The Next Century’s Big Demographic Mystery

Experts can’t agree on how many humans will be on Earth by 2100. The implications could be profound.

The Atlantic

The first modern, comprehensive attempt to predict the human population’s long-term trajectory took place in 1945. The number of people on Earth had more than doubled in the previous century and a half, to more than 2 billion, and experts worried that food production would not be able to keep pace. Frank Notestein, the founding director of the Office of Population Research at Princeton, estimated that about 3.3 billion humans would be on the planet by the year 2000.

He was only about 3 billion off. The global population topped 6 billion by the millennium’s end, and has grown by nearly 2 billion more since. Nevertheless, Notestein’s work was foundational. In 1946, he was appointed director of the newly formed United Nations Population Division (UNPD), which continues to make global-population projections today. These forecasts help national leaders anticipate demand for food, water, and energy, as well as plan infrastructure projects and support systems for children and the elderly. They also assist environmental scientists in forecasting climate change.

The UNPD has traditionally had sparse competition. But over the past several years, two other prominent forecasts—one from the Centre of Expertise on Population and Migration (CEPAM) and another from researchers at the Institute for Health Metrics and Evaluation (IHME), at the University of Washington—have offered alternative narratives about humanity’s future. The UNPD’s most recent forecast predicts that the global population will reach about 10.9 billion by 2100 and could stabilize or even decline slowly thereafter. CEPAM projects that the population will peak at 9.8 billion around 2070–80 and then drop to about 9.5 billion by the end of the century. According to IHME, the global population will peak earlier still, at 9.7 billion in 2064, and then plummet by nearly 1 billion before the century is out. (Though IHME’s stats are widely reported in the media, in August of last year, The Lancet published a letter signed by more than 150 experts calling for greater scrutiny of IHME’s forecast.)

If, as the saying goes, “demography is destiny” and population trends determine the prosperity of nations and the world, then these forecasts imply conflicting prophecies of our future challenges. The UNPD’s points to a much more crowded planet, which some fear threatens to deplete natural resources and increase carbon emissions. The IHME’s—and, to a lesser extent, CEPAM’s—envisions extreme population aging, where, without considerable immigration, some countries could end up with an “inverted age pyramid” in which the old outnumber the young and the needs of elderly dependents strain the workforce supporting them. Scientists all seem to have different ideas about what to do next.


In the short term, demographic forecasts are quite reliable, John Wilmoth, the UNPD’s director, told me. You don’t need a fancy statistical model to know how many women of childbearing age are alive, or how old a baby born in 2021 will be in 30 years. That’s why, through about 2050, all three global-population forecasts mostly agree. Only in the second half of the century do they begin to diverge.

A number of assumptions underlie these discrepancies. For example, the fertility rate is currently very high in sub-Saharan Africa, though it is declining. Although most demographers expect that women there will continue to have fewer children as the region develops economically, they don’t agree on how steep the decline will be. The UNPD model posits that the best way to predict the future is to study the past, Wilmoth said. In other words, fertility should fall in places such as sub-Saharan Africa at about the same rate it’s fallen elsewhere. But demographers at both CEPAM and IHME suspect that population shifts in 21st-century Africa won’t resemble those of, say, late-20th-century Asia or Latin America. Instead, the rapid expansion of women’s access to modern education and effective contraception may accelerate the fertility decline.

Another source of contention is what will happen to fertility in wealthy nations such as the United States, Finland, and Japan. In many of these countries, fertility has already fallen below replacement level—the rate needed for a population to replace itself from one generation to the next without immigration, which is about 2.1 children per woman. The UNPD and CEPAM models both expect fertility to eventually reach a global average of about 1.75. But that would require a significant rebound in many nations—in 2020, the fertility rate was about 1.4 in Finland—which Christopher Murray, the director of the IHME, told me there’s little evidence to support. Based on his organization’s calculations, fertility will converge closer to 1.3 in most countries. By the end of the century, that adds up to billions fewer people, Murray said.

In reality, raising fertility is very difficult. Countries have implemented all manner of so-called pronatalist policies—including “baby bonuses” of 3,000 Australian dollars and generous parental leave in Scandinavia—with little sustained success. It’s not exactly clear why low fertility is so resistant to reversal, but seemingly once people get used to the idea of having just one or two children (or none), persuading them to have more is very hard. This is particularly evident in countries where the fertility rate has fallen below 1.5 or so. “The risks of ignoring low fertility for those countries who are affected are enormous,” Murray said.


All major forecasts agree on one thing: The massive population growth that began during the industrial era will continue, but it will end within the century. The world will inevitably get older in the process.

The key disagreement is when exactly the population will peak, and how much it will fall. Some researchers believe that the world cannot support the 11 billion people that the UNPD predicts. The planet is already straining under human activity, Jane O’Sullivan, a sustainability researcher at the University of Queensland who’s also been critical of the IHME’s methodology, told me. One of her major concerns is climate change, which is driven by greenhouse-gas emissions that tend to rise with population growth. The UN’s Intergovernmental Panel on Climate Change has warned that further warming could lead to more heat waves, droughts, floods, and wildfires. And extreme weather is already threatening to destabilize the food supply, all while demand for food increases. Overpopulation, in short, might lead to more conflict over ever-scarcer resources. Life on this planet could involve not just more unemployment but also its messy knockdown effects—violence from a restless populace, unbalanced migration from rural areas to cities, a lack of housing leading to unsanitary informal settlements, O’Sullivan told me.

Then again, if the IHME is correct, the planet won’t be as crowded as the UNPD predicts—but its people will be much older. Countries tend to spend much more on the elderly than on other age groups: Older people are more prone to illness, and many rely on publicly funded pensions and eventually require caregiving. All of this relies on a system of younger taxpayers, workers, and family members. Many countries, including the U.S., are already struggling to meet the needs of the rapidly growing elderly population. There’s a critical shortage of long-term-care workers in many countries and a rising number of people in the U.S. unable to afford their services. Estimates vary, but according to one literature review, somewhere from one-third to two-thirds of workers in America are already at risk of having inadequate income to maintain their living standards in old age, while our Social Security system relies on funds that are facing insolvency. In Japan, the oldest country in the world, officials are already concerned about the high number of elderly people dying alone in their homes in part because of an aging society and weaker family ties.

These potential outcomes seemingly point to different solutions. Researchers such as Murray believe that countries on the path to an inverted age pyramid should implement policies that make raising kids easier—such as publicly funded child care or child allowances—in order to prevent fertility from plunging too far. O’Sullivan, who believes that concerns about aging are overblown, thinks that staving off the worst effects of overpopulation will require normalizing small families. Achieving that could involve policies such as enhancing sex education in schools, door-to-door birth-control delivery, and encouraging girls in low-income countries to finish school. Still, some demographers believe that a fixation on population numbers is misplaced. The considerable discrepancies among long-term population forecasts reflect real unpredictability, Nico Keilman, a former demography professor at the University of Oslo who spent his career studying uncertainty, told me.

Regardless of when precisely Earth’s population peaks and falls, the problems caused by aging and climate change will be dire if left unaddressed. Accepting as much could be an asset if it directs policy makers toward what they actually can control. “It’s the habit of governments to try to solve demographic problems, quote, unquote, with demographic solutions,” Stuart Gietel-Basten, a social-science and public-policy professor at the Hong Kong University of Science and Technology who helped design CEPAM’s model, told me. Governments concerned about carbon emissions, for example, might focus on pushing fertility down rather than finding ways to minimize the average person’s carbon footprint. He admits that population growth and aging raise challenges for society, and can be the consequence of problems such as gender inequality, lack of education, and poor support for working parents. But treating fertility—which is as stubborn as it is unpredictable—as the problem can set governments up for failure.

Gietel-Basten’s perspective seems to offer a more hopeful spin on the idea that demography is destiny. Many aspects of our fate may not be within our command. The question is how well we can cope with what we have.