New Republic: In June 1975, a Yale economist named William Nordhaus published a paper for the International Institute for Applied Systems Analysis, an Austrian think tank. In the paper, he put forward a theory about a potentially globe-altering climate-change Red Line--a threshold that, if crossed, could result in a fusillade of environmental dangers. Research suggested that a rise in carbon-dioxide emissions from burning fossil fuels might melt the Arctic Sea ice, prompting a dramatic increase in rain and...