evidence: Scientists and policymakers have long agreed that global warming beyond 2° C above the pre-industrial average would pose large and escalating risks to human life as we know it on Earth, and governments have used that number as an organizing principle. [...] In the 1970s, William Nordhaus, an economist at Yale, suggested in several papers that if global warming were to exceed 2° C on average, it would push global conditions past any point that any human civilization had experienced.1,2 At the time, Nordhaus's idea was a simple suggestion of what rise in temperature could cause extreme conditions, based on the historical record of past average temperatures, but it gained new importance a decade later. [...] "At two degrees we see dramatic alterations to the ability of the Earth's system to maintain the conditions that allow for human life and indeed other species’ life," says Maria Ivanova, a professor of global governance at University of Massachusetts Boston.