Only months before make-or-break UN climate talks in Copenhagen, an extraordinary conclave of climate scientists gathered here Tuesday to warn that global warming is accelerating more quickly than forecast by a key UN report for policymakers.

The UN Intergovernmental Panel on Climate Change (IPCC) concluded in early 2007 that global warming, if unchecked, would unleash a devastating amalgam of floods, drought, disease and extreme weather by century's end.

But a welter of new research suggests the impact could be even worse, and will arrive sooner rather than later.

"We need a much stronger sense in our societies of urgency," John Ashton, Britain's top climate negotiator, told journalists as the meeting got under way.

Most worrying, scientists say, is the possibility that human activity -- mainly the burning of oil, gas and coal -- could trigger natural drivers of global warming which, once unleashed, would be nearly impossible to reverse.

The shrinking of the Arctic ice cap, and the release of billions of tonnes of greenhouse gases trapped in melting permafrost are two such "positive feedbacks" that could become both cause and consequence of global warming.

The three-day conference is also likely to unveil a new scientific consensus that sea levels are set to rise at least a metre by 2100, more than double the IPCC estimate, which failed to take melt-off from the Greenland Ice Sheet into account.

"We need to look at what is a 'reasonable worst case' in the lifetime of people alive today," said Ashton, noting even rich nations had yet to take such scenarios seriously.

"A sea level rise of one or two meters would not just be damaging for China, it would be an absolute catastrophe. And what is catastrophic for China is catastrophic for the world," he said.

More than 2,000 researchers from 80 countries responded to the open invitation to present their findings, which were then vetted by a panel of climate experts, many of them top figures in the IPCC.

"I and a lot of scientists see this meeting as an opportunity to update the science that has come out since the last IPCC report," said William Howard, a researcher from the University of Tasmania in Hobart, Australia.

Howard will present evidence showing for the first time that ocean acidification caused by climate change is stripping away the calcium-based shells of tiny organisms, called forams, that play a vital role in absorbing huge amounts of carbon pollution from the atmosphere.

"The huge response from scientists comes from a sense of urgency, but also a sense of frustration," said Katherine Richardson, head of the Danish government's Commission on Climate Change Policy and a co-organiser of the meeting, sponsored by the University of Copenhagen and nine other schools.

"Most of us have been trained as scientists to not get our hands dirty by talking to politicians. But we now realise that what we are dealing with is so complicated and urgent that we have to help to make sure the results are understood," she told AFP.

Richardson said the IPCC report was an invaluable document, but will be five years out of date by the time negotiators convene in December to hammer out a global climate treaty.

"There is a whole lot more knowledge available today," she said. "When you make decisions on what you are going to do about the problem it is important to know what trajectory you are on."

Connie Hedegaard, Denmark's minister for climate and energy, agreed that political decisions should be driven by science.

"As policymakers, we can't ignore what the scientists are telling us, nor can we close our eyes to reality," she told AFP.

The head of the UN panel, Rajendra Pachauri -- who shared the 2007 Nobel Peace prize with Al Gore -- will address the proceedings, along with Danish Prime Minister Anders Fogh Rasmussen, European Commission President Jose Manuel Barroso, and top climate economist Lord Nicholas Stern.