The extent of sea-level rise from polar ice-melt will be hotly debated. Credit: J. MCCONNICO/AP

Bathing in the still-warm glow of receiving the 2007 Nobel Peace Prize, the United Nations Intergovernmental Panel on Climate Change is meeting in Valencia, Spain, this week to negotiate the summation of its report on global warming.

The IPCC has issued three scientific reports in the past year, and its fourth 'synthesis report' is scheduled for release on Saturday. The report aims to condense the three volumes ? nearly 2,800 pages ? into roughly 100 pages for policy-makers and government delegates who will be gathering in Bali, Indonesia, next month for the latest round of global-warming treaty talks (see page 319).

Scientists began arriving in Valencia last week to work out a consensus draft before the arrival of the government delegations. Stephen Schneider, a climatologist at Stanford University in California, says that the most-recent draft received 2,000 comments from governments. That compares to 5,000 comments from individual scientists on the previous draft.

Given that the report will contain no new science, the challenge will be to get the scientific community and international governments to agree, paragraph by paragraph, on concise language that lays out the facts without downplaying or overstating the problem at hand.

Many have criticized the process as being too political. The conservation group WWF is already warning that ?politically inspired trimming? could unnecessarily dilute the main body of science.

?Some governments will propose emphasizing the more dangerous aspects of the findings. Some will emphasize the uncertainties,? says Schneider, who is one of those working on the final draft. ?It's our job not to let either one of them go too far.?

There will be much after-hours debate over what should be included in the summary, and what can be omitted. The report must also overcome legitimate divides between the three working groups, each of which approached the science from a different perspective. For instance, the first working group, which based its forecasts on climate models, was necessarily cautious about sea-level rise, saying that sea levels could rise between 18 and 59 centimetres during the twenty-first century. The second working group, however, had to look at the possibility that accelerated warming could cause sea levels to rise much faster than expected.

After a year of negotiations, the two groups agreed to a statement that ?risks of metres of sea-level rise in centuries cannot be excluded,? Schneider says. ?That is weaker than working group II wants, stronger than working group I, but it's a synthesis of the reports.? The question now, he adds, is whether such language will be accepted by the government delegations.