To tackle big risks requires big imagination and big bucks. And, except for annihilation from global thermonuclear warfare, no set of risks loom larger than the continuing atmospheric buildup of greenhouse gases, chiefly carbon dioxide. Earth is responding to this buildup through rising temperatures, melting glaciers, expanding seas, migrating species and disease vectors and a host of other impacts. Scientists and policymakers believe that the Paris accord goal of restraining warming to 1.5