By Dylan Walsh
As millions of acres of Canadian forest burned in the summer of 2025, the second-worst fire season in the country’s history, smoke drifted south, settling in the Upper Midwest, prompting air quality alerts from local regulatory agencies. For several hours on the last day of July, Chicago had the dirtiest air in the world, a scumble of smoke dimming the tall-towered skyline.
During these hazy days, David Keith, professor of geophysical sciences at the University of Chicago, was completing the move into his new office on the ground floor of Ryerson Hall. It is a high-ceilinged space with pale wood floors and old doors large enough to ride a horse through. Keith had been recruited to the University from Harvard just over two years earlier, in April of 2023, and charged with establishing the Climate Systems Engineering initiative.
Keith is tall, narrow, bearded, bespectacled; a loping walker; and a warm and engaged conversationalist. When he alights upon a topic of acute interest, he hunches in his chair and begins to expound while gazing at the floor.
He is a controversial figure in the field of climate science—and in the broader world of environmental policy and activism—because he has devoted nearly all of his academic career to studying methods by which humans might deliberately intervene in the climate system to mitigate the effects of global warming. Much of this work has focused on adding sulfur to the stratosphere to reduce the amount of sunlight that reaches Earth—a skywide umbrella of aerosols to cool us just a bit, just enough.
Keith came to this work sideways. He studied physics as an undergraduate at the University of Toronto. After graduation he worked odd jobs, lived in a 12-person co-op, and spent time rock climbing in New York’s Shawangunk Mountains. He then went on to a doctorate in experimental physics at MIT where, under the guidance of David Pritchard, he helped build one of the world’s first atom interferometers. The work was “a thrill,” Keith has written, but, in some ways, hollow. If it sated his intellectual appetite, it did little else. He was troubled, too, that the Office of Naval Research funded the work: One of the interferometer’s immediate applications was improving the locational accuracy of submarines carrying ballistic missiles.
Keith was doing this work in the late ’80s, as global environmental catastrophe gained a public audience. The British Antarctic Survey reported on a large hole in Earth’s ozone layer. NASA scientist James Hansen testified before the Senate about the warming effects of greenhouse gas emissions. These were topics that Keith engaged with, vigorously, in an informal group of students from Boston-area colleges who met to discuss environmental science and policy.
The topics enthralled him, drawing on a deep personal connection to the natural world. His father was a field biologist researching toxic chemicals for the Canadian Wildlife Service. As a teenager Keith was a member of the Macoun Club, the youth arm of the Ottawa Field Naturalists’ Club, alongside a handful of bookish biology students who liked to camp. He hiked the Appalachian Trail solo from Maine to Vermont as a 17-year-old, and shortly after skied alone across Ontario’s Algonquin Provincial Park. Climate change, as he began to learn about it, inspired complex questions in precisely the way atomic interferometry did not. It raised policy implications of the largest magnitude; it was rich with intriguing and contentious scientific uncertainties.
Through a family connection, Keith met and talked with Hadi Dowlatabadi, a professor who was studying climate change in Carnegie Mellon University’s Department of Engineering and Public Policy. It was a productive chat. In 1991 Keith moved to Pittsburgh, leaving interferometry behind to begin a postdoc with Dowlatabadi.
Keith published his first paper on geoengineering one year later. The general idea had been around for decades. A 1965 report to President Lyndon B. Johnson—which, notably, spends a good deal of time fretting over human-induced climate change—discusses the potential of blanketing oceans in reflective particles. Soviet and American scientists in the ’70s wrote about creating a “stratospheric smog” to deflect sunlight. The idea surfaces and sinks, again and again, in academic literature, in government reports.
Geoengineering is a catchall term for a set of technologies and processes that range from planting trees to launching a quilt of light-deflecting mirrors the size of Brazil high into orbit between Earth and the sun. But the subject, in its diversity, can be broken into two broad approaches: those intended to pull carbon dioxide from the air, and those that cool Earth but do nothing about carbon.
Among the many ways to deflect the sun—formally known as solar radiation modification, or SRM—injecting sulfur into the stratosphere is the approach that’s likely best understood. Atmospheric dynamics in the stratosphere have been studied ever since scientists in the 1950s, eager to monitor nuclear fallout, used high-elevation U-2 flights to measure the concentration of radionuclides. Volcanic eruptions, meanwhile, have afforded centuries of insight into the cooling effects of sulfates in the stratosphere. Researchers have theorized about SRM aerosols that are potentially less pernicious than sulfates, which are known to deplete the ozone and cause pollution at ground level, but sulfates are “the devil we know,” Keith says.