Man's catastrophic damage to the environment and disparities between rich and poor head the daunting challenges facing the Rio Summit in June, experts said on Monday.
The summit must sweep away a system that lets reckless growth destroy the planet's health yet fails to help billions in need, they said.
"This century is special in the Earth's history. It is the first when one species — ours — has the planet's future in its hands," said Martin Rees of the Royal Society, Britain's academy of sciences.
"We've invented a new geological era: the Anthropocene," he said referring to an epoch shaped by Man, not nature.
The four-day London meeting gathers 2,800 scientists, economists, business executives and policymakers in the goal of issuing a snapshot of the planet's health ahead of Rio.
The June 20-22 UN conference is the 20-year followup to the famous Earth Summit.
That year, political leaders declared they would nail sustainable development to their agenda and set up two UN institutions for tackling global warming and species loss.
But many experts on Monday painted a grim tableau of threat and called on governments to ditch strategies based obsessively on GDP growth.
The drivers of the peril are a world population set to balloon from seven billion today to nine billion by mid-century — but also voracious, inefficient and damaging consumption of resources, they said.
Will Steffen, head of the Climate Change Institute at the Australian National University, said the Anthropocene was pushing several of Earth's ecological systems towards "tipping points."
Within a few decades, these vital buffers could suffer lasting or irreversible damage through man-made warming, he said.
Worry spots include the Greenland icesheet, the Amazonian forest as well as Siberia, where billions of tons of natural greenhouse gas could be freed from melting permafrost. The Arctic Ocean would probably become ice-free "this century," he said.
"Under a worst-case scenario, it's very likely, I think, that the Earth's system will move to a new state of some sort, with a very severe challenge to contemporary civilisation," said Steffen. "Some people have even talked about a collapse."
Twenty leading figures and organisations, all of them past winners of the prestigious Blue Planet Prize for environmental work, called for Rio to look at problems through fresh eyes.
The UN's goal of limiting global warming to 3.6 degrees Fahrenheit is already out of reach, said Bob Watson, former head of the UN's climate panel and chief advisor to Britain's environment ministry, as he presented the laureates' study.
"If you look at the commitments today from governments around the world, we've only got a 50-50 shot at a 5.4 F world, almost no chance of a 3.6 F world, and to be quite honest I would say it's not unlikely that we will hit a 9.0 F world," said Watson.
"That is clearly a world with significant adverse consequences for ecological systems, for socio-economic systems and for human health."
He added: "We have to realise that we are looking at a loss of biodiversity that is unprecedented in the last 65 million years... We are clearly entering the (planet's) sixth mass extinction."
"The challenges we face today are exactly the same challenge of Rio 20 years ago," Watson said bluntly.
"We just have not acted. The need for action is becoming more and more urgent with every day that passes."
Diana Liverman, a professor at the University of Arizona, said the news was not entirely bleak.
Since 1950, "we have have seen a great acceleration in human impacts but there are some signs that some drivers are slowing or changing," she said.
Liverman pointed to a slowdown and eventual stabilisation in population growth, gains in energy efficiency and reforestation in some countries.
But, she added, "many people still struggle to meet basic needs. Many of them see a profoundly unequal world. And many still aspire to increase their consumption."