The world's surging population is a big driver of environmental woes but the issue is complex and solutions are few, experts at a major conference here say.
Answers lie with educating women in poorer countries and widening access to contraception but also with reforming consumption patterns in rich economies, they say.
The four-day meeting on Earth's health, Planet Under Pressure, is unfolding ahead of the Rio+20 Summit in June.
Scientists taking part have pinpointed population growth as a major if indirect contributor to global warming, depletion of resources, pollution and species loss.
But they also mark it as an issue that has disappeared almost completely off political radar screens.
This is partly because of religious sensitivities but also because of traumatic memories of coercive fertility controls in poorer countries in the 1970s that no-one wants to repeat.
Diana Liverman, a professor at the University of Arizona, said the link between population growth and environmental damage arose in the mid-20th century.
"The 50 years from 1950 to 2000 were a period of dramatic and unprecedented change in human history," she said.
During that time, the planet's human tally doubled from three billion to six billion. It now stands at seven billion, and by some estimates could reach around nine billion by 2050.
The good news is that the fertility rate -- the number of children a women is likely to have -- has halved from five to 2.5 since 1950 and will fall below the replacement rate of 2.1 around 2025, Liverman said.
"It means that there is a strong probability that population growth will level off around nine billion and may in fact fall thereafter," said Liverman.
Others caution that raw statistics mask many complexities.
"The world's carrying capacity isn't a single headline figure but depends on lifestyle, technology, and so forth," said Lord Martin Rees of the Royal Society, whose report on demography and the environment will be issued next month.
The population is stabilising or falling in rich countries.
But these economies remain -- in per capita terms -- by far the biggest sources of environmental damage, with for instance greenhouse gas emissions per head that are double or quadruple those in a developing country.
The big population growth will happen in developing countries, especially in sub-Saharan Africa.
These countries bear least responsibility for climate change but will be hit worst by it, because they lack money and skills to adapt. Thus the higher their population, the more of their people who will be hit by drought, storms, rising seas and floods.
Strategies for working on the demographic drivers of environmental damage are essentially two-pronged, said specialists.
One is to change consumption patterns, so that the rich countries -- and the emerging giants rushing to catch up with them -- use energy and resources more sustainably.
The other is to protect women's rights, education for women and their access to jobs and contraception.
"If you have economic development and you educate women, and women get labour market opportunities, they tend not only to reduce the number of children but crucially to delay when they start having children," said Sarah Harper, director of the Institute of Population Ageing at the University of Oxford.
"And if you delay the start of having children, you tend to have smaller families."
Such changes can have a "surprisingly fast" effect on reducing birthrates, said Stephen Tyler, who works with group called the Asian Cities Climate Change Resilience Network (ACCCRN). He gave the fast-shrinking families in India as an example.
On Sunday, a group of scientists and policymakers that have won the Blue Planet Prize, a top environmental award, made a pre-conference appeal to intensify green action.
Looking at demography, they said more than 200 million women in developing countries still have unmet needs for family planning.
But funding for access to contraception fell by 30 percent between 1995 and 2008, "not least as a result of legislative pressure from the religious right in the USA and elsewhere," they said.
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