As they expand, many of the world’s fast-growing cities end up building over floodplains, forests, and wetlands that could absorb rising waters during a storm or hold reservoirs of water during a drought.

As with temperatures, sea levels have in the past varied dramatically. At the end of the Wisconsin, as the great ice sheets were breaking up, there were periods when they rose at the astonishing rate of a foot a decade. (It’s been proposed that one of these “meltwater pulses” inspired the account of the deluge in Genesis.) Obviously, our ancestors