 The idea that humans could change Earth’s climate by emitting large quantities of carbon dioxide and other gases is not a new one (Fleming 1998; Weart 2008 [2017, Other Internet Resources]). In the late nineteenth century, Swedish chemist Svante Arrhenius calculated that doubling the levels of carbonic acid (i.e., carbon dioxide) in the atmosphere would warm Earth’s average surface temperature by several degrees Celsius (Arrhenius 1896). By the mid-twentieth century, oceanographer Roger Revelle and colleagues concluded: 