Already straining to host seven billion souls, Earth is set to teem with billions more, and only a revolution in the use of resources can avert an environmental crunch, experts say.

As early as 1798, Thomas Malthus gloomily forecast that our ability to reproduce would quickly outstrip our ability to produce food, leading to mass starvation and a culling of the species.

But an industrial revolution and its impact on agriculture proved Malthus and later doomsayers wrong, even as our numbers doubled and redoubled with accelerating frequency.

"Despite alarmist predictions, historical increases in population have not been economically catastrophic," notes David Bloom, a professor in the Department of Global Health and Population at Harvard.

Today, though, it seems reasonable to ask if Malthus wasn't simply a couple of centuries ahead of the curve.

On October 31, the world's population is officially scheduled to hit seven billion -- a rise of two billion in less than a quarter century.

Over six decades, the global fertility rate has roughly halved, and amounts to a statistical 2.5 children per woman today.

But this varies greatly from country to country. And whether the planet's population eventually stabilises at nine, 10 or 15 billion depends on what happens in developing countries, mostly in Africa, with the fastest growth.

Diminishing resources

As our species has expanded, so has its devouring of the planet's bounty, from fresh water and soil richness to forests and fisheries.

At its current pace, humankind will need, by 2030, a second planet to satisfy its appetites and absorb its waste, the Global Footprint Network (GFN) calculated last month.

And through the coal, oil and gas that drive prosperity, we are also emitting greenhouse gases that alter the climate, potentially maiming the ecosystems which feed us.

"From soaring food prices to the crippling effects of climate change, our economies are now confronting the reality of years of spending beyond our means," GFN's president, Mathis Wackernagel, said.

French diplomat Brice Lalonde, one of two coordinators for next June's UN Conference on Sustainable Development, dubbed "Rio+20," said Earth's population rise poses a fundamental challenge to how we use resources.

"In 2030 there will be at least another billion people on the planet," Lalonde said.

"The question is, how do we boost food security and provide essential services to the billion poorest people but without using more water, land or energy?"

This is why, he said, Rio+20 will focus on practical things such as increasing cleaner sources in the world energy mix, smarter use of fresh water, building cities that are environmentally friendlier and raising farm yields without dousing the soil with chemicals.

But such options dwell far more on the impact of population growth than on the problem itself.

Braking fertility rates would help the human tally stabilise at eight billion and haul poor countries out of poverty, ease the strain on natural resources and reduce climate vulnerability, say advocates.

For some experts, voluntary birth control is the key.

Geoff Dabelko, director of the Environmental Change and Security Programme at the Woodrow Wilson Center in Washington, cites Somalia as a case study of what happens when women have no access to contraception.

Racked by civil war and poverty, its population is projected to grow from about 10 million today to 22.6 million by 2050. It has the eighth-highest birth rate in the world and an average of seven children per family.

Even before the country fell into a full-fledged crisis, a third of its children were severely underweight, according to UNICEF. Ninety-nine percent of married Somali women have no access to family planning.

Many economists, though, argue that the answer lies more in reducing poverty and boosting education, especially of women.

A 2010 study in Colombia found family planning explained less than 10 percent of the country's fertility fall. The real driver was improved standards of living.

Don't mention the 'P' word

Even so, at summits that seek to shape Earth's future, tackling population growth head-on is almost taboo.

"When I attended the UN environment conference in Stockholm (in 1972), the No. 1 item on the agenda was out-of-control population growth," recalled Paul Watson, head of Sea Shepherd Conservation Society, a radical green group.

"When I attended the 1992 conference (in Rio), it wasn't even on the agenda. No one talked about it any more."

Demography was similarly absent from the UN's 2002 Johannesburg Summit, when Earth's population had climbed to six billion.

Why does "how many is too much" remain absent from the top tables?

One perceived reason is the opposition by religious conservatives to contraception or abortion. Politicians, too, may see no mileage in addressing an issue that will only cause them headaches and yield benefits several decades away.

But for some critics, population measures are synonymous with the mistakes of coercive sterilisation in India in the 1970s or China's "one child" policy, which has led to a gender imbalance in favour of boys.