Headlines warned of economic doom after the U.S. government released its fourth National Climate Assessment last week. Yet a close reading of the report shows that the overall economic impact of human-caused climate change is expected to be quite small.

Projecting human-caused changes in the global climate is a major scientific challenge; estimates of the temperature increases due to rising greenhouse-gas concentrations are uncertain by a factor of three. Trying to make projections for a particular region—such as the contiguous...