The great defining event of the 21st century — one of the great defining events in human history — will occur in three decades, give or take, when the global population starts to decline. Once that decline begins, it will never end. We do not face the challenge of a population bomb, so rampant in the popular imagination, but of a population bust — a relentless, generation-after-generation culling of the human herd. Nothing like this has ever happened before.

If you find this news shocking, that's not surprising. The United Nations forecasts that our population will grow from 7 billion to 11 billion in this century before leveling off after 2100. But an increasing number of demographers around the world believe the UN estimates are far too high.