.
“I don’t know whether that stuff will ever get used,” said Gates, a major investor in climate technology. “I do believe that doing the research and understanding it makes sense.”
Then in 2009, Keith founded Carbon Engineering, a company that developed a process for pulling carbon dioxide from the atmosphere. Last year Carbon Engineering was acquired by Occidental Petroleum for $1.1 billion.
Occidental is now building carbon capture plants. It plans to sell carbon credits to big companies 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,” Keith said, adding that he plans to give away most of his profits from the sale of Carbon Engineering, perhaps to a conservation group.
“I’m more motivated even now to push on solar geo because the rationalist case for it is looking stronger,” he 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.”

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