Just as there is a carbon tax on polluting industries, there should be a “migration tax” on the nations who got rich while emitting greenhouse gases. The United States is responsible for one-third of the excess carbon in the atmosphere; Europe, another one-quarter. A hundred million refugees fleeing hurricanes and droughts will have to be resettled by the end of the century. The United States should take a third, and Europe another quarter.

A huge bill would come to the West, but it is one it should look forward to paying. Without immigration, America’s economic growth would have been 15 percent lower from 1990 to 2014; Britain’s would have been a full 20 percent lower. Immigrants are 14 percent of the American population, but they started a quarter of all new businesses and since 2000 earned over a third of the American Nobel Prizes in chemistry, physics and medicine.

Migrants are 3 percent of the world’s population but contribute 9 percent of its gross domestic product. Their taxes prop up the pension systems of the wealthy nations, which are not making enough babies of their own.