NASA / Reuters

For most of the time that humans have existed, our ranks have grown really, really slowly. There were an estimated 4 million people on Earth in 10,000 B.C., and after the following 10 millennia, the planetwide population had only reached 190 million. Even in 1800, the total number of humans was still under 1 billion.

The climb since then—made possible by advances in medicine, sanitation, and food production—has been astounding. By 1900, there were 1.65 billion people; by 2000, there were more than 6 billion. Just two decades later, the global population sits at 7.7 billion.

But soon—or at least, soon in the context of human history—the number of people on Earth will stop growing. Based on the latest figures from the United Nations, demographers’ best guess for when this will happen is about 2100. By then, the global population is projected to have risen to just shy of 11 billion.

Humanity has experienced population drop-offs before—the Black Death is thought to have killed about 200 million people—but this time will be different. “In the past, when the world population experienced a decline,” Tom Vogl, a development economist at UC San Diego, told me, “it was because a lot of people died.” This coming transition, meanwhile, will be the result of people having fewer kids—a product of rising incomes and levels of education, especially for women and especially in less-wealthy countries.

There’s always some uncertainty to making predictions, but Vogl said that population projections are usually “less uncertain” than other social and economic projections. This is because researchers already know roughly how many humans there are now, as well as how old everyone is, so they can guess, with some confidence, how many people will be of childbearing age in the next couple of decades—which means they can then guess how many children those people will have.

Even if future fertility rates stray a bit from expectations, Vogl said, it wouldn’t “change the fact that sometime in the next [100 years], the world’s population is going to peak.” And his hunch is that the population is unlikely to go up from there, barring some major increase in fertility rates (perhaps as a result of a political movement that encourages people to have a lot of babies, which is what happened in China in the mid-20th century).

Because some determinants of what the population will be 80 years from now are locked in today, it’s possible to anticipate broad demographic shifts. “By the time the world population stabilizes, Africa is going to be the largest world region in terms of population … and Islam is going to be the world’s largest religion,” Vogl said.

And crucially, the human population will, as a whole, get older. The UN’s data suggest that during the eight decades from 2020 to 2100, the number of people aged 80 or older will rise from 146 million to 881 million; during roughly that same span of time, humans’ median age will increase from 31 to 42.

When the population of a single country gets older like this, “that typically poses big problems for the country’s politics,” Vogl said. In this scenario, working people have to support a growing number of retirees, both on the society level, in terms of funding national retirement-benefits programs, and on the household level, where aging relatives might need family members’ care. (Japan is a frequently cited example of a country currently facing these issues as a result of low fertility rates and long life spans.)

“When that happens on a global level, it means that that pension crisis is going to happen in many countries independently, at different points along that global path,” Vogl said. As each country encounters this problem, immigration—bringing in younger, work-ready people from countries with a lower concentration of older people—could counteract the aging dynamics. But today’s politics indicate that immigration is not a simple fix. Less controversial ways for countries to offset this problem include growing their economy (because there would be more money to go around) and creating more opportunities for women in the labor market (which would alter a country’s ratio of workers to retirees).

The population trends of the rest of the century will also alter the texture of family life. As the fertility rate declines in many parts of the world, families will get smaller. This means children will tend to have fewer siblings, and parents can invest more of their resources and attention in each child, perhaps paying more to send them to a better school, notes Parfait Eloundou-Enyegue, a development-sociology professor at Cornell University. “And also, the culture moves in the direction in which the families become nuclear rather than extended,” he adds. “So in Africa, for instance, you have extended family systems where people would live with cousins and in-laws.” In many households, that family structure might start to give way to a smaller one.

This will have both emotional consequences—people without many relatives may struggle to find familial companionship—and economic ones. Large extended families, Eloundou-Enyegue notes, are effective at diffusing financial responsibilities, because family members can help one another out. “As families shrink, we have to think about that dimension of economic welfare: How do we manage risk?” he says.

When Eloundou-Enyegue thinks about the coming demographic shifts, he also wonders how they will alter the world’s cultural centers of gravity. “Because the young shape a lot of the large segments of the culture—let’s say, artistic culture, or sports culture—it would be interesting to see where most of the young people [will be],” he says. According to his calculations based on the UN’s data, the proportion of all humans on Earth under the age of 25 who live in Asia will drop from 56 to 37 percent between next year and 2100. Meanwhile, Africa’s share of the global population of young people will shoot up, from 25 to 48 percent. (The proportion living in the rest of the world will not fluctuate much.)

Amid all this, the planet itself will change too. “We’ve done a whole bunch of damage to the environment,” said Vogl of the effects of the global population’s precipitous rise in the 20th century. When I asked him how concerned he is about the environmental toll of further growth, he said, “I’m pretty worried, mostly by the total inability of the global political apparatus to grapple with this issue and to try to find solutions.”

Unfortunately, the demographic changes of the 21st century will not do much to help matters. Eloundou-Enyegue points out that the problems that come up—whether they have to do with the environment, shifting family structures, or any number of other things—will hit different countries and regions at different times. “Just looking at the average global picture is a statistical simplification, because the world is really, really diverse in terms of the [fertility] rates that we have now,” he said. And because the leveling off of the population by about 2100 will arrive incrementally across countries, and not all at once, “that may make it difficult to have a uniform debate or consensus about what the situation is and what the remedies are.” Uniform consensus has never been humanity’s strong suit, but at least we have the rest of the century to prepare for what’s ahead.

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