A leaden cloak of responsibility lies on the shoulders of UN scientists as they put the final touches to the first volume of a massive report that will give the world the most detailed picture yet of climate change.

Due to be unveiled in Stockholm on September 27, the document will be scrutinised word by word by green groups, fossil-fuel lobbies and governments to see if it will yank climate change out of prolonged political limbo.

The report will kick off the fifth assessment by the Intergovernmental Panel on Climate Change (IPCC), an expert body set up in 1988 to provide neutral advice on global warming and its impacts.

Six years ago, the IPCC's fourth assessment report unleashed a megawatt jolt of awareness. It declared that the planet was warming, that this was already starting to affect Earth's climate system and biosphere, and that there was overwhelming evidence that humans, especially by burning coal, gas and oil, were the cause.

It earned the IPCC a share in the Nobel Peace Prize with former US vice president Al Gore and stoked momentum that led to the 2009 climate conference in Copenhagen, the biggest summit in UN history.

Yet that was the high point. The near-fiasco of Copenhagen combined with a financial crisis that struck Western economies... and climate change vanished off politicians' radars. Then came damage to the IPCC's own reputation, when several errors were found in the landmark report, prompting a fightback by gleeful climate sceptics and a painful investigation of the panel itself.

A draft of the leviathan new work, seen by AFP, will amplify the 2007 warning in several ways.

The panel will declare it is even more confident that global warming is man-made and starting to affect extreme weather events, such as flooding, drought, heatwaves and wildfires. It also warns of a potential rise in sea levels that, by century's end, would drown many coastal cities in their current state of preparedness.

"Changes are projected to occur in all regions of the globe, and include changes in land and ocean, in the water cycle, in the cryosphere, in sea level, in some extreme events and in ocean acidification. Many of these changes would persist for centuries. Limiting climate change would require substantial and sustained reductions of CO2 [carbon dioxide] emissions," warns the draft.

The document, focussing on the science of climate change, will be followed next year by two volumes, on impacts and on how to tackle the problem, followed by a synthesis of all three texts.

The main text is written and approved by scientists, and cannot be modified by national governments, who also have representatives on the IPCC.

The governments do have a say, though, in the all-important summary for policymakers, which in its present form runs to 31 pages. So far, they have raised 1,800 reservations about the summary, and these will be hammered out in a line-by-line appraisal over four days before next month's release.

Defenders of the laborious system say approval by governments amounts to a "buy-in" from all the world's nations -- a consensus ranging from huge carbon polluters China and the United States and vulnerable small-island states such as the Maldives to major oil and gas exporters like Qatar and Saudi Arabia.

"I am greatly in favour of this process of comments followed by adoption," Jean Jouzel, a leading French climate scientist who is vice president of the IPCC group in charge of the upcoming volume, told AFP. "The adoption is what gives the IPCC report its success and visibility, and enables its effective use by governments."

Others are not so sure. Inclusiveness, transparency and nitpicking mean the process is horribly slow.

Almost every week, new evidence of climate damage is published in peer-reviewed scientific journals. But the most recent scary stuff -- the discovery, for instance, that melting permafrost is starting to leak methane, a potent greenhouse gas -- will not be included in the new report because of the cutoff date for reviewing material.

"It [the summary] is a powerful document because it is signed off by all governments," said a source who follows the process closely. "But the IPCC has become such a conservative organisation. The report is really science at the lowest common denominator."

Michael Mann, a professor at Penn State University and author of a book, "The Hockey Stick and the Climate Wars," blames this in part on campaigning by well-funded sceptics who either deny global warming or pin it on natural causes, such as fluctuations in solar heat.

They intimidate individual scientists and exploit areas of scientific uncertainty to claim there is no expert consensus, he said. As a result, the IPCC compilers are driven to even greater caution, with the risk that they deliver a message that is fuzzy or larded with doubt.

"I believe that these pressures combine with the innate tendency of scientists to be reticent about drawing strong conclusions," said Mann.

As a result, "assessment reports like the IPCC report almost inevitably end up understating the conclusions and, in this case, the risks of human-caused climate change."