But to conserve our climate requires more than mere inaction. To do nothing—to let greenhouse gas emissions accelerate as they would if we kept burning coal and oil and gas heedlessly—is to welcome warming of four or five or six degrees Celsius. These are numbers that diverge from the climate of the eighteenth century as sharply as the climate of the eighteenth century diverges from the Ice Age.1 These are numbers inside which the planetary systems that sustain us break.
