When reticent and circumspect scientists feel compelled to declare a climate emergency, the world had best listen. Last month, more than 11,000 scientists signed onto a declaration stating “clearly and unequivocally” that the planet is facing such an emergency—and that conventional indicators are underestimating its scale. In parallel, the European Union recently made such a declaration, the largest entity to do so to date, joining more than 1,200 city councils and parliaments around the globe. The time for concerted and coordinated action has arrived: massive investments in renewable energy and energy efficiency, stronger regulations, shifting diets and consumption patterns, and decarbonizing all aspects of the economy. Next year, when the signatories to the Paris Accords submit revised climate plans, will they heed the scientist’s warning? Will the US electorate?