In 2014 Henrik Karlsson, a Swedish entrepreneur whose startup was failing, was lying in bed with a bankruptcy notice when the BBC called. The reporter had a scoop: On the eve of releasing a major report, the United Nation’s climate change panel appeared to be touting an untried technology as key to keeping planetary temperatures at safe levels. The technology went by the inelegant acronym BECCS, and Karlsson was apparently the only BECCS expert the reporter could find.

Karlsson was amazed. The bankruptcy notice was for his BECCS startup, which he’d founded seven years earlier after an idea came to him while watching a late-night television show in Gothenburg, Sweden. The show explored the benefits of capturing carbon dioxide before it was emitted from power plants. It’s the technology behind the much-touted notion of “clean coal,” a way to reduce greenhouse gas emissions and slow down climate change.

Karlsson, then a 27-year-old studying to be an operatic tenor, was no climate scientist or engineer. Still, the TV show got him thinking: During photosynthesis plants naturally suck carbon dioxide from the air, storing it in their leaves, branches, seeds, roots, and trunks. So what if you grew crops and then burned those crops for electricity, being sure to capture all of the carbon dioxide emitted? You’d then store all that dangerous CO 2 underground. Such a power plant wouldn’t just be emitting less greenhouse gas into the atmosphere, it would effectively be sucking CO 2 from the air. Karlsson was enraptured with the idea. He was going to help avert a global disaster.

The next morning, he ran to the library, where he read a 2001 Science paper by Austrian modeler Michael Obersteiner theorizing the same idea, which was later dubbed “bioenergy with carbon capture and storage”—BECCS. Karlsson was sold. He launched his BECCS startup in 2007, riding the wave of optimism generated by Al Gore’s first climate change movie. Karlsson’s company even became a finalist in Richard Branson’s Virgin Earth Challenge, which was offering $25 million for a scalable solution for removing greenhouse gases. But by 2014, Karlsson’s startup was a failure. He took the BBC’s call as a sign that he shouldn’t give up.

In the report, the UN’s Intergovernmental Panel on Climate Change—universally known by yet another acronym, IPCC—presented results from hundreds of computer-model-generated scenarios in which the planet’s temperature rises less than 2 degrees Celsius (or 3.6 degrees Fahrenheit) above preindustrial levels, the limit eventually set by the Paris Climate Agreement.

The 2°C goal was a theoretical limit for how much warming humans could accept. For leading climatologist James Hansen, even the 2°C limit is unsafe. And without emissions cuts, global temperatures are projected to rise by 4°C by the end of the century. Many scientists are reluctant to make predictions, but the apocalyptic litany of what a 4°C world could hold includes widespread drought, famine, climate refugees by the millions, civilization-threatening warfare, and a sea level rise that would permanently drown much of New York, Miami, Mumbai, Shanghai, and other coastal cities.

But here’s where things get weird. The UN report envisions 116 scenarios in which global temperatures are prevented from rising more than 2°C. In 101 of them, that goal is accomplished by sucking massive amounts of carbon dioxide from the atmosphere—a concept called “negative emissions”—chiefly via BECCS. And in these scenarios to prevent planetary disaster, this would need to happen by midcentury, or even as soon as 2020. Like a pharmaceutical warning label, one footnote warned that such “methods may carry side effects and long-term consequences on a global scale.”