Jim Franke pulls back the cover page of a presentation on his wraparound desk, revealing an illustration of an odd-looking aircraft with massive wings stretching out from a stubby fuselage. It's the kind of plane that looks like it was designed by someone who really, really wanted to make a paper airplane but had access to aerospace engineering software.
The uncrewed vehicle would soar thousands of meters higher than commercial jets fly - so high you can see the curvature of the Earth. 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 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. Because when you're trying to hack the planet's atmosphere, you want the best bet, not just a pretty good one.
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, which have historically been excellent at reducing global temperatures by blasting sulfur dioxide into the stratosphere, where it converts into sunlight-scattering particles. Hundreds of studies have suggested that a human attempt to mimic this mechanism would work quickly and efficiently - at least within the confines of climate models, which are basically the Sims version of Earth.
But these computer simulations 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 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. You know, minor details.
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 to building out the monitoring infrastructure we'll need 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, arguing that we can't possibly predict the spiraling consequences of manipulating such large, complex, interconnected planetary systems. Critics say the building momentum will make it ever more likely that someone, somewhere, will eventually pull the trigger on geoengineering, no matter the remaining unknowns.
"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." Because as we all know, the history of technology is a story of responsible restraint and careful consideration of consequences.
But proponents 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 prominent geoengineering researcher David Keith.
Franke, a professional engineer before earning his doctorate in geosciences, is overseeing a series of overlapping research projects 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, 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."
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 to figure out just how much we still don't know. The process began by outlining what the organization 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 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 early-stage geoengineering scenarios because 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, because 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. 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 and track what changed. 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.
Dakota Gruener, the chief executive officer of Reflective, 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's also important 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. 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. "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."
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 for 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. Because if there's one thing we've learned, it's that when you're planning to potentially tinker with the entire planet's climate system, you should definitely start with wildfire monitoring. Baby steps.