Earth Population has been a staple of dystopian fiction for decades, with stories predicting an unmitigated spread of humanity pushing Earth’s resources to breaking point. A fresh look at the numbers paints a very different scenario.

A team of researchers estimate by the 2060s there’ll be maybe another two billion people on Earth. Just a few decades later, numbers will drop as fertility rates decline and nations like Japan and Italy lose as much as half of their population.

Just how this overall decline will impact society and the planet is hard to say.

We might assume fewer mouths to feed and fewer bodies to house would be less taxing on the environment. But the reality of a shrinking population may be a bleak one.

“While population decline is potentially good news for reducing carbon emissions and stress on food systems, with more old people and fewer young people, economic challenges will arise as societies struggle to grow with fewer workers and taxpayers, and countries’ abilities to generate the wealth needed to fund social support and health care for the elderly are reduced”, says first author of the new study Stein Emil Vollset, a biostatistician from the Institute for Health Metrics and Evaluation (IHME) in Washington.

In the new study, Stein and a team from IHME and the University of Washington’s School of Medicine used data collected as part of the 2017 Global Burden of Disease Study to assess population growth around the world.

Their results suggest that we expect today’s figure of 7.8 billion to rise until sometime around 2064, where it will level off at 9.7 billion. Then the pool will shrink, bringing us back to 8.8 billion by the century’s end.

Such figures are in significant contrast to other estimates made in recent years, some venturing numbers as high as 12.3 billion by 2100 with no plateau in sight. So who should we believe?

Knowing which predictions to bank on depends on the mathematical tools you have the most confidence in.

Forecasting the size of a population across any one region relies on having a model that can accurately take into account the diverse constraints and incentives affecting the number of children people produce in a lifetime.

Past attempts have relied on what’s known as a total fertility rate, which is simply the total number of children born to an individual in a lifetime. An alternative is to use completed cohort fertility – an average of the number of children born to a group of women by a given age. In this case, by age 50.

This study uses the later, and also takes a slightly different approach in determining migration patterns that affect the trajectory of population growth for any one area.

“By making explicit the pathways through which fertility, mortality, and migration patterns can change, our model is able to identify where future time trends might be different from past trends,” the team writes in their paper.

The results provide more than just an updated number for humans living on Earth in coming decades. The researchers describe a fluctuating map of population densities and critical changes in the makeup of citizenship in various countries.

Today, sub-Saharan African nations average around 4.7 births per woman, a figure set to drop below replacement rates by 2100. Niger held the record in 2017, for example, with an astonishing seven births per mother. That figure should drop to 1.8 by 2100, according to this newest model.

In other places numbers are set to plummet unless something drastic changes. Japan’s population could fall from 128 million in 2017 to 60 million by 2100. Even China could dip well below a billion by then, to just over 730 million.

Right now, China is likely to overtake the US in gross domestic product in coming years. But if its working age population drops as predicted, economic growth could easily swing back to give the top spot back to the US by the start of the next century.

That is, of course, if America maintains a working population through immigration and increasing support for reproductive health services.

It’s a prediction that could be more hopeful thinking than confident forecasting.

“For high-income countries with below-replacement fertility rates, the best solutions for sustaining current population levels, economic growth, and geopolitical security are open immigration policies and social policies supportive of families having their desired number of children”, says IHME Director and research leader Christopher Murray.

“However, a very real danger exists that, in the face of declining population, some countries might consider policies that restrict access to reproductive health services, with potentially devastating consequences.”

Such ‘devastating’ consequences of a shrinking population also depend largely on how nations protect worker’s rights and redistribute wealth.

Individuals over the age of 80 will boom in the second half of the century, increasing six fold. Not only will this be a challenge in terms of social support, it will bring into focus resources for hospitalisation and health care.

The study won’t be the final word on population sizes. Models are only as good as the data we have at hand at any one time, and if a global pandemic has taught us anything it’s that population-shaking events are anything but predictable.

Still, it’s a timely warning. Endless growth is still a doomsday scenario as far as our planet’s ecology goes, but a shrinking global population could be just as stark for humanity, at least under current economic regimes.