Image copyright Getty Images Image caption Nargis, born in 2011, was named the world's seven billionth person by Plan International

Restricting population growth will not solve global issues of sustainability in the short term, new research says.

A worldwide one-child policy would mean the number of people in 2100 remained around current levels, according to a study published in the Proceedings of the National Academy of Sciences.

Even a catastrophic event that killed billions of people would have little effect on the overall impact, it said.

There may be 12 billion humans on Earth by 2100, latest projections suggest.

Concerns about the impact of people on the planet's resources have been growing, especially if the population continues to increase.

'Can't stop it'

The authors of this new study said roughly 14% of all the people who ever existed were alive today.

These growing numbers mean a greater impact on the environment than ever, with worries about the conversion of forests for agriculture, the rise of urbanisation, the pressure on species, pollution, and climate change.

Even if we had a third world war in the middle of this century, you would barely make a dent in the trajectory over the next 100 years Prof Corey Bradshaw, University of Adelaide

The picture is complicated by the fact that while the overall figures have been growing, the world's per-capita fertility has been declining for several decades.

The impact on the environment has increased substantially, however, because of rising affluence and consumption rates.

Many experts have argued the best way of tackling this impact is to facilitate a rapid transition to much lower fertility rates.

To work out the impact on population, the team constructed nine different scenarios for population change up to the year 2100, using data from the World Health Organization, and the US Census Bureau's international database.

They also used "catastrophe scenarios" to simulate the impacts of climate disruption, wars or global pandemics on population trends.

According to the study, attempts to curb our population as a short-term fix will not work.

If China's much criticised one-child policy was implemented worldwide, the Earth's population in 2100 would still be between five and 10 billion, it says.

Image copyright Getty Images Image caption Rising population impacts the environment through deforestation and pressure on natural resources

"We've gone past the point where we can do it easily, just by the sheer magnitude of the population, what we call the demographic momentum. We just can't stop it fast enough," said Prof Corey Bradshaw from the University of Adelaide.

"Even draconian measures for fertility control still won't arrest that growth rate - we're talking century-scale reductions rather than decadal scale, because of the magnitude."

In their paper, the researchers also look at the impact on numbers of a global catastrophe in the middle of this century. They found that even an event that wiped out two billion people would still leave about eight and a half billion in 2100.

"Even if we had a third world war in the middle of this century, you would barely make a dent in the trajectory over the next 100 years," said Prof Bradshaw, something he described as "sobering".

'Difficult to tackle'

The scientists said the issue of population and its impact on global consumption was often described as the "elephant in the room" - a problem that the world ignores as it is politically and ethically difficult to tackle.

But the research shows that curbing numbers will not deal with environmental challenges in the short term.

"Our work reveals that effective family planning and reproduction education worldwide have great potential to constrain the size of the human population and alleviate pressure on resource availability over the longer term," said Prof Barry Brook from the University of Tasmania.

"Our great-great-great-great grandchildren might ultimately benefit from such planning, but people alive today will not."

As a result of this long-term impact, the world should focus on curbing consumption and designing ways to conserve species and ecosystems.

"Society's efforts towards sustainability would be directed more productively towards reducing our impact as much as possible through technological and social innovation," says Prof Bradshaw.

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