On his way to the United Nations climate talks in Bonn, Germany, this week, Jerry Brown stopped over at the Vatican, where a doleful group of climate scientists, politicians and public health officials had convened to discuss calamities that might befall a warming world. The prospects were so dire—floods and fires, but also forced migration, famine and war—that some of the participants acknowledged difficulty staving off despair. California’s doomsayer governor did not express much optimism either. Seated between an economist and an Argentine bishop at the Pontifical Academy of Sciences, Brown leaned into his microphone and said, “It is despairing....