By James Temple
Jim Franke pulls away the cover page of a presentation on the wraparound desk in his office, revealing an illustration of an odd-looking aircraft with massive wings stretching out from a stubby fuselage.
The uncrewed plane is soaring thousands of meters higher than commercial jets fly—so high you can see the curvature of the Earth. It’s precisely the type of aircraft one would need to begin artificially cooling the planet. Those outsize wings would keep the plane and its payload aloft in the stratosphere, about a dozen miles (or 20 kilometers) above the surface, where the air is much thinner—as little as 5% the density near the ground. Once at altitude, the plane would release materials that could, after a few steps of chemistry, reflect sunlight back into space.
“If you want to get to 20 kilometers in the near term, this is probably the best bet,” says Franke, a research assistant professor at the University of Chicago.
The concept came from volcanoes. Massive eruptions in the past have reduced temperatures worldwide by blasting sulfur dioxide and other compounds into the stratosphere, where they convert into sunlight-scattering particles. Hundreds of studies in recent decades have suggested that a human attempt to mimic this mechanism would work quickly and efficiently—at least within the confines of climate models.
But these computer simulations are approximations of how the real world works. They gloss over numerous challenges. Like the fact that aircraft capable of carrying the necessary loads to the necessary altitudes don’t exist. Or that we don’t know for sure how to release material so that most of it turns into tiny reflective aerosols instead of, say, clumping together and falling out of the sky. Or even what specific substance we would want to load onto an aircraft, given open questions about safety, cost, and effectiveness.
Amid these compounding unknowns, more and more research on solar geoengineering is moving beyond computer simulations, delving into the detailed design and practical engineering work that would be needed before we could carry out a campaign to dial down temperatures. The tasks required range from inventing high-altitude aircraft to mastering the precise chemistry and delivery mechanisms for dispersing materials to building out the monitoring infrastructure that we’ll need in order to know if any of it actually works.
The question of whether we should geoengineer the planet has no clear-cut answer. It might save millions of lives by reducing the dangers of catastrophic heat waves, floods, droughts, and famines. But many fear it’s too dangerous to even consider, much less seriously study, arguing that we can’t possibly predict the spiraling consequences of manipulating such large, complex, interconnected planetary systems.
Critics argue that the building momentum in this phase of research will make it ever more likely that someone, somewhere in the world, will eventually pull the trigger on geoengineering, no matter the remaining unknowns or the dangers for certain parts of the world.
“I do think it’s very dangerous because of what we know about science and technology,” says Jennie Stephens, a professor of climate justice at Maynooth University in Ireland. “The more investment that’s made, the further the advances, the more likely it is that it will be deployed.”
But proponents of this practical research argue that playing out how we’d mount a solar geoengineering program will improve our understanding of the potential benefits and risks, helping to ensure that if anyone does try to tweak the climate, they might at least do so in an informed and potentially safer way.
It’s still very much a niche field. Much of the work now underway is happening at the Climate Systems Engineering Initiative (CSEi) at the University of Chicago, which formally launched in 2024 under the leadership of the prominent geoengineering researcher David Keith.
Franke, a professional engineer before earning his doctorate in geosciences, is overseeing a series of overlapping research projects and collaborations aimed at resolving many of the engineering uncertainties. That includes working out the designs now on his desk—renderings of the type of aircraft that could be used in the initial phase of a geoengineering program.
Franke argues that more computer simulations are simply not going to answer the big remaining questions in the field, including the most compelling one: the “boogeyman” of what could go wrong.
“I’m kind of personally skeptical that additional model development or more simulations are going to satisfactorily resolve those things,” he says. “And so I’m not really that interested in turning the crank on more models.”
For Franke, it’s time for the next step: “We’re interested in seeing how you’d actually do this thing if you wanted to do it.”
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The program
David Keith’s group, CSEi, is still coming together.
The University of Chicago unveiled the research initiative in 2024 and has committed to hiring 10 additional faculty members to advance scientific understanding of various forms of geoengineering and explore the thorny questions related to policy, ethics, and governance. It had hired two of them as of press time.
The university saw an opportunity to step up as a leader in a field that wasn’t getting adequate academic attention despite its potential to address the dangers of climate change, says Michael Greenstone, a climate economist and the founding director of the university’s Institute for Climate and Sustainable Growth.
“Universities, as a whole, were committing academic malpractice by not investigating the technical, the social, the political, and the even kind of humanist elements of geoengineering,” Greenstone says.
He helped recruit Keith to lead the initiative.
Keith, 62, previously spent nearly 13 years as a professor of applied physics and public policy at Harvard, where he led the establishment of the university’s Solar Geoengineering Research Program. More famously, he strove to carry out what could have been the first solar geoengineering experiment to release material in the stratosphere, known as SCoPEx. But after years of work and multiple delays, the research team finally scrapped the project in early 2024, following mounting criticism from environmental and Indigenous groups and the eventual intervention of the Swedish government.
Keith has long argued that researchers should seriously study geoengineering because it might substantially reduce the dangers of climate change, alleviating death, destruction, and suffering on massive scales.
He says that the overarching goal of the Chicago initiative is to expand the field by bringing together “enough independent professors and other research professionals” to “build a community around climate engineering as a broad field of inquiry.”
“Solar geoengineering certainly has complex and potentially dangerous political consequences, but so do a host of other emerging ideas and technologies.” David Keith, geoengineering researcher
“The University of Chicago was the first big university to try and build this as a field in a serious way, to make it not about one person,” he tells me. “It’s a giant commitment.”
Keith himself has become a divisive figure, the face of geoengineering to some. He says he now wants to help build a larger, sustainable research program that will outlive his involvement. He told the administrators that he shouldn’t run the program for more than five years.
“It’s important to have a generational handover,” he says, adding: “I think it’s really important that this not be ‘the David Keith Show.’”
The CSEi researchers are now exploring nearly every engineering challenge that Reflective highlighted in its analysis. In addition to the work on novel aircraft and in situ observations, the group is designing small “cube” satellites with optical sensors optimized for observing the stratosphere. It is also studying which materials might prove most practical to ship to the stratosphere and how best to release them.
The goal is “producing public information which can be independently assessed, critically assessed, so policymakers can understand more about what’s possible and not,” Keith says.
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The particles
Mingyi Wang, an assistant professor at the University of Chicago, leads me down the hall to a square, white lab room in the Henry Hinds Laboratory for Geophysical Sciences.
He pulls open the doors to a gray Haier biomedical freezer just inside the entrance, revealing a transparent flow tube hanging vertically and tapering at the bottom.
It’s a miniature stratosphere, chilled below −50 °C and filled with the same mix of oxygen, nitrogen, and other air molecules you’d find 20 or so kilometers above us. A series of Teflon and stainless-steel tubes run into the vessel, allowing Wang and his team to add various gases or particles and observe how they react.
Wang is an atmospheric scientist who studies how aerosols form, and he is now exploring what materials might be the most effective for reducing temperatures.
Most modeling experiments focusing on solar geoengineering explore the impact of adding sulfuric acid to the stratosphere, because that’s what ultimately ends up there after a volcanic blast.
But it would be costly and complicated to simply haul sulfuric acid up there and release it, because it’s heavy and sticky. So Wang and his team are conducting experiments in that chilly flow tube to determine what substances, including precursors to the acid, might do the best job of producing aerosols of the ideal size for reflecting away sunlight—and how best to prevent the materials from simply clumping together with existing particles and falling out of the stratosphere.
Wang, whom Keith refers to as a “young star,” has arrived at a novel solution to this problem, though he’s not ready to share the full details yet. He and his team are feeding the findings from their experiments into computer simulations of stratospheric plumes that they’ve developed. These, in turn, can be plugged into large-scale climate models to improve their simulation of smaller-scale effects—and thus enhance our understanding of stratospheric chemistry.
Wang says that it’s important to do this detailed research because until now, climate models simply assumed you’d wind up with the right aerosols of the right size.
“Scientifically, we may understand it reasonably well, but on the engineering perspective—do we really know how to do it right?” he asks. “That’s a big question.”
What’s next
As I began reporting on CSEi, I assumed that some of the engineering and design work would lead to new proposals for stratospheric experiments, picking up from where SCoPEx left off.
Keith, though, insists he has no interest in reliving that experience, given the weight the experiment took on as the focal point for a broader societal debate over solar geoengineering. He doesn’t see any of the other “practical engineering” work at the initiative leading toward field experiments either, at least at this stage.
Much of the work, in fact, is focused on a step beyond that: exploring what it would take to start a geoengineering campaign, if a nation or group of them eventually decides to. Franke notes that we already have balloons and other aircraft that could get to the lower bounds of the stratosphere to release an experimental amount of, say, sulfur dioxide.
“We’re thinking of it right now as: We’re trying to develop, we think, the tools should someone want to start doing SAI,” he says.
He and Keith are quick to stress that the research group does not intend to actually build the physical hardware that would be needed to deploy solar geoengineering—not even the aircraft that Langford’s company is designing.
Indeed, most of the researchers at the University of Chicago stress that they are not advocating for use of geoengineering; they’re doing the research to inform the public and policymakers about its benefits and risks.
But after decades closely studying the topic, Keith, at least, has evolved in his thinking on this point, and his public comments reflect that.
“As a scientist, I think the evidence [indicates] that early deployment—careful, hemispherically balanced, slow, monitored early deployment—would have benefits that are higher than the risks,” he says. “I think that evidence is very strong.”
Keith adds that if there were somehow a global referendum on whether to start, he would vote yes.