Ben Wattenberg's Fewer: How the New Demography of Depopulation Will Shape Our Future, begins with a momentous observation. In 2002 the U.N. predicted that the Total Fertility Rate (TFR) of the world would soon drop to 1.85 children per woman. The replacement rate - the average number of children that each woman has to have to keep the population numbers steady - is 2.1.
Wattenberg calls this a Copernican revolution in the history of demography. The entire field of demography has been built on the assumption of growing world population. In the 20th century alone, world population grew from 1 billion to 6 billion. Population alarmists were predicting for the past forty years that world population would reach 11, or 15, or even 20 billion by the end of the 21st century. They usually then predicted a catastrophe.
Instead, the fertility rate has been falling below replacement level in country after country. In all the industrialized world, the rate has fallen to an alarming low level - well below 1.85 in most European countries and Japan. The US, almost alone of rich countries, has a near-replacement level of fertility. Now, the poor countries are getting in the act, too. China, with its brutal one-child policy, is down to 1.7. Mexico and Brazil, two population powerhouses, are probably below replacement. India has cut its TFR in half in the past generation, and will probably go down past replacement, too. And many other countries, large and small, even the quite poor ones, are headed the same way.
If the world TFR is 1.85 for even a generation, it is inevitable that the world population will decline. Wattenberg predicts that after hitting a peak of perhaps 8 billion in mid-century, the population will fall back to 6 billion (the current number) by the end of the 21st century. And it could be much lower.
The last time world population declined was due to the Black Death.