One study by the First Street Foundation, a research firm that studies climate threats to housing, found that roughly 3.2 million Americans have already migrated, many over short distances, out of flood zones, such as low-lying parts of Staten Island, Miami and Galveston, Texas. Over the next 30 years, 7.5 million more are projected to leave those perennially flooded zones, according to the study.

All of this suggests a possible boom for inland and Northern cities. But it also will leave behind large swaths of coastal and other vulnerable land where seniors and the poor are very likely to disproportionately remain.