But the potential impact on climate change of a planet teaming with up to ten billion souls has again forced the issue into the open ahead of the December 7-18 UN climate conference in Copenhagen.

In a sign of change, the UN Population Fund (UNFPA) has declared that braking the rise in Earth's population would be a major contribution to fighting greenhouse gases.

"Slower population growth... would help build social resilience to climate change's impacts and would contribute to a reduction of greenhouse-gas emissions in the future," the agency said in report in November.

If, by 2050, Earth's population stood at eight billion rather than nine billion, that would save between one and two gigatonnes of carbon per year, buying precious time for cleaner technology and other policies, its report said.

That figure is comparable to savings in emissions by 2050 if all new buildings were constructed to the highest energy-efficiency standards and if two million one-gigawatt wind turbines were built to replace today's coal-fired power plants.

The 104-page document is the first by a UN agency to address the climate-population link so explicitly.

It highlights the option of a gentle, voluntary decline in population expansion, thanks to access to contraception and empowerment of women.

In a slew of papers published in September by Britain's Royal Society, University of California scientist Malcolm Potts pointed at the impact of an expected population rise in the United States, from 300 million today to between 450 million and 500 million in 2050.

"Every unintended birth prevented in the US will permit the rest of the world to breathe a little easier," said Potts.

The United States is not only the world's No. 2 polluter in volume terms, it is the seventh highest in per-capita terms: around 23 tonnes of carbon per head per year, or roughly 10 times that of the world's poorest countries.

In the emerging giant economies, a challenging picture emerges, say experts.

China's one-child policy, for instance, has purportedly averted 300 million births. But it also provided a huge spur to the economy, which in turn generated greenhouse-gas emissions, helping to make China the world's No. 1 carbon culprit.

In the world's poorest countries, where 99 percent of the growth of the world's population will occur over the next four decades, reduced fertility would be a boon for adaptation.

It would mean fewer demands on the environment and fewer people exposed to water stress, floods, poor harvests, bad storms and loss of their homes.

"How Niger is going to feed a population growing from 11 million today to 50 million in 2050 in a semi-arid country which may be facing climate change is unclear," Lord Adair Turner, a British businessman and academic, observed crisply.

Some experts say the population debate has been sidelined thanks to natalists in rich countries, who argue vociferously that growing populations are an asset.

These voices also maintain that plunging fertility is a risk, exposing a country to a pensions time bomb as a smaller workforce struggles to fund a growing population of greyheads.

Yet population policies also carry the taint of Thomas Malthus, whose dire 18th predictions of hunger, disease and death through overpopulation fell apart when mechanised farming and the "Green Revolution" fed the swelling billions.

More recently, cases of forced sterilisation in India in the 1970s and controversy over China's one-child policy helped to make population a no-go area for some.

"Very few people can even discuss" population curbs, said Bob Engelman, vice president for programmes at the Worldwatch Institute, who lead-authored the UNFPA report.

"They don't believe it arrives at a reduction of population growth through humane, rights-based methods -- through men's and women's intentions to have fewer babies," he told AFP.

In the UN climate talks, 37 developing countries have already included population issues in national plans for addressing global warming, while the European Union (EU) has suggested factoring in "population trends" in efforts to tackle emissions.

Will population policies one day become a climate-change bargaining chip?

That possibility seems remote, although Engelman believes that eventually the world cannot escape the core issue of carbon justice -- how many tonnes of CO2 should be emitted per head of population.

"In Copenhagen, most people will be saying, 'please, it's going to be hard enough to do anything at all right now, the last thing you want to do is bring up an issue as controversial as population'," he said.

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