"The scientific definition of that is lacking, and so we are operating within the construct of, again, strong agreement among world leaders that urgent action is warranted," said Jim Connaughton, chairman of the Council of Environmental Quality.

Connaughton, a top White House environmental adviser, was speaking late Friday during a conference call from a meeting of the Intergovernmental Panel on Climate Change in Valencia, Spain.

The Nobel-winning IPCC was to release the fourth and final piece of its assessment report on climate change later Saturday, summarising the first overview on the effect of greenhouse gases since 2001.

"The scientific community has offered a wide range of perspectives in these documents," Connaughton said.

"We are operating consistent with the G8 leaders' consensus that the issue warrants urgent action, and we need to bring forward, in a more accelerated way, the technology that will make a lasting solution possible."

The IPCC report is styled as a guide for politicians facing tough decisions on cutting pollution from fossil fuels, shifting to cleaner energy, bolstering defences against extreme weather, and other problems set to intensify through climate change.

The IPCC was to adopt a 20-page "summary for policymakers" and a 70-page technical document.

UN secretary general Ban Ki-moon warned Saturday that the world was on the verge of a "catastrophe" due to global warming after the draft IPCC report said the evidence of a human role in observed warming was now "unequivocal."

Retreating glaciers and snow loss in alpine regions, thinning Arctic summer sea ice and thawing permafrost show that climate change is already on the march, the draft report said.

By 2100, global average surface temperatures could rise by between 1.1 C (1.98 F) and 6.4 C (11.52 F) compared to 1980-99 levels, while sea levels will rise by between 18 and 59 centimetres (7.2 and 23.2 inches), it forecasted.

Head US delegate in Valencia, Sharon Hays, cited recent American studies by made on the basis of the last IPCC report, in which US researchers stated "very clearly" that "value judgments" still have to be made in determining what the dangers of climate change really are.

"So the science simply can't tell us what that number is," Hays stated. "There are always going to be value judgments associated with it."

"That is a political judgment, as it's been made," added US negotiator on climate issues Harlan Watson. "It's their interpretation."

Watson expressed the view that the conference in Valencia would prompt heads of state and government to seek a new agreement on combating climate change that would replace the Kyoto Protocol due to expire in 2012.

The US delegation also expressed optimism about the prospects of a conference on climate change in Bali, Indonesia, on December 3 to discuss an agenda for future talks on an agreement that would eventually replace Kyoto.

The United States continues to oppose establishing strict legal limits on greenhouse gas emissions.

Instead it is arguing for voluntary measures, new technologies and increased efforts to persuade fast-developing countries like China and India to do their share of regulating greenhouse gases.

"We actually, in the conversations we had, we think there's a broad consensus around these elements," Watson pointed out. "It's just a matter of coming up with the right wording to get it done, and we're very confident it's going to happen."

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