Debates over climate change are filled with dire estimates of its cost. This many trillions of dollars of damage, that large a share of gross domestic product destroyed, so-and-so many lives lost, etc. Where do such figures come from? Mostly from laughably bad economics.

This has nothing to do with the soundness of climate science. The games begin when economists get their hands on scientific projections and try to translate temperatures into human impacts. They conduct statistical analyses of the effects that small year-to-year...