Today Guy Callendar is a historical footnote, but tomorrow he will have a chapter of his own. Born in 1898, Callendar was the son of Britain’s leading steam engineer, a successful academic and inventor who raised his children in a 22-room mansion. A greenhouse on the grounds was converted into a laboratory for the children until one of Callendar’s three brothers blew it up trying to make TNT. The same brother put out Callendar’s left eye. Undeterred by the subsequent lack of depth perception, he became his father’s successor as the nation’s most important steam engineer.

None of this is why Guy Callendar’s name will be boldfaced in tomorrow’s textbooks. Instead it will be because he was willing to delve into fields he knew nothing about, atmospheric science among them. Nobody knows why he got so interested in the air. Callendar himself attributed it to ordinary curiosity: “As man is now changing the composition of the atmosphere at a rate which must be very exceptional on the geological time-scale, it is natural to seek for the probable effects of such a change.”

In the early 1930s Callendar began collecting measurements of the properties of gases, the structure of the atmosphere, the sunlight at different latitudes, the use of fossil fuels, the action of ocean currents, the temperature and rainfall in weather stations across the world, and a host of other factors. It was a hobby, but a remarkably ambitious one: He was producing the first rough draft of the huge climate models familiar today. After years of calculation, in 1938 he came to a surprising conclusion: People were dumping enough carbon dioxide into the air to raise the world’s average temperature.

This article is excerpted from Charles Mann's new book, The Wizard and the Prophet, published this month by Penguin Random House.

Callendar did not have a PhD, but he had enough academic status to be allowed to present his ideas that year in front of a panel of six professional climate scientists at the Royal Meteorological Society. The pros were familiar with the claim that carbon dioxide affected climate, which other researchers—notably Sweden’s Svante Arrhenius—had made in previous decades. But these ideas, in their view, had been thoroughly debunked. Years before Callendar’s presentation, British Meteorological Office head George Clarke Simpson had stressed the scientific consensus that carbon dioxide in the air had “no appreciable effect on the climate.” Now he was one of Callendar’s commentators. Callendar, unlike his predecessors, had a coherent model and decades of new data. Nonetheless Simpson was not kind. The problem with people like Callendar, he sniffed, was that “non-meteorologists” simply didn’t know enough about climate to be helpful. The other five commentators were no more appreciative. Although Callendar had spent years gathering evidence, they were “very doubtful” that his work meant anything.

Aside from snobbery, the biggest reason for skepticism was that there simply wasn’t—and isn’t—very much carbon dioxide in the air. When Callendar was scribbling away, carbon dioxide comprised about .03 percent of the atmosphere by volume (the level has risen slightly since then). If somebody collected 10,000 scuba tanks of air, the carbon dioxide in them would be enough to fill up three tanks. How could anything so tiny be important to a huge, super-complex system like the atmosphere? It was like claiming that a toy bulldozer could level Manhattan. The idea seemed absurd on its face.

A few were grappling with an even crazier idea: that people were pumping enough carbon dioxide into the air to reshape the face of the Earth and put human existence at risk.

Undeterred, Callendar kept working on what came to be called the “Callendar effect.” This was not because he feared the impacts of rising carbon dioxide. In fact, Callendar believed that this warming business sounded like a good thing. “Small increases in mean temperature” would help farmers in cold places, he argued. Better yet, they would “indefinitely” postpone “the return of the [ice ages’] deadly glaciers.”

Callendar died in 1964. By that time, many climate scientists had reconsidered their opposition to his ridiculous-sounding belief that slightly increasing the small amounts of carbon dioxide in the air could affect global temperatures. A few were grappling with an even crazier idea: that people were pumping enough carbon dioxide into the air to reshape the face of the Earth and put human existence at risk. But nobody was imagining that the possible solution to our inadvertent transformation of the planet would be to transform the planet even more.

Stupid Bad Luck

Climate change entered the modern public arena on June 23, 1988, when NASA researcher James E. Hansen testified before the US Senate about its potential effects. Colorado Senator Tim Wirth, an avid environmentalist and unusually cerebral politician, had learned about the Callendar Effect and wanted Hansen, a climate expert, to ring the bell and warn the nation. To bolster the narrative, Wirth deliberately scheduled the hearing for what historically was the summer’s hottest day and shut off the room’s air conditioning.