Even a Pandemic Wouldn't Create a 'Sustainable' Population, Study Says

Overcrowded trains prepare to leave Dhaka, Bangladesh, on January 26, 2014. Andrew Biraj / Reuters

Environmental scientists generally agree that the growth rate of the world's human population and its current rate of consumption are unsustainable. For that reason, many researchers and policy-makers have called for family planning and birth control to slow growth in various countries.

A good example is China's one-child policy. Beginning in 1979, families in China were largely limited to a single child. That policy was extremely controversial and helped lead to a gender imbalance in Chinese society, but it also helped avert 400 million births.

But a new study, by Corey Bradshaw and Barry Brook, published today in the Proceedings of the National Academy of Sciences, suggests that reducing the Earth's human population won't be a "quick fix" for environmental problems. Why not? Because there are already so many people that even drastic actions would take many generations to have a significant impact, the researchers wrote.

In the study, the scientists examined several hypothetical scenarios, such as declining fertility and mass casualties, to see what impact these would have on subsequent population growth rates.

"Even one-child policies imposed worldwide and catastrophic mortality events would still likely result in [5 billion] to 10 billion people by 2100," the scientists, both from Australia's University of Adelaide, wrote in the paper.

For comparison, there are currently more than 7 billion people on Earth (14 percent of all humans who have ever existed are alive today), and median estimates put the population at 11 billion by 2100.

In another scenario, the researchers tested what would happen if 2 billion people died over the course of a five-year period in the mid-21st century, for example by a war or pandemic. They calculated the world's population would still grow to 8.5 billion by 2100.

The paper doesn't suggest that efforts to have fewer children shouldn't be pursued. On the contrary, the researchers wrote that reducing fertility rates (defined as the number of offspring the average woman has over her lifetime) to 2 from the current rate of 2.37 by 2020 would lead to 777 million fewer "people to feed planet-wide by 2050," for example. But they emphasized that worldwide population would take a long time to stabilize.

Several other studies suggest that a world population of between 1 billion and 2 billion "might ensure that all individuals [live] prosperous lives, assuming limited change in per capita consumption and land/materials use." If humans reduced fertility rates to one child per woman on average by 2100, there could be as few as 2 billion people by 2153, they calculated.

"Our great-great-great-great grandchildren might ultimately benefit from [family] planning, but people alive today will not," said Brook in a statement.

But before that time, climate change and biodiversity loss are likely to cause "unacceptable" losses to the environment, humans and the Earth's many ecosystems, they wrote.

"The corollary of these findings is that society's efforts towards sustainability would be directed more productively towards reducing our impact as much as possible through technological and social innovation," as opposed to focusing solely on population, said Bradshaw. A primary end goal for these innovations would be to produce energy without greenhouse gases (by embracing renewable energy) and to quit destroying forests and other ecosystems, they added.