Late last month, about 100 researchers from around the world gathered at Logan International Airport in Boston. A fleet of buses appeared to whisk them to a remote and luxurious ski resort in northeastern Maine. They met to talk, drink, and cogitate off the record for five days about a messy solution to one of the world’s most challenging problems. They had gathered to discuss how to provide humanity one last line of defense against catastrophic global warming: solar geoengineering.

The idea behind solar geoengineering is simple. For the last four decades, humanity has struggled to reduce the amount of greenhouse gas entering the atmosphere. We have decommissioned nuclear plants, introduced millions of new gasoline-burning cars to the roadway, and dawdled through treaty after treaty. Meanwhile, the concentration of carbon dioxide in the atmosphere has only risen.

It sure seems like we’ll need some more time to get our act together. So maybe we should toy with another variable: While we try to reduce the planet’s heat-trapping gas, maybe we can also try to reduce the amount of heat entering the atmosphere in the first place.

Interest in the technique has spiked recently. An administration hostile to climate mitigation has taken power in the United States, and some countries risk falling short of the promises they made under the Paris Agreement. It seems suddenly plausible that the industrialized world will not succeed in staving off two degrees of temperature rise.

Governments and private donors have opened their pockets in advance of that failure. This summer, China’s national Ministry of Science and Technology announced it will fund a 15-person, $3 million geoengineering research program at Beijing Normal University. It will join several government-funded teams working on the same problem in Germany as well.

This fall, Harvard University will also launch its own solar geoengineering research with $7.5 million in funding from private sources. Its leaders hope to eventually amass a budget of $20 million.

So the conference, organized by Gordon Research Conferences, was well-timed. It is thought to be the largest gathering of geoengineering scientists ever assembled, and its participants included almost every senior researcher in the field. It was held under the Chatham House rule, which proscribed attendees from disclosing who said what, but its agenda and attendees list are available online.

Leaders and participants described a strategy-focused gathering that allowed the rapidly growing field to settle on common research questions.

“We had a really, really good conference,” said Trude Storelvmo, a professor of atmospheric chemistry at Yale University and one of the vice chairs of the conference. The attendees included physicists, chemists, biologists, economists and social scientists, reflecting the degree to which the problem “touches a lot of disciplines.”

Nearly everyone involved described their work as a way to find a stopgap to the looming problem of climate mitigation.

“We all agree that climate change is real and that the solution is to reduce the emissions of the gases that cause global warming,” said Alan Robock, a professor of atmospheric chemistry at Rutgers University and one of the co-chairs of the Maine meeting. “The Paris Agreement was a good start, but those pledges aren’t enough, and we have to reduce more. Even then it [won’t be] fast enough. So what we’re looking at is: If global warming is so dangerous, could we shave off a little warming while we continue to mitigate greenhouse gases?”

There are several ways of holding off that warmth. They all involve bouncing sunlight back into space before it penetrates too far into the lower atmosphere. Over the past decade, scientists have discussed some different ways to do this: by brightening clouds over the ocean; by pushing cirrus clouds to form in the high atmosphere; or by spraying a reflective gas into the sky at high altitudes, mimicking the effect of a large volcanic eruption.

Last month’s meeting arrived at the consensus that this final technique—called stratospheric aerosol injection—is the best bet going forward. Researchers don’t see a technological impediment to developing seeding tools, seeing the few remaining problems as within the capability of any large aerospace company. There are plenty of natural precedents for stratospheric aerosols, too—volcanoes have gone off hundreds of time during human history—and they see it as the most reversible and easy to model.

“When you put particles in the stratosphere, it’s simpler to calculate what the effect would be on the energy budget and on the temperature,” said Storelvmo. “Anything that involves clouds generally becomes much more complicated.”

It also benefits from some built-in economies of scale. “With the physics [of stratospheric injection], you can have huge multipliers. You put one particle in the stratosphere and it can deflect trillions of photons,” said Ken Caldeira, a senior scientist at the Carnegie Institution for Science and a presenter at last month’s meeting.

This neat efficiency separates solar geoengineering from its sibling, “carbon geoengineering,” which aims to directly scrub the atmosphere of carbon dioxide and other heat-trapping gases. If perfected, carbon geoengineering could permanently solve the problem of global warming, but researchers consider the technological advances needed to accomplish it far-off, and the meeting didn’t address it.

“With carbon-dioxide removal, you basically need one molecule of the reactant to touch each molecule of carbon dioxide you capture. So carbon geoengineering has to be the scale of the [global] energy system,” Caldeira told me. “And if you have to do something on the scale of the energy system, why don’t we just build a better energy system?”

Yet this makes the most feasible form of geoengineering (dimming the sun) only a stopgap—albeit a tentatively effective one. “Most of the climate models project that solar geoengineering could offset most climate change for most people most of the time. But in climate models, if something goes wrong, you can run the model again. In the real world you don’t have that option,” Caldeira said.

So much of the meeting was devoted to the natural follow-up question: What could go wrong?

The most glaring concerns are about how stratospheric aerosol injection could affect the water cycle, “whether it could induce droughts or floods or things like that,” said Storelvmo. Many researchers describe that as the riskiest aspect of solar geoengineering right now.

“There’s no consensus on it, really,” said Storelvmo. “If we had a better understanding of the risks there, that would make a big difference.”

But those will be difficult issues to resolve, even aside from the complexities of modeling aerosols at the scale of geoengineering. Climate models generally struggle to predict how precipitation patterns will change years or decades from now, even as they have excelled at projecting future temperature change.

It’s also unclear how much solar geoengineering will be required to hold off the worst symptoms of climate change. Because the equator receives stronger, more frequent sunlight than the poles, stratospheric aerosols would chill the tropics more than they would the higher latitudes. This could trap wannabe geoengineers in a dosage dilemma.“If you’re interested in stopping ice sheets from melting, you can’t just make global temperature constant. You have to overcool it to reach the high latitudes,” said Robock.

A similar mechanism could create even nastier water-cycle problems. Stratospheric aerosols may cool the continents more than they cool the oceans, equalizing the normal pressure difference between land and sea—and, in turn, killing the seasonal monsoons of Asia and Africa. Food prices worldwide would skyrocket.

Some researchers are investigating whether these secondary problems could be addressed by adjusting where or how aerosols are sprayed, but Robock said this was “like putting a Band-Aid on a Band-Aid.”

“We still have to see if that’s better than not doing it at all,” he told me.

Other sessions at the conference focused on other unknowns about solar geoengineering beyond weather. Little research has been conducted into how ecosystems would respond to prolonged global dimming. Some researchers also worry that spraying sulfur dioxide into the high stratosphere could damage the ozone layer, though recent research has suggested this risk is smaller than once thought.

And, of course, there are outstanding questions about who will get to make the call—morally, politically, and technologically—that it’s time to deploy solar geoengineering.

The existence of a large, organized field to study geoengineering is itself a recent development. Eleven years ago, the Dutch climate scientist and Nobel laureate Paul Crutzen called for researchers to seriously investigate solar geoengineering “as an escape route against strongly increasing temperatures” in an article in the journal Climatic Change. Today, the paper is considered the founding article in the field, allowing researchers to seriously study solar geoengineering.

“It was a big deal—not in anything that [the paper] said, but in that it said anything about solar geoengineering at all,” said Gernot Wagner, a co-director at the Harvard solar-geoengineering program, and an attendee at last month’s meeting. “There was a decades-long taboo to work on this topic.”

“Now, there are hundreds of peer-reviewed papers on this topic. It’s developing as a research field that people take seriously, where people are interested in formulating testable hypotheses, and where people work on advancing the state of knowledge across the field,” he said.

The new Chinese program is a testament to the scientific seriousness of the field, he said. “That it’s funded through the regular Chinese scientific apparatus, that in itself is important. These things don’t happen overnight.”

It’s still unclear whether solar geoengineering will ever be deployed. Some researchers, like Wagner, consider it a virtual certainty. “It’s not a question of if, it’s a question of when someone will pull the trigger,” he told me.

In his book Climate Shock, written with the economist Martin Weitzman, he argues that it will be a principle in the 21st century:

Geoengineering is so cheap to do crudely, and it has such high leverage, that it almost has the exact opposite properties of carbon pollution ... It’s so cheap that someone will surely do it based on their own self-interest, broader consequences be damned.

Ken Caldeira wasn’t so sure—though he said that last month’s meeting helped solar geoengineering seem real to him. “The meeting made me take it a little more seriously as something practical, and not just theoretical,” he said.

He continued: “I think it really rests on this question of: Is climate change going to be catastrophic, or is it going to be a nuisance and an ongoing cost? If it’s a nuisance, probably people will muddle through. But if climate change does turn out to be catastrophic, then solar geoengineering is pretty much the only way that our political system could start cooling the Earth in a few years or decades.”

The next off-the-record Gordon conference on solar geoengineering has already been planned for the summer of 2020.