New forecast: The Earth could have 11 billion people by 2100

There are currently 7.2 billion people on the planet. We may need to make room for 11 billion.

The world's population is unlikely to peak this century, as once thought

A new study in Science argues that the world's population is likely to grow to nearly 11 billion by 2100 if current trends hold. What's more, the global population now seems unlikely to stabilize this century.

This in contrast to previous forecasts, which had suggested that the world's population was on track to peak at around 9 billion people by 2050 and then start falling. One big reason for the change? Birth rates in Africa are falling more slowly than expected.

The Science analysis was led by researchers at the United Nations and University of Washington, and it helps clarify the latest UN population forecasts. The authors calculate that there's now an 80 percent chance the global population will be between 9.6 billion and 12.3 billion by 2100:

UN

UN 2012 world population projection (solid red line), with 80% prediction interval (dark shaded area), 95% prediction interval (light shaded area), and the traditional UN high and low variants (dashed blue lines). (Gerland et al 2014)

Here's a second graph showing UN population projections for each continent. Asia's population is expected to peak by mid-century, but Africa is now expected to keep growing:

(Gerland et al 2014)

These are probabilistic forecasts, so there are lots of possible outcomes here. But by using new statistical techniques, the authors of the Science paper were able to better assess the likelihoods of different outcomes. They argue that there's now a 70 percent chance the world's population will keep growing this century — without peaking.

Birth rates in Africa haven't fallen as quickly as expected

Why have these forecasts changed over the past few years? The researchers note that UN population projections are based on two big things — life expectancy and fertility rates. If either of those change, the projections shift.

In particular, the researchers note that birth rates in sub-Saharan Africa haven't fallen as quickly as previously thought. And deaths from HIV/AIDS have gone down significantly. Currently, Africa has 1.1 billion people. By the end of the century, the researchers think there's an 80 percent chance that the continent will have between 3.5 and 5.1 billion people.

That said, not everyone agrees with this assessment of Africa. A separate paper recently published in Population and Development Review argues that African fertility rates are likely to decline more sharply in the future as education rates rise, particularly among women. Indeed, researchers generally that increased access to contraception and education among women tend to reduce birth rates over time. So that's a key variable here.

What's more, these projections also don't take into account whether the Earth has the resources to support 9 billion or 10 billion or 11 billion people. The authors of the Science paper concede that fast population growth in Africa could end up affecting food availability or migration patterns — all of which could, in turn, alter population trends.

The overall number of people will matter a lot. Food researchers, for instance, have pointed out that it will be an enormous challenge to feed 9 billion without destroying the planet. But that task becomes even tougher if it turns out we'll have 11 billion or 12 billion people.

Why population forecasts are so difficult

What's new about the Science paper is that it provides a more detailed statistical assessment of the different population analyses that are currently out there — allowing the researchers to provide more precise confidence intervals. This is in contrast to the much cruder scenarios of old.

Demographic projections often turn out to be wrong

But even with these new techniques, there are still plenty of caveats: Demographic forecasts are notoriously difficult — and very often turn out to be wrong. Note that we can't even be sure of how many people are currently on the planet, particularly in places like China, where families are believed to be hiding tens of millions of babies.

And making predictions about future fertility rates is even tougher, as small assumptions about things like education levels can make big differences. Analysts at Deutsche Bank, for instance, have previously argued the world's population will peak in mid-century at 8.7 billion people and then decline thereafter — because they expect fertility rates in Africa to fall faster than the UN does.

The latest Science study is an attempt to sharpen the UN's current forecasts and try to quantify all that uncertainty more precisely. But surprises are always possible, and these numbers are quite likely to change in the future.

Further reading: How to feed 3 billion extra people — without trashing the planet

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