University of Chicago’s Jim Franke says models won't settle geoengineering’s biggest “boogeyman”

Jim Franke, a research assistant professor at the University of Chicago, is making a blunt bet that computer simulations will not resolve the biggest unknown in solar geoengineering. In fact, he says he is personally skeptical that “additional model development or more simulations are going to satisfactorily resolve” what he calls the “boogeyman” of what could go wrong. Instead, he wants the field to do the next step he describes as seeing how you’d actually do the thing if you wanted to do it.

That reality check is already showing up on his desk. Franke pulls away the cover page of a presentation, revealing an illustration of an uncrewed aircraft soaring thousands of meters above commercial jets, high enough that you can see the curvature of the Earth. The design is all about altitude: massive wings stretched out from a stubby fuselage so the plane can operate in the stratosphere, about a dozen miles, or 20 kilometers, above the surface. The reason is simple physics. At that height the air is far thinner, as little as 5% the density near the ground. At altitude, the aircraft would release materials designed to, after a few steps of chemistry, reflect sunlight back into space.

If this sounds like science fiction, the field is insisting it is actually engineering triage. Solar geoengineering is the controversial idea that we could deliberately intervene in the climate system to counteract global warming. The concept borrows from volcanoes. When massive eruptions blast sulfur dioxide and other compounds into the stratosphere, they convert into sunlight-scattering particles, which can reduce temperatures worldwide. Hundreds of studies in recent decades have suggested that mimicking this mechanism could work quickly and efficiently in climate models.

But models are approximations. Franke and others in the small but growing cohort say the simulations gloss over problems that only show up when you try to build and operate a system: aircraft that can carry the necessary loads to the necessary altitudes do not exist yet. There are also open questions about how to release material so most of it becomes tiny reflective aerosols instead of clumping and falling out of the sky. Even the substance is still contested, with safety, cost, and effectiveness all tied up in unresolved choices.

This is why more research on solar geoengineering is moving beyond “what the model predicts” toward detailed design and practical engineering. The tasks range from inventing high-altitude aircraft to mastering the chemistry and delivery mechanisms for dispersing materials. They also include building monitoring infrastructure, because you need to know if any of it is working, and how well, in the real world. The stakes here are not abstract. The question of whether we should geoengineer the planet has no clear-cut answer. It might save millions of lives by reducing dangers of catastrophic heat waves, floods, droughts, and famines. But many fear it is too dangerous to even consider or seriously study, arguing that the spiraling consequences of manipulating such a large, complex, interconnected planetary system cannot be predicted reliably.

The debate is also about momentum and incentives. Jennie Stephens, a professor of climate justice at Maynooth University in Ireland, argues the research itself raises risk: “The more investment that’s made, the further the advances, the more likely it is that it will be deployed.” Proponents of practical engineering research respond with a different incentive logic. Playing out how a solar geoengineering program could be mounted, they argue, improves understanding of potential benefits and risks, so that if anyone ever does try to tweak the climate, it might be done in an informed and potentially safer way.

All of this is playing out within a concrete organizational structure. Much of the work underway is happening at the Climate Systems Engineering Initiative (CSEi) at the University of Chicago, formally launched in 2024 under the leadership of David Keith, a prominent geoengineering researcher. Franke, who was a professional engineer before earning his doctorate in geosciences, is overseeing overlapping research projects and collaborations. One focus is the aircraft designs now on his desk, aimed at resolving engineering uncertainties.

And then there is the money question, which gets even more real when you move from hardware to scenario planning. Solar geoengineering is often portrayed as relatively cheap and easy, but researchers looking at the “nuts and bolts” say they are finding considerable uncertainties, missing tools, and unbuilt infrastructure. None of this is necessarily a showstopper, but it does imply time and money are required to develop the components needed even for early stages. That is the core idea of a project involving Reflective, a young San Francisco nonprofit that pools money from donors to fund geoengineering studies.

Reflective recently worked with scientists to outline what the organization describes as a “well-managed, moderate” scenario. In 2035, a nation or group of nations begins a small-scale geoengineering deployment, spraying an equal amount of sulfur dioxide or hydrogen sulfide. These gases would 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 since the start of the industrial era.

The poles are central in the scenario because the stratosphere starts as low as seven kilometers there, compared to around 18 to 20 kilometers at the equator. That makes it easier to reach, enabling existing aircraft with modifications to carry sizable payloads up there. But the cooling effect would be more pronounced at the northernmost and southernmost latitudes. The reason is that higher temperatures in the tropical stratosphere would mostly prevent aerosols released around the poles from drifting toward the equator. Deploying near the poles would likely mean milder effects on the hotter and poorer nations around the tropics, which are also among the areas most vulnerable to climate change. To cool the world evenly and fairly, Reflective’s scenario suggests flights closer to the equator later.

Over the following decade or so, it 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 Reflective’s researchers examined is not “will this work in theory,” but “what would we still need to do to pull it off.” For decision-makers in adjacent roles, from research leadership to funding and governance, the implied message is that geoengineering is becoming less of a climate-model conversation and more of an operational one: aircraft capability, material release, monitoring, and the safety case all have to exist as a system.

That is the reality check. Even if the idea originates in volcanoes, the future is engineering. And for executives watching risk, reputation, regulation, and capital allocation around climate tech, the strategic stakes are straightforward: if the field can translate concepts into deployed infrastructure, it will also force regulators, governments, and donors to confront accountability, measurement, and unintended consequences much sooner than most plans anticipate.