Fewer babies’ cries. More abandoned homes. Toward the middle of this century, as deaths start to exceed births, changes will come that are hard to fathom.

All over the world, countries are confronting population stagnation and a fertility bust, a dizzying reversal unmatched in recorded history that will make first-birthday parties a rarer sight than funerals, and empty homes a common eyesore.

Maternity wards are already shutting down in Italy. Ghost cities are appearing in northeastern China. Universities in South Korea can’t find enough students, and in Germany, hundreds of thousands of properties have been razed, with the land turned into parks.

Like an avalanche, the demographic forces — pushing toward more deaths than births — seem to be expanding and accelerating. Though some countries continue to see their populations grow, especially in Africa, fertility rates are falling nearly everywhere else. Demographers now predict that by the latter half of the century or possibly earlier, the global population will enter a sustained decline for the first time.

A planet with fewer people could ease pressure […]