IT’S getting overcrowded here on Earth. More than 7 billion people are taking their toll on the planet, and the number is rising. What would it take to defuse this population time bomb?

Drastic times could, in theory, call for drastic measures. But according to a new study, not even a global one-child policy would make much difference, and neither would a killer plague.

Work by Corey Bradshaw and Barry Brook of the University of Adelaide in Australia suggests that protecting the planet will take more than condoms and microbes. Escalating death rates or fertility crashes could only curb human numbers in the long term, they say, so reducing the human population won’t work as a quick fix for our environmental woes.

The two demographers modelled the impact on the human population of extreme changes to birth and death rates. They found that, even if the world rapidly switched to a China-style one-child policy, numbers wouldn’t shrink much at all – there would still be close to today’s 7.2 billion people at the end of this century.

And a mid-21st century plague wouldn’t have much effect either. Even if 2 billion people died, the models suggest there would still be about 8.5 billion people in 2100.

The analysis is based on United Nations estimates that predict there could be 11 billion people by 2100. These estimates assume continued high birth rates in Africa, with Europe and China tending towards about two children per woman.

The problem is the strong momentum from past high birth rates, which would sustain population size even under Bradshaw and Brook’s extreme scenarios. The global baby boom of the last four decades of the 20th century means there is a large generation of women who will be fertile for decades to come.

“Humanity has now exceeded the planet’s carrying capacity,” says Bradshaw. “The demographic momentum of the current human population certainly precludes massive [population] declines in the next few decades.”

Not everyone thinks the population will continue to increase in this way. Global fertility has halved in a generation to fewer than 2.5 children per woman, thanks to social trends such as education and urbanisation. Some researchers say that this decline should deliver falling human numbers within a few decades, even allowing for demographic momentum.

Wolfgang Lutz of the International Institute for Applied Systems Analysis of Laxenburg, Austria, has argued that the world population could peak at 9.4 billion as soon as 2070, and be back down to 9 billion by 2100, without population control or a great epidemic. He says that improvements to women’s education in particular reduce fertility rates, by overcoming obstacles to using contraception and lowering desired family sizes, and that UN population projections ignore this.

Whether it’s a natural decline, a strict policy or a disastrous plague, none of these population drops would be enough to fix environmental problems like climate change. The sticking point instead lies in our high consumption of natural resources.

Bradshaw and Brook suggest that a sustainable human population, given current Western consumption patterns and technologies, would be between 1 and 2 billion people.

That’s an impossibly small target, but it shows where the real problem is. “Human behaviour is more important than human numbers,” says Lutz. “It’s not just the head count that matters, but what is inside the heads.”