“If we were having this debate in 1990, I would be on the other side. We knew enough by then to warrant strong action to cut emissions. And if we had started then, we would have had time to stop the train,” said Ted Parson, a professor of environmental law at UCLA and one of the debaters arguing for more research into solar geoengineering. “But it’s 2019. And 30 years of delay have let that opportunity slip out of reach. It is still possible to hold climate change to manageable limits. But it’s no longer possible to do this confidently, relying only on technologies and policies that are familiar, comfortable, uncontroversial.”

At this point, he explained, any credible path to keeping warming in check relies on scrubbing huge volumes of carbon dioxide from the atmosphere, which is a herculean task. “Removing CO2 from the atmosphere is like draining a lake through a straw. It will work, but it’ll take a long time,” Parson said. “So deep emission cuts and carbon removal are both essential, but they may not be enough soon enough, even with extreme efforts. We need something else, and geoengineering might be that something else.”

Parson’s ally in the debate, Harvard physicist David Keith, argued that it is all but inevitable that a future generation will deploy solar geoengineering, so it is vital to understand its full implications now. “We can’t bind our children’s hands. Decisions about deployment will still be made, but they will be made without adequate understanding of what the risks are, without an exploration of technologies that could substantially reduce those risks, without knowledge about how to monitor it adequately, and without enough time for nations to discuss how they might govern this technology,” he said.

While Keith and Parson made a compelling case, they elided real worries about the geopolitical risks of solar geoengineering. One concern is that developing a cheap technology to cool the planet would discourage countries from addressing the root cause of climate change — planet-warming, ocean-acidifying carbon dioxide, a substance that will linger in the atmosphere for centuries, far longer than the chemicals used in solar geoengineering, which would need to be refreshed every year or so.

Parson dismissed this concern by citing research showing that people are actually more likely to support cutting emissions after learning about solar geoengineering, because, he said, “They don’t see it as a get out of jail free card. They see it as a signal of alarm.” This may be the case, but public opinion is difficult to measure, and other research suggests that solar engineering could fuel apathy. More importantly, public opinion is malleable, argued Parson’s debate opponent Clive Hamilton, a professor of public ethics at Charles Sturt University.

“Let me tell you about my worst nightmare. It’s a very short one. And that is that Rupert Murdoch watches this debate and says, ‘This sounds like a great idea,’ and he sends a tweet out and his… editors around the world say, ‘Rupert has spoken. We now support geoengineering’… They say, ‘Screw cutting emissions. We’re going to put up a solar shield.’ Then, we’re really in trouble,” Hamilton said.

There is also the more potent concern about how solar geoengineering would shape geopolitics. Keith and Parson were optimistic that world leaders could manage its use through a “UN-style global institution,” despite the fact that decades of diplomatic wrangling on climate change have yielded little meaningful cooperation. Hamilton and his debate ally, Anjali Viswamohanan, a Chevening scholar at the Blavatnik School of Government at Oxford University, were more skeptical. “Without complete global scientific and political cooperation, there are high chances of misuse of solar geoengineering,” Viswamohanan said.

Hamilton painted a bleak picture of how easily solar geoengineering could undermine global stability. “Who would you trust to have their hand on the global thermostat? That is the power to turn the Earth’s temperature up a bit, down a bit, up a bit more, down a bit more, to change the weather in ways that may benefit Chinese people at the expense of Indian people, Americans at the expense of Africans. Who should make the decision?” he said. “Should the Kremlin make the decision? Should the Politburo of the Chinese Communist Party make the decision? Should Donald Trump make the decision? Could we expect to see a tweet one morning, ‘Hey, this heat wave down at Mar-a-Lago is getting out of control, so I have instructed the U.S. climate regulatory authority to turn it down.’”

Solar geoengineering promises to upset global stability by creating winners and losers. “Think about it from the point of view of the peasant farmer in Pakistan,” he said. “The rains have failed. People are starting to go hungry. You know that someone, somewhere in the world is messing with the climate system — it’s not an act of God. And a political stirrer comes along and tells you that America, the great Satan, is messing with your climate. You’ve got a massive political problem.”

At the end of the evening, the audience was asked to choose between two ultimately unsatisfying arguments. Opponents of solar engineering want to prevent a geopolitical nightmare by halting scientific research, which is unlikely to happen. Champions of the technology believe it could be used without incident, an equally implausible outcome. The latter side won the night, but they certainly did not end the debate.

Solar geoengineering is so risky as to be unthinkable. And yet, the situation is so dire that it feels almost inevitable. Experts will likely argue about the idea without resolution for years to come. If there is one conclusion to be drawn from the debate, it’s that when Plan B is looks this bad, the only good option is Plan A.