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.
Franke is one of a small but growing cohort of scientists focused on the engineering challenges associated with solar geoengineering, the controversial idea that we could deliberately intervene in the climate system to counteract global warming.
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.
The Climate Systems Engineering Initiative (CSEi) at the University of Chicago formally launched in 2024 under the leadership of the prominent geoengineering researcher David Keith.MIT TECHNOLOGY REVIEW | JUSTIN SAGLIO
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.”
What we don’t know
Solar geoengineering is often portrayed as a relatively cheap and easy fix for climate change. But as researchers take a harder look at the nuts and bolts, they’re finding considerable uncertainties, missing tools, and unbuilt infrastructure.
None of that may be a showstopper, but we’ll need time and money to develop the components necessary to implement even the early stages of a solar geoengineering program. What this research is about, at its core, is not actually launching something, but figuring out what it would take to do so.
A young San Francisco nonprofit, Reflective, recently worked with scientists in the field to figure out just how much we still don’t know.
The process began by outlining what the organization, which pools money from donors to fund geoengineering studies, describes as a “well-managed, moderate” scenario: In 2035, some nation or group of nations begins a small-scale geoengineering deployment, spraying an equal amount of sulfur dioxide or hydrogen sulfide—gases that should convert into reflective aerosols in the stratosphere—near both the North and South Poles. The initial program would release enough material to reduce temperatures by about 0.1 °C, shaving off a fraction of the roughly 1.4 °C of worldwide warming that’s occurred since the start of the industrial era.
The poles figure prominently in this and other early-stage geoengineering scenarios, for a simple reason: The stratosphere starts as low as seven kilometers there—as opposed to around 18 to 20 kilometers at the equator. That makes it easier to reach, enabling existing aircraft, with some modifications, to carry sizable payloads up there.
The wrinkle is that the cooling effect would be more pronounced in the northernmost and southernmost latitudes. That’s because, among other complicated mechanisms, higher temperatures in the tropical stratosphere would mostly prevent aerosols released around the poles from drifting toward the equator. So deploying geoengineering in those areas would likely have milder effects on the hotter and poorer nations around the tropics, which are also some of the areas most vulnerable to climate change.
To cool the world evenly—and fairly—you’d eventually want to add flights closer to the equator. Over the following decade or so, under Reflective’s scenario, the program would scale up, shift to novel aircraft flying above the subtropics, and release enough material to achieve global cooling of 0.5 °C.
The question the researchers then examined was: If we wanted to carry out such a scenario, what would we still need to do to pull it off?
Quite a bit, it turns out. Earlier this year, Reflective published its SAI Uncertainty Database (SAI stands for “stratospheric aerosol injection”), highlighting a variety of scientific unknowns and six engineering obstacles.
Among them: sorting out how hard or expensive it would be to retrofit existing aircraft to carry out the early stages of the project. Deploying at the poles could also require constructing new airports, establishing new shipping lanes or railways to transport supplies, and building facilities that could process raw materials—by, for example, combusting elemental sulfur to produce sulfur dioxide.
We would also need to build more instruments and send them up to the stratosphere aboard balloons, drones, or other aircraft to observe the baseline chemistry, reflectivity, and distribution of compounds there—and to track what changed once new materials were released.
Finally, the main satellites that observe the stratosphere from space are set to go out of commission in the coming years, creating the risk of an “imminent data desert,” as a 2025 paper in the Bulletin of the American Meteorological Society warned. Several new instruments are in development or available for launch, but there could be a gap in observations at a point where we’d want to have a clear picture of the baseline conditions, Reflective notes.
Dakota Gruener, the chief executive officer of the nonprofit, stresses that the organization isn’t advocating the use of solar geoengineering. But she says it’s important for the field to begin addressing engineering uncertainties now because it stress-tests the assumptions in climate models. It helps us determine whether the scenarios explored in silico are feasible in the real world.
It’s also important to do this, she says, because it may take a long time to resolve all these unknowns while the climate grows steadily warmer. “If we aren’t putting adequate attention to them now, we might be caught flat-footed,” Gruener told MIT Technology Review.
A 2024 analysis in the journal Earth’s Future highlighted just how expensive and time-consuming it might be to develop the aircraft and infrastructure required for an initial deployment. The study explored what it would take for a geoengineering program around the poles, capable of reducing temperatures by 2 °C in the northernmost and southernmost parts of the planet, to be up and running by 2040. The conclusion: It could require at least a decade of work and a $35 billion investment.
Wake Smith, a research fellow at Harvard and lead author of the study, also says that researchers need to move forward with engineering studies now, because the urge to use the technology will likely grow stronger as climate change becomes increasingly catastrophic.
“The risk I worry about is needing it before we understand it and therefore doing it badly,” he says, later adding: “The sooner we get going with it, the better decisions we’ll be able to make a few decades hence in terms of whether to do it, how to do it, when to do it.”
A novel aircraft
The aircraft pictured on Franke’s desk, which is still just a concept, could reach just beyond the threshold of the stratosphere above the tropics when fully loaded. A fleet of 270 of them could disperse about a million metric tons of material per year, enough to ease global surface temperatures by about 0.26 °C.
The CSEi outsourced the work of designing it to John Langford, a well-known aeronautical engineer and entrepreneur. Langford’s company, Electra.aero, had previously collaborated with the MIT Department of Aeronautics and Astronautics to develop autonomous, solar-powered aircraft that could carry out extended scientific missions in the stratosphere. He is now spinning out a new business, Iris Aero, to produce those planes, which are assembled from a single, continuous wing covered in solar panels and suspended above a tiny fuselage.
Langford expects the solar plane to find its main initial commercial applications in wildfire monitoring and forecasting. But by swapping in a different set of instruments, it could be used to monitor how materials dispersed in the stratosphere might alter conditions there, he says.
The novel aircraft is a variation on the observational plane, with the added space and thrust necessary to carry these materials to the stratosphere and release them. It has a wider wingspan and swaps out those solar panels for a pair of Rolls-Royce AE 3007 engines.
The aircraft would also include a detachable tank that would function something like a trailer on a semi. This would make it possible to load materials between flights and prevent any damage to the plane itself from those materials, some of which are corrosive, Langford says.
He says he and his team have completed the initial designs and are now doing more detailed engineering and cost analyses. They intend to publish the findings when the effort is complete.
“We’d love to build a prototype of such an airplane and feel we could do so relatively quickly,” Langford says. “But that all depends on what David’s group wants to do.”
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.
Normalizing a dangerous idea
The debate around solar geoengineering is quickly moving beyond the academic and theoretical realm. A handful of startups, some more serious than others, have begun testing technologies that could one day be used to cool the planet.
Yet to critics, solar geoengineering is the peak of techno-solutionism, affixing a high-tech Band-Aid to a global crisis caused by earlier technologies instead of addressing the root cause. Further, they argue that there’s no way to deploy or govern it in a globally equitable way, because any use of it will prove more advantageous to some regions than others.
Even if solar geoengineering succeeded in reducing the average global temperature by 1 °C or so, that would mean very different things in different regions.
It could keep farmers prosperous and cities safe across, say, much of the US and the world’s temperate zones. But the lower temperature might be too cool for Russia to boost its agricultural productivity, while it might still be too hot for subsistence farming in northern Africa.
Some studies also suggest that high levels of solar geoengineering could create new dangers in some regions, potentially altering monsoon rains, decreasing agricultural output, shifting the range of infectious diseases, and more.
These complications raise a long list of thorny and divisive ethical questions. Even if solar geoengineering produced better conditions across most of the planet relative to a world with unchecked climate change, would it still be acceptable if it unleashed deadly famines or floods in a few regions? What kind of global consensus should be required to decide it’s okay to deploy it? And how should we determine where to set the planet’s temperature—and when, if ever, to shut the technology off?
Stephens argues that the answers, like so much else in the world, will come down to wealth and power. Countries, corporations, or even wealthy individuals with the resources to deploy such a system would have every incentive to tune it for their optimal benefit, no matter what it might mean for others.
“It will be certain people who have a lot of wealth and power deciding when and how, and who should benefit and who will get screwed,” she says. “That’s the fundamental reason I think any advance in this technology is so dangerous.”
Duncan McLaren, an environmental researcher and political scientist, argues that the shift into practical engineering studies demands more oversight of the research field.
For many critics of outdoor experiments like SCoPEx, he explains, the major concern wasn’t the environmental or safety risks, which were minimal; the issue was the normalization of a concept that could reduce pressures to cut greenhouse-gas emissions.
He says that any advance in research—whether it’s on paper, in the lab, or in the stratosphere—raises a similar risk: undermining progress on climate action by allowing the fossil-fuel sector and other business interests to say there’s an easier solution in development that doesn’t require overhauling our energy systems. A policy paper that the Texas Conservative Coalition Research Institute released in March advanced this very argument, citing the far lower costs of solar geoengineering relative to the “staggering costs” of a “forced transition.”
Given this so-called moral hazard risk, design and engineering work should demand the same level of scientific supervision that outdoor experiments do, including ethical review, risk assessments, and public engagement, McLaren says.
“It ought to be more onerous,” he says. “There ought to be more barriers to researchers saying they want to do this.”
“The next ethical step”
Keith pushes back forcefully on that assertion, condemning as “profoundly illiberal” the idea that we should regulate open academic research posing no physical risks.
“Solar geoengineering certainly has complex and potentially dangerous political consequences, but so do a host of other emerging ideas and technologies,” he said in an email. “The best chance to manage these challenges is to debate them openly and freely.”
Keith is all for keeping solar geoengineering technology in the public domain, and he agrees that the first line of climate defense must be rapid and deep reduction of greenhouse-gas emissions. But the world has made little progress in cutting climate pollution, carbon dioxide can persist for thousands of years in the atmosphere, and the planet is heating up fast. So, he argues, we may need to pursue other measures to temper the growing threats.
The bar for restricting research in this field should be “very high,” he says, given the potential promise of the technology.
After visiting flood-devastated villages in Bangladesh, Keith underscored this point in an interview with the director of Plan C for Civilization, a recent documentary that profiles his work. “I think people have to take the next ethical step,” he said. “Because if you are really going to withhold access to and knowledge of a technology that could potentially save enormous numbers of lives—real lives, people we’ve met in the last few days—you’ve got to be very confident that that technology is going to be misused.”
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.
This rendering illustrates the type of high-altitude aircraft that could one day be used to deliver Earth-cooling material into the stratosphere.COURTESY OF IRIS AERO CORP.
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.
“I think that this field needs to stop being so ashamed of using the ‘deployment’ word,” he says.