Has the UN climate panel now outlived its usefulness?

The scientists dug in. "I sat for five hours defending this paragraph," said Reto Knutti of ETH Zurich, a Swiss science university, who was the coordinating lead author for the relevant chapter on "projections, commitments, and irreversibility."

"There was very strong opposition from many governments. It was obviously political, though they were using strange scientific arguments," Knutti said. The governments saw this statement as, in effect, scientists imposing emissions restrictions through the back door. "I am proud to say we didn't lose any figures," he added, "though some of the text was rewritten a bit."

Drew Shindell of NASA's Goddard Institute for Space Studies, another lead author on the report, was also in the room. "Some people were very sensitive to this cumulative carbon issue," he said. "They had agendas beyond the science."

Another contentious topic was how the report should deal with the recent warming hiatus. The draft acknowledged the scientists' concerns and noted that climate models "do not generally reproduce the observed reduction in surface warming trend over the last 10-15 years." This was reportedly met with opposition from some delegates who wanted to remove all references to a slowdown. Some argued that the hiatus had not lasted long enough to be considered a temperature trend. Perhaps they also felt it would be seized on by climate-change deniers.

"We looked at this very carefully," said Stocker. There was, he noted, "not a lot of published literature" on the phenomenon. This was a problem, since the IPCC does not do its own research and can only review published literature. But again, the authors of that passage stuck to their guns, and retained most of the message, though the direct statement about the failings of the models does not appear in the report.

Science is not naturally a consensual process. Reaching agreement is hard for people more used to spending their time refuting each others' hypotheses. So the question arises: Is the IPCC's self-imposed task of producing massive consensual documents about every aspect of climate science - and then resisting politicians' efforts to change them - worth it?

For one thing, the consensus even among scientists is creaking. In interviews with Yale Environment 360 in recent weeks, a number of past and present IPCC authors have expressed strong dissatisfaction with what they saw as the conservatism of the emerging text for the scientific assessment. (There is, if anything, even more contention over the two companion reports that will be published next year, covering the impacts of climate change and what to do about it.)

Some researchers are angered about the marginal reduction in predicted warming. They say that may be justified by the outputs of the climate models, but that those models do not include some worrying positive feedbacks that could accelerate warming in coming decades. Other critics say that, even though the report has upped its estimates of sea level rise this century to as much as one meter, the lead authors did not accept findings from reputable researchers suggesting that a rise of as much as two meters was possible.

The problem, in essence, is that factors that climatologists cannot yet successfully model are left out of the modeling studies that deliver the headline predictions.

Michael Mann of Penn State University, a past IPCC lead author, is concerned about the sidelining of the potential for higher sea level rises due to collapsing ice sheets on land. Before the report's publication, he said the report "should not be dismissing impacts with lower probability, but higher threat potential. Such potential outcomes are a critical part of the societal risk." For instance, people designing flood risk defenses want to know about the worst expectations of possible sea level rise, not those scientifically most likely.

One lead author of the IPCC chapter on sea level rises, spoken to after the report's publication, conceded the point. "I agree there can be a conflict between good science and what policymakers and engineers like flood designers want to know," said Tony Payne of the University of Bristol, England.

Another concern is methane, a potent greenhouse gas that could escape into the atmosphere as Arctic permafrost melts and sea beds warm. The methane is the frozen product of rotting vegetation in centuries past. The IPCC estimates that up to 80 percent of the Arctic permafrost could melt this century.