When we’re faced with threats of inundation, our reaction has traditionally been to build walls. Sea-level rises, storms and floods have been held back with solid barriers, seawalls and dykes. We have used walls to keep out people, too: the fact that this has failed throughout the ages has not stopped its recent revival in the United States.

The climate crisis threatens global sea-level rises of well over half a metre if we fail to act, while tidal storm surges will reach many times that height. Fiercer and more frequent hurricanes will batter us, and millions of people who live in areas where crops have failed and wells run…