Eventually, the two produced an equation-dense paper in which they argued that self-replicating machines could solve the world’s energy problem and, more or less at the same time, clean up the mess humans have made by burning fossil fuels. The machines would be powered by solar panels, and as they multiplied they’d produce more solar panels, which they’d assemble using elements, like silicon and aluminum, extracted from ordinary dirt. The expanding collection of panels would produce ever more power, at a rate that would increase exponentially. An array covering three hundred and eighty-six thousand square miles—an area larger than Nigeria but, as Lackner and Wendt noted, “smaller than many deserts”—could supply all the world’s electricity many times over.

This same array could be put to use scrubbing carbon dioxide from the atmosphere. According to Lackner and Wendt, the power generated by a Nigeria-size solar farm would be enough to remove all the CO 2 emitted by humans up to that point within five years. Ideally, the CO 2 would be converted to rock, similar to the white sand produced by Carbon Engineering; enough would be created to cover Venezuela in a layer a foot and a half deep. (Where this rock would go the two did not specify.)

Lackner let the idea of the self-replicating machine slide, but he became more and more intrigued by carbon-dioxide removal, particularly by what’s become known as “direct air capture.”

“Sometimes by thinking through this extreme end point you learn a lot,” he said. He began giving talks and writing papers on the subject. Some scientists decided he was nuts, others that he was a visionary. “Klaus is, in fact, a genius,” Julio Friedmann, a former Principal Deputy Assistant Secretary of Energy and an expert on carbon management, told me.

In 2000, Lackner received a job offer from Columbia University. Once in New York, he pitched a plan for developing a carbon-sucking technology to Gary Comer, a founder of Lands’ End. Comer brought to the meeting his investment adviser, who quipped that Lackner wasn’t looking for venture capital so much as “adventure capital.” Nevertheless, Comer offered to put up five million dollars. The new company was called Global Research Technologies, or G.R.T. It got as far as building a small prototype, but just as it was looking for new investors the financial crisis hit.

“Our timing was exquisite,” Lackner told me. Unable to raise more funds, the company ceased operations. As the planet continued to warm, and carbon-dioxide levels continued to climb, Lackner came to believe that, unwittingly, humanity had already committed itself to negative emissions.

“I think that we’re in a very uncomfortable situation,” he said. “I would argue that if technologies to pull CO 2 out of the environment fail then we’re in deep trouble.”

Lackner founded the Center for Negative Carbon Emissions at A.S.U. in 2014. Most of the equipment he dreams up is put together in a workshop a few blocks from his office. The day I was there, it was so hot outside that even the five-minute walk to the workshop required staging. Lackner delivered a short lecture on the dangers of dehydration and handed me a bottle of water.

In the workshop, an engineer was tinkering with what looked like the guts of a foldout couch. Where, in the living-room version, there would have been a mattress, in this one was an elaborate array of plastic ribbons. Embedded in each ribbon was a powder made from thousands upon thousands of tiny amber-colored beads. The beads, Lackner explained, could be purchased by the truckload; they were composed of a resin normally used in water treatment to remove chemicals like nitrates. More or less by accident, Lackner had discovered that the beads could be repurposed. Dry, they’d absorb carbon dioxide. Wet, they’d release it. The idea was to expose the ribbons to Arizona’s thirsty air, and then fold the device into a sealed container filled with water. The CO 2 that had been captured by the powder in the dry phase would be released in the wet phase; it could then be piped out of the container, and the whole process re-started, the couch folding and unfolding over and over again.

Lackner has calculated that an apparatus the size of a semi trailer could remove a ton of carbon dioxide per day, or three hundred and sixty-five tons a year. The world’s cars, planes, refineries, and power plants now produce about thirty-six billion tons of CO 2 annually, so, he told me, “if you built a hundred million trailer-size units you could actually keep up with current emissions.” He acknowledged that the figure sounded daunting. But, he noted, the iPhone has been around for only a decade or so, and there are now seven hundred million in use. “We are still very early in this game,” he said.

The way Lackner sees things, the key to avoiding “deep trouble” is thinking differently. “We need to change the paradigm,” he told me. Carbon dioxide should be regarded the same way we view other waste products, like sewage or garbage. We don’t expect people to stop producing waste. (“Rewarding people for going to the bathroom less would be nonsensical,” Lackner has observed.) At the same time, we don’t let them shit on the sidewalk or toss their empty yogurt containers into the street.

“If I were to tell you that the garbage I’m dumping in front of your house is twenty per cent less this year than it was last year, you would still think I’m doing something intolerable,” Lackner said.

One of the reasons we’ve made so little progress on climate change, he contends, is that the issue has acquired an ethical charge, which has polarized people. To the extent that emissions are seen as bad, emitters become guilty. “Such a moral stance makes virtually everyone a sinner, and makes hypocrites out of many who are concerned about climate change but still partake in the benefits of modernity,” he has written. Changing the paradigm, Lackner believes, will change the conversation. If CO 2 is treated as just another form of waste, which has to be disposed of, then people can stop arguing about whether it’s a problem and finally start doing something.

Carbon dioxide was “discovered,” by a Scottish physician named Joseph Black, in 1754. A decade later, another Scotsman, James Watt, invented a more efficient steam engine, ushering in what is now called the age of industrialization but which future generations may dub the age of emissions. It is likely that by the end of the nineteenth century human activity had raised the average temperature of the earth by a tenth of a degree Celsius (or nearly two-tenths of a degree Fahrenheit).

As the world warmed, it started to change, first gradually and then suddenly. By now, the globe is at least one degree Celsius (1.8 degrees Fahrenheit) warmer than it was in Black’s day, and the consequences are becoming ever more apparent. Heat waves are hotter, rainstorms more intense, and droughts drier. The wildfire season is growing longer, and fires, like the ones that recently ravaged Northern California, more numerous. Sea levels are rising, and the rate of rise is accelerating. Higher sea levels exacerbated the damage from Hurricanes Harvey, Irma, and Maria, and higher water temperatures probably also made the storms more ferocious. “Harvey is what climate change looks like,” Eric Holthaus, a meteorologist turned columnist, recently wrote.