Action is urgently needed to limit population growth - and rich nations must curtail gluttonous consumption - or else humanity faces a terrifying future, scientists have warned.
The warning goes against what many governments and academics have espoused for centuries in the industrialised world. Yet more than 100 scientific academies around the globe have broken the taboo, saying if birthrates and overconsumption continue unabated, large-scale famine, drought, disease and war lie ahead.
Scientists say proposals at the Rio+20 Earth Summit on sustainable development barely mention the twin solutions of population and consumption control, which would ease pressure on fast-diminishing resources.
"For too long the dual issues of population and consumption have been left off the table due to political and ethical sensitivities," said Professor Mohamed Hassan, a co-chair of InterAcademy Panel (IAP), a global network of 105 science academies.
Warning that irreversible damage was being inflicted on the planet, the scientists called on world leaders in Rio de Janeiro to implement "urgent and coordinated international action" to rein in the number of human beings on the planet, and for high-income nations to restrain their voracious appetites.
Current patterns of consumption are wiping out the planet's biodiversity. With the population now at more than seven billion, human beings have cleared 43 per cent of Earth's surface for urban development or agriculture, scientists say. By 2025, the usage level is expected to exceed 50 per cent, when the population reaches eight billion. Ecological studies of other species show that once the 50 per cent threshold is surpassed, populations quickly begin to decline.
"If current patterns of production and consumption of natural resources prevail and cannot be reversed … then governments will preside over unprecedented levels of damage and degradation," said UNEP Executive Director Achim Steiner.
With the population reaching an estimated 9.3 billion by 2050, crucial biological mechanisms - such as forests providing clean air and water, insects pollinating crops, and untouched landscapes holding cures for disease - would disappear forever, scientists say.
An interdisciplinary group of 22 scientists also recently highlighted the need for reduced fertility rates and consumption levels. They warned of an upheaval on Earth not seen since the dinosaur extinctions 65 million years ago.
If population growth rates remain what they were between 2005 and 2010, 27 billion people will inhabit the planet by the end of the century, said Anthony Barnosky from the University of California at Berkeley. That would lead to the disappearance of most large and small animals, and the collapse of food chains.
But many scientists, politicians, and religious groups are opposed to any talk of "limiting growth", whether referring to humans themselves or the ways we consume.
"The theory that human well-being is inversely proportional to human numbers is completely contrary to all historical data," Robert Zubrin, a senior fellow with the Center for Security Policy, told Al Jazeera.
Opponents of population control policy say it could lead to forced abortions and sterilisations, as well as one-child-only measures such as those imposed in China.
"The idea of 'overpopulation' is not science at all, but a pseudo-scientific cover for racism and genocide," Zubrin said. "If there are 11 billion people in 2030, the world will be much richer as a result."
Professor Charles Godfray with the InterAcademy Panel said "coercive" methods of population control were not being advocated. Providing universal access to reproductive health and family planning programmes, and investing in education - particularly of girls - was the best way to reduce high birthrates, he said.
"Our statement is focused on the need to stabilise the global population on a voluntary basis with full recognition of human rights," Godfray said.
"If every person used as many resources as the average North American, more than four Earths would be required to sustain the total rate of consumption, depletion and waste assimilation."
Making free contraception available to women in the developing world who want it could prevent more than 20 million births annually, research suggests.
For some scientists, the threat of overconsumption far outweighs overpopulation. Professor Paul Ehrlich, author of the 1968 best-seller The Population Bomb, said burning through non-renewable resources must stop.
"If we don't get off the fossil fuel standard, it hardly matters how many people there are," Ehrlich told Stanford News Service.
Even though about 80 per cent of humans live in the developing world, people in rich areas have a far greater per capita ecological impact.
If every person used as many resources as the average North American, more than four Earths would be required to sustain the total rate of consumption, depletion and waste assimilation, according to an environmental "accounting system" developed by researchers William Rees and Mathis Wackernagel.
While the warnings are dire, some scientists are optimistic that human beings can turn things around through research, technology, and widespread human recognition that there are limits to growth.
"That's my hope for the future, that we realise where we are, that we're standing at the crossroads, and that we mobilise and do the right thing," said Barnosky.
But Professor Mikael Fortelius of the University of Helsinki noted that overpopulation and its potential devastating results have long been ignored.
"This is what scientists saw in the 1960s and 1970s. We've never been quite sure when it would happen. We're there now," said Fortelius. "I'm not personally particularly optimistic."