When William Nordhaus, who would later win a Nobel prize in economics, modelled the interaction between the economy and the atmosphere he represented the “damage function”—an estimate of harm done by an extra unit of warming—as a wiggly line. So little was known about the costs of climate change that he called it “terra incognita”, unknown land, compared with the “terra infirma”, shaky ground, of the costs of preventing it. Eventually, a rough calculation gave him an estimate that 1-2% of global GDP would be lost from a 3°C rise in temperature. This was no more than an “informed hunch”, he wrote in 1991.