For decades, debate over whether to limit global population growth was stifled or ignored, branded as immoral and a return to heartless Malthusian logic.

But the potential impact on climate change of a planet teaming with up to 10 billion souls has again forced the issue into the open ahead of the UN climate conference in Copenhagen in December.

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.