When world leaders came together in 2015 to settle on a framework for action on climate change, the resulting agreement set a goal of limiting the rise in global average temperatures from preindustrial levels to 2 degrees Celsius by 2100. The climatologist Michael Mann, voicing what has been the scientific consensus for some time, has warned that warming beyond that could lead to “environmental ruin.” Recent research suggests, in fact, that if all carbon emissions worldwide ceased tomorrow, the Earth would still warm as much as 1.3 degrees by this century’s end.