It’s a popular theme—from Thomas Malthus to Paul R. Ehrlich to Thanos of Avengers fame: Only drastic action will avert catastrophe. We must consume less, accept less, be less. We’re warned in ever more strident terms that we must act now or in 12 years (or 10 years or 18 months) there will be devastating consequences. That’s the message of the G-7 as it met in Biarritz, France, and it’s the message of every Democratic presidential hopeful...