Phillip Longman points out something most people don’t quite get about global population growth over the next few decades:

Over the next 40 years, according to the UN, world population will grow from 6.9 billion to 9.1 billion, which may sound like more of the same robust growth that we saw throughout the 20th century. But this will be a very different kind of population growth from anything humankind has seen before. The rate of growth is perpetually diminishing toward zero, and more than half of the remaining increase in population (56 percent) will be among people over 60 — among people, that is, who have already been born.

This may seem impossible, but when calculating population growth, declining death rates are just as important as rising birthrates. Today’s children are more likely than their parents to live to advanced ages. Even without any new children being born, this decline in mortality by itself would add to the number of people on the planet. Today’s population explosion among those over 60 will be echoed in twenty years by a population explosion among those over 80. Most of the predicted 2.2 billion in world population growth between now and 2050 will not come from children. Indeed, over that period, the population of young children (0 to 4) is expected to fall by 49 million.

Do you follow this? The world will not have so many more people because that many more people are being born. It will have so many more people because the elderly are not dying. There will be fewer young workers around to take care of them, and to pay for their pensions. With so much of the money the productive younger workers make going into taxes to pay for pension and medical benefits for the elderly masses, there will be far less for those young people and their young families to buy goods and invest. Which is why Longman says:

Perhaps there is an economic system that can preserve prosperity even in the face of an aging, stagnating population, but it has not yet been devised.