For most of human history, the world’s population grew so slowly that for most people alive, it would have felt static. Between the year 1 and 1700, the human population went from about 200 million to about 600 million; by 1800, it had barely hit one billion. Then, the population exploded, first in the United Kingdom and the United States, next in much of the rest of Europe, and eventually in Asia. By the late 1920s, it had hit two billion. It reached three billion around 1960 and then four billion around 1975. It has nearly doubled since then. There are now some 7.6 billion people living on the planet.
Just as much of the world has come to see rapid population growth as normal and expected, the trends are shifting again, this time into reverse. Most parts of the world are witnessing sharp and sudden contractions in either birthrates or absolute population. The only thing preventing