But beneath its newly-won fame, the Intergovernmental Panel on Climate Change (IPCC) is under intensifying scrutiny about some of its key processes.

Some voices, including from within the IPCC itself, fear the panel's grand report will be badly out of date before it is even printed. Others quietly criticise the organisation as being too conservative in its appreciation of the climate threat.

The document to be issued in Valencia next Friday boils down a 2,500-page, three-volume assessment issued earlier this year, the first IPCC review of climate change since 2001.

The upcoming "synthesis report," comprising a summary for policymakers of 25 pages, and a technical document of around 70 pages, puts in a nutshell the evidence for climate change, its likely impacts and the options for tackling it.

The analysis carries huge political weight. It will be a compass for guiding action on climate change for years to come, starting with a crucial UN conference in Bali next month.

But some experts are worried, fearing that the IPCC's ponderous machinery, which gives birth to a new review only every five or six years, is falling dangerously behind with what's happening to Earth's climate systems.

The new report notably fails to take into account a batch of dramatic recent evidence, including the shrinkage of the Arctic ice cap, glacier loss in Greenland, a surge in levels of atmospheric carbon dioxide (CO2) and an apparent slowing of Earth's ability to absorb greenhouse gases, they say.

Taken together, say the sources, these phenomena suggest climate change is be occurring faster than expected -- and may even unleash "tipping points" that could uncontrolably accelerate the damage.

"Over the past several years we have realized ... that the speed at which changes can occur -- such as ice sheet disintegration and resulting sea level rise -- is much faster than IPCC has estimated," leading climatologist James Hansen, who heads the NASA Goddard Institute for Space Studies in New York, told AFP.

"We are now in a situation where the luxury of super-caution and reticence poses a danger for the planet and all its creatures," he said.

French scientist Jean Jouzel, who has been deeply involved in the IPCC report, acknowledges that its estimates about rising sea levels -- which could affect hundreds of millions of people -- are half of what more recent studies indicate.

Another weakness of the IPCC, say others, is a tendency to shy away from controversy.

The more forceful the panel's conclusions, the more pressure it will put on policymakers to adopt measures -- some of them politically costly -- ranging from carbon taxes and mandatory caps on CO2 emissions to massive investment in renewable energy.

Too often, draft text that highlights the possible consequences of warming gets watered down in the final version, say these critics.

British scientist James Lovelock blames the consensus rule that governs IPCC proceedings, enabling government representatives to meddle with "forthright and inconvenient forecasts" made by experts.

"The IPCC has a history and a habit of ignoring many of the big issues that hint at policy or policy analysis," agreed Tom Downing, director of the Oxford Office of the Stockholm Environment Institute and a lead author of the report.

He pointed to the draft report to be hammered out in Valencia, saying it had "barely a page" on vulnerability to climate change and how to cope with it.

The IPCC, jointly launched 20 years ago by the UN Environmental Programme (UNEP) and the World Meteorological Organisation (WMO), shared this year's Nobel Peace Prize with former US vice president Al Gore.

Pep Canadell, executive director of the Global Carbon Project based in Australia, says the panel's caution and rigour had helped create awareness that climate change was a genuine and pressing issue.

But it was time for the IPCC to move to a faster and more assertive track, Canadell suggested.

"We are no longer in the business of convincing governments that the problem is real," he told AFP. "The issue now is what to do and how fast it needs to be done."

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