sn-fertilityHThe planet’s large, growing, and over-consuming human population, especially the increasing affluent component, is rapidly eroding many of the Earth’s natural ecosystems. However, society’s only real policy lever to reduce the human population humanely is to encourage lower per capita fertility. How long might fertility reduction take to make a meaningful impact? We examined various scenarios for global human population change to the year 2100 by adjusting fertility and mortality rates (both chronic and short-term interventions) to determine the plausible range of outcomes. Even one-child policies imposed worldwide and catastrophic mortality events would still likely result in 5–10 billion people by 2100. Because of this demographic momentum, there are no easy ways to change the broad trends of human population size this century.

An asteroid impact that wiped out hundreds of millions of people would barely slow down human population growth. That’s one of the surprising results of a new computer model, which still finds that there may be a couple of things we can do to keep our numbers in check.

Every dozen years or so, we add another billion people to the planet. If the trend continued, we’d eventually run out of food and water, and we’d be unable to handle the massive amounts of waste and pollution we produce. Yet we know that population growth is already levelling-off due to a combination of family planning programs and education for women. Is it possible to slow population growth even more in the next few decades? Corey Bradshaw decided to find out.

Bradshaw, a population biologist at the University of Adelaide in Australia, studies population ecology in animals. But when he gives talks at scientific meetings on declining biodiversity, audience members increasingly ask, “What about the elephant in the room? What about human population size?” he says. “I’ve modelled changing populations in other species for years,” he says, “but I never applied [those models] to human beings.” 

corey.bradshawSo Bradshaw and University of Adelaide climate biologist Barry Brook decided to see how much momentum the human population has. They also wanted to see how sensitive population growth is to factors like mortality and fertility. The duo obtained data on death rates, average family size (i.e., fertility), and regional population size from the World Health Organization and the U.S. Census Bureau International Data Base. They created a computer model that projects human population growth from 2013 to 2100. They added variables to the model that they could modify to create different scenarios. Their goal was to assess how sensitive human population growth is to changes in mortality, life span, family size, and a mother’s age when she has her first baby.

The team then created 10 scenarios, including a “business-as-usual” scenario in which death and fertility rates stayed the same as they were in 2013. The other scenarios projected the effects of alterations such as longer life spans, mothers having their first children at older ages, the imposition of a global one-child policy, and catastrophic deaths due to war or pandemics. Using the regional data, the researchers also examined the effects of population growth on biodiversity hotspots in different parts of the world.

The business-as-usual model matched U.N. projections of 12 billion people by 2100, giving the researchers confidence in their model. But they also saw booming population growth even when they introduced global catastrophic deaths of up to 5% of the population, the same seen in World War I, World War II, and the Spanish flu. When the computer model population lost half a billion people, the total population was still 9.9 to 10.4 billion people by 2100, the team reports online today in Proceedings of the National Academy of Sciences. “It actually had very little effect on the trajectory of the human population,” Bradshaw says.

Some economists argue that shrinking populations create an unsupportable burden of elderly dependents that leads to economic collapse. But the team’s model showed otherwise. When the population is growing, more of the dependents are children, and when the population is shrinking, more are older adults, the model indicates. A dependent is always supported by 1.5 to two workers. The idea that shrinking populations cannot support older adults is a “fallacy,” Bradshaw says.

Two factors that did have an impact

Two factors did have an impact on human population growth: eliminating unwanted pregnancies, which make up about 16% of all live births, and adopting a global one-child policy. Eliminating those births year after year resulted in population sizes in 2050 and 2100 that are comparable to those produced with a global one-child policy—about 8 billion and 7 billion, respectively.

The models also confirmed that the worst human impacts on biodiversity hotspots will occur in Southeast Asia and Africa, which by 2100 will likely have the highest human densities in the world. Pressures in those parts of the world, Bradshaw says, will be higher than anywhere else in the world. Elephants, rhinos, and lions will likely disappear faster. “So, will my 7-year-old daughter ever see an elephant in Africa unless I get her there very quickly?” Bradshaw says. “I don’t know.”

Bradshaw and Brook are not advocating any particular policy. But they make the case that population momentum in humans is strong, says Richard Cincotta, a political demographer at the Stimson Center, a Washington, D.C., think tank. The study’s results aren’t surprising, Cincotta says: Demographers have known this for years—they just don’t communicate it very well.

Human population reduction is not a quick fix for environmental problems: