Since the dawn of humans, we have faced one inexorable challenge — how to support the rise and — in the last half century or so — explosion of the population. But, in a momentous reversal, that age-old challenge is changing: the population of most countries is shrinking — for many of them at an alarming pace — and at the same time aging.

Much of the world teeters on the cusp of a childless, elderly future.

Why it matters: A growing, youthful population is typically a bedrock sign of vitality. In the industrial age, that's included a growing economy, greater opportunity, advancing technology, and a more comfortable retirement for older people. The turnaround on all continents except Africa means supporting an increasing number of retired people with many fewer workers, and confronts the world with two primary solutions, both of them controversial.

Loosen up currently fraught politics around migration from still-growing countries.

Or populate our countries with robot helpers.

Whichever the case, almost no one seems prepared. People are going to have to work longer, experts say. And the U.S. requires "major reforms" in its programs for the elderly, says Richard Cincotta, director of the Global Political Demography Program at the Stimson Center, and formerly a lead demographer for the U.S. intelligence community. But the government "seems to be nowhere near making the social security reforms that are needed."

Largely because of Africa, the global population is forecast to continue rising through the century past 11 billion. But according to the United Nations, there is a 23% chance the world population will stabilize or fall by then in a phenomenon that some demographers call “peak human.”

Digging underneath the numbers:

By 2100, the population of the world's poorest countries will more than triple, from 954 million in 2015 to 3.2 billion.

Minus immigration, the populations of the U.S., Japan and all of Europe are shrinking. By 2050, 48 countries or areas will have fewer people. Several will see a 15% decline, including Japan, the Balkans and the Baltics, and some much more.

In 2016, the fertility rate in the United States was the lowest it has ever been, according to the Centers for Disease Control. It was 1,765 births per 1,000 women of child-bearing age, below the replacement rate of 2,100.

At the same time, there will be more old people: forecasts include a spike in the number of people 80 and over.

Global life expectancy will rise from about 70 in 2015 to 83 in 2100, according to UN figures.

Except Africa, by 2050 about a quarter of the world population will be 60 or older. At about 900 million now, their numbers will rise to about 3.2 billion in 21oo. By 2080, those 65 or older will be 29.1% of the global population — and 12.7% will be 80 or over, Eurostat said.

The most worrying figure: The world will not have sufficient working-age people to support the elderly. Currently, North America has just under four workers per retired person. Seven European countries have three, and Japan has just a bit more than two.

By 2050, seven Asian countries, 24 European and four Latin American will fall below two workers per retired person, the UN says.

This means “a rising fiscal burden and slower economic growth than if the population was not aging,” says Richard Jackson of the Global Aging Institute.

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Editor's note: This deep dive was first published in July of 2018.