Not everyone agrees the proposal would work. A 2013 paper in Science, led by MIT atmospheric scientist Dan Cziczo, concluded that the formation of ice crystals around dust, known as heterogeneous ice nucleation, is already the dominant mechanism creating cirrus clouds. That might mean adding more dust would, on balance, create thicker clouds that trap more heat. The larger problem with the idea, Cziczo argues, is that clouds are the least understood part of the climate system. We do not have nearly enough knowledge about cloud microphysics, or accurate enough measurements, to precisely manipulate climate in this way, he says.

But Mitchell’s most recent research, relying on observations of ice crystal concentrations from NASA’s Calipso satellite, has further convinced him that cloud seeding could work, as long as it’s done in regions where cirrus clouds form primarily without dust particles. On the monitor in his office, Mitchell pulls up a page of maps from a paper he presented at the National Center for Atmospheric Research in late February. Navy- and light-blue dots, representing Cziczo’s heterogeneous clouds, dominate the mid-latitudes, covering much of South America and Africa. But the higher latitudes are covered in red, yellow, orange, and green dots that indicate the sorts of clouds Mitchell has in mind.

The satellite images suggest that in very cold and humid conditions, toward the poles and particularly during winter, tiny ice crystals can form on their own, spontaneously, without dust. That suggests that cloud seeding could work, if it’s targeted to those areas during those months. Mitchell even thinks he’s come up with a way to get nature to carry out a field experiment to test his theory. During spring and winter, strong winds regularly stir up major dust storms in the deserts of Mongolia and the western edge of China. The fine particles blow across the Pacific and run into an atmospheric wave that rolls over the Rocky Mountains.

If Mitchell is correct, the dust should promote thinner cirrus clouds in an area where the thicker type otherwise tends to dominate. There was no way to properly observe this phenomenon—until late last year, when the National Oceanic and Atmospheric Administration launched a satellite equipped with some of the most powerful imaging technology ever launched into space, as well as sensors that can measure the temperatures of clouds. The satellite should be able to capture exactly what happens as the dust rides over the Rockies, detecting the subtle shifts under way in cloud microphysics.

Mitchell submitted a research proposal to NOAA last year, asking the agency to use the satellite to make such observations. He knows it’s a long shot, particularly in light of the Trump administration’s efforts to slash funding for climate science. But if NOAA agrees, the test could lend weight to his theory—or, of course, contradict it.

Another outdoor geoengineering experiment should occur even sooner.

By this time next year, Harvard professors David Keith and Frank Keutsch hope to launch a high-altitude balloon from a site in Tucson, Arizona. This will mark the beginning of a research project to explore the feasibility and risks of an approach known as solar radiation management. The basic idea is that spraying materials into the stratosphere could help reflect more heat back into space, mimicking a natural cooling phenomenon that occurs after volcanoes blast tens of millions of tons of sulfur dioxide into the sky (see “A Cheap and Easy Plan to Stop Global Warming”).

Scientists generally believe the technique would ease temperatures, but a lingering question is: what else will it do? Notably, volcanic eruptions have also significantly altered rainfall patterns in certain areas, and sulfur dioxide is known to deplete the protective ozone layer.

“The most likely scenarios for climate over longer time scales are devastating to future generations, absolutely devastating.”

Keith has done extensive climate modeling to explore whether other materials, including alumina, diamond dust, and calcium carbonate, might have a neutral or even positive impact on ozone. During a conversation in his office at Harvard, he stressed that the experiments wouldn’t constitute a test of geoengineering itself. But they would allow his group to subject its models to real-world data, revealing more about the relevant stratospheric physics and chemistry. “Theory alone doesn’t tell you what will happen in the atmosphere,” Keith says. “You can fool yourself if you don’t go out and make direct measurements.”

Keith has already begun design work with the balloon company World View Enterprises, as well as discussions about the appropriate transparency and oversight for such outdoor experiments. The early flights would test the basic workings of the balloon, which would be tethered to a gondola equipped with propellers, sprayers, and sensors. But eventually the experiment would involve releasing a fine plume of materials, probably calcium carbonate, into the stratosphere. The balloon would then track that trail in reverse, allowing the sensors to measure how well the particles scatter sunlight, whether they coalesce or disperse, and how they interact with precursors to ozone.

Unknown unknowns

Full-scale geoengineering would inevitably involve some level of risk. We are likely to face a terrible choice between accepting the clear dangers of climate change and risking the unknowns of geoengineering. Alan Robock, a professor of environmental sciences at Rutgers, has published a list of 27 risks and concerns raised by the technology, including its potential to deplete the ozone layer and to decrease rainfall in Africa and Asia.

Ultimately, Robock worries that geoengineering may simply be too risky to ever try. “We don’t know what we don’t know,” he says. “Should we trust the only planet known to have intelligent life to this complicated technical system?” MIT’s Cziczo is blunter. “We know the problem is greenhouse gas, so the solution is you take the greenhouse gas out,” he says. “You don’t try to do something that we completely don’t understand.”

The reservations surrounding geoengineering research were on full display in late March as dozens of notable climate and social scientists gathered at the Carnegie Endowment for International Peace in Washington, D.C., for the Forum on U.S. Solar Geoengineering Research. Speakers highlighted a long list of unanswered, and perhaps unanswerable, questions about international governance: Who gets to decide when to pull the trigger? How do we determine “correct” average temperatures when the same ones will affect different nations in markedly different ways? Can one nation be held responsible for the negative effects of its geoengineering scheme on another country’s weather? Could these tools be used to deliberately attack a neighboring nation? And could conflicts over these questions tip into war?

“I have yet to hear any description of a future solar-geoengineered world that sounds to me anything other than dystopian or highly unrealistic,” said Rose Cairns, a research fellow at the University of Sussex, who joined the morning discussion from England by Skype.