Two point one: That’s how many children everyone able to give birth must have to keep the human population from beginning to fall. Demographers have long expected the world will dip below this magic number—known as the replacement level—in the coming decades. A new study published last month in The Lancet, however, puts the tipping point startlingly near: as soon as 2030.

It’s no surprise that fertility is dropping in many countries, which demographers attribute to factors such as higher education levels among people who give birth, rising incomes, and expanded access to contraceptives. The United States is at 1.6 instead of the requisite 2.1, for example, and China and Taiwan are hovering at about 1.2 and one, respectively. But other predictions have estimated more time before the human population reaches the critical juncture. The United Nations Population Division, in a 2022 report, put this tipping point at 2056, and earlier this year, the Wittgenstein Centre for Demography and Global Human Capital, a multidisciplinary research organization dedicated to studying population dynamics, forecasted 2040.

Christopher Murray, co-author of the new study and director of the University of Washington’s Institute for Health Metrics and Evaluation (IHME), suspects his study’s forecast is conservative. “With each passing year … it’s becoming clearer that fertility is dropping faster than we expect,” he says. Because the 2030 figure is already a hastening of IHME’s previous estimate of 2034, “I would not be surprised at all if things unfold at an even faster rate,” he says.

SIGN UP FOR THE SCIENCEADVISER NEWSLETTER The latest news, commentary, and research, free to your inbox daily A drop below replacement fertility does not mean global population will immediately fall. It will likely take about 30 additional years, or roughly how long it takes for a new generation to start to reproduce, for the global death rate to exceed the birth rate. Even then, because countries’ fertility may vary dramatically, global fertility rate is a “very abstract concept that doesn’t mean much,” says Patrick Gerland, chief of the Population Estimates and Projection Section of the U.N. Population Division. But he says the trend points to a world increasingly split between low-fertility countries, in which a diminishing number of young people support a burgeoning population of seniors; and high-fertility countries, largely poorer sub-Saharan African nations, where continued population growth could hamper development.

Estimating when the world will reach the turning point is challenging. The new model from IHME is based on how many children each population “cohort”—people born in a specific year—will give birth to over their lifetime. It captures changes such as a move to childbirth later in life. But full cohort fertility data are thus far only available for generations of people older than 50, and so the IHME model builds projections within itself to try to capture trends as they are unfolding.

A steady decline Global fertility has been dropping for several decades. Low-income countries in sub-Saharan Africa and high-income countries such as the United States and Japan are expected to dip below the level needed to sustain the human population in the coming decades. But a new model says the global fertility rate could drop below the replacement level as soon as 2030.

D. AN-PHAM/SCIENCE In contrast, the U.N. and Wittgenstein models are based on each country’s total fertility rate, or the sum of age-specific fertility rates, typically for those between the ages of 15 and 49, which is considered reproductive age. As a result, temporary fluctuations in childbearing behaviors—say, people decades ago delaying giving birth to children so they could advance in their education and careers—can throw off their projections, and they can miss longer term changes in childbearing behaviors. These models may have been prone to undercounting fertility in the past, then finding a temporary rebound in fertility rate, and therefore predicting a longer time frame for world population decline.

ADVERTISEMENT This is one reason that Wittgenstein is considering moving to a cohort model, says Anne Goujon, director of the Population and Just Societies Program at the International Institute for Applied Systems Analysis, one of the three institutions that form the Wittgenstein Centre.

Other factors also contribute to the differences between the projections, including how the IHME model accounts for four variables that impact fertility, including access to contraceptives and higher education among those who give birth. (The other two models generally do not, although Wittgenstein considers education.)

Regardless of when the turning point comes, “growing disparity in fertility levels could contribute to widening of [other] disparities,” says Alex Ezeh, a global health professor at Drexel University, who was not involved in the Lancet study. For middle- to high-income, low-fertility countries, falling below replacement level could mean labor shortages and pressure on health care systems, nationalized health insurance, and social security programs. Meanwhile, low-income countries that still have high fertility are at heightened risk of falling further behind on the world’s economic stage, Ezeh says. “They will not be able to make the necessary investments to improve health, well-being, and education” with too few resources to support a booming population.

Although some experts, including Goujon, think there isn’t yet reason for alarm, others call for urgency. “This is going to be a very big challenge for much of the world,” Murray says. “There’s a tendency to dismiss this as sort of like, yeah, we’ll worry about it in the future. But I think it’s becoming more of an issue that has to be tackled sooner rather than later.”