VALENCIA, Spain, Nov. 16 - In its final and most powerful report, a United Nations panel of scientists meeting here describes the mounting risks of climate change in language that is both more specific and forceful than its previous assessments, according to scientists here.
Synthesizing reams of data from its three previous reports, the United Nations Intergovernmental Panel on Climate Change for the first time specifically points out important risks if governments fail to respond: melting ice sheets that could lead to a rapid rise in sea levels and the extinction of large numbers of species brought about by even moderate amounts of warming, on the order of 1 to 3 degrees.
The report carries heightened significance because it is the last word from the influential global climate panel before world leaders meet in Bali, Indonesia, next month to begin to discuss a global climate change treaty that will replace the Kyoto protocol, which expires in 2012. It is also the first report from the panel since it was awarded the Nobel Peace Prize in October, an honor that many scientists here said emboldened them to stand more forcefully behind their positions.
As a sign of the deepening urgency surrounding the climate change issue, the report, which was being printed Friday night, will be officially released by Secretary General Ban Ki-moon on Saturday.
The full report was embargoed from news organizations until Saturday. But drafts have been circulating for weeks, and descriptions of its findings began to appear on Web sites and in news agency reports on Friday. Bush administration officials held a news conference to discuss the report but insisted that their comments be withheld until after its official release.
"This document goes further than any of the previous efforts," said Hans Verolme, director of the World Wildlife Fund's Global Climate Change Program. "The pressure has been palpable - people know they are delivering a document that will be cited for years to come and will define policy."
The previous three sections, released between February and April, focused on one issue at a time: the first on science, the second on how the world could adapt to warming and the third about how countries could “mitigate,” or reduce the greenhouse gases produced.
This fourth and final assessment - the so-called synthesis report - seeks to combine lessons from all three. Its conclusions are culled from data contained in the thousands of pages that were essentially technical supplements to the panel's previous publications. How that data is summarized and presented to the world is a powerful guide to what the scientists consider of utmost importance at the end of a five-year process, offering concrete guidelines for policy makers.
"You look to a synthesis report to provide clarity, to clarify what was obscure in previous reports," said Michael Oppenheimer, a climate scientist at Princeton University. "Now, how can we take these findings and formulate a policy response that's quick enough and big enough?"
While drafts of the panel's reports are written by panels of scientists, the language is reviewed and often altered by delegates from 130 governments who meet before their final approval and release. Those negotiations took place here this week, and were often contentious, with the United States, China and India raising many objections, said scientists who spoke on condition of anonymity because they are not allowed to publicly refer to any countries by name.
The scientists and country representatives who had flocked here this week to participate in negotiations on the final wording applauded as the panel’s chairman, Rajendra Pachauri, declared the panel's years of work concluded, just after 10:30 p.m. on Friday.
Even though the synthesis report is more alarming than its predecessors, some researchers believe that it still understates the trajectory of global warming and its impact. The I.P.C.C.'s scientific process, which takes five years of study and writing from start to finish, cannot take into account the very latest data on climate change or economic trends, which show larger than predicted development and energy use in China.
"The world is already at or above the worst case scenarios in terms of emissions," said Gernot Klepper, of the Kiel Institute for World Economy in Kiel, Germany. "In terms of emissions, we are moving past the most pessimistic estimates of the I.P.C.C., and by some estimates we are above that red line."
The panel presents several scenarios for the trajectory of emissions and climate change. In 2006, 8.4 gigatons of carbon were put into the atmosphere from fossil fuels, according to a study in the proceedings of the National Academy of Science, which was co-written by Dr. Klepper. That is almost identical to the panel's worst case prediction for that year.
While the United States, Saudi Arabia and China tried to change the text in order to play down the consequences of global warming, developing nations - which will bear the initial brunt of climate change - were much more forceful than at previous meetings in opposing these efforts, one scientist who was in the negotiating room said.
"I suspect that will continue," he said. "As they feel more and more threatened by the sea and the storms they will insist that, as one of them put it, 'We do not want this report to be warm and fuzzy when the reality is cold and risky,' or something like that," he said.
One novel aspect of the report is a specific list of "Reasons for Concern." It includes items that are thought to be very likely outgrowths of climate change that had been mentioned in previous reports, like an increase in extreme weather events.
But it for the first time includes less likely but more alarming possibilities, like the relatively rapid melting of polar ice. Previous reports focused more on changes the scientists felt were "highly likely."
"This time, they take a step back and look at the totality," Dr. Verolme said. "Saying it is less likely to occur, but if it does we are fried."
One such area is the future melting of ice sheets in Greenland and western Antarctica. In earlier reports, the panel's scientists acknowledged that their computer models were poor at such predictions, and did not reflect the rapid melting that scientists have recently observed. If these areas melt entirely, seas would rise 40 feet, scientists said. While scientists are certain that the sheets will melt over millennia, producing sea-level rises, there is now evidence to suggest that it could happen much faster than this, perhaps over centuries.
"In my view that would make it not just difficult, but impossible to adapt successfully, some of my colleagues would say catastrophic," said Dr. Oppenheimer. "If they say that it's possible that melting could occur in centuries leading to meters of change, that's a headline."
This final report also puts more emphasis on the ripple effect of small degrees of temperature change, some of which are already being seen, such as species extinctions and loss of biodiversity.
"A relatively modest degree of warming - one to three degrees - spells a lot of trouble and I think that was not clear in the previous report," Dr. Oppenheimer said. He said part of the reason for the lack of clarity was that governments had "messed around" with the language and structure of the report during the approval process.
This time around, the consequences of different degrees of climate change will be better laid out so that the ministers who meet in Bali next month will understand the options and the consequences of inaction. "This should light a fire under policy makers," Dr. Oppenheimer said.
Andrew C. Revkin contributed reporting from New York.