The United States and China are signatories to Kyoto, but Washington has not ratified the treaty, and China, along with other developing countries, is not bound by its mandatory emissions caps.

“Today the world’s scientists have spoken, clearly and in one voice,” Mr. Ban said of the report, the Synthesis Report of the United Nations Intergovernmental Panel on Climate Change. “In Bali, I expect the world’s policymakers to do the same.”

He added, “The breakthrough needed in Bali is for a comprehensive climate change deal that all nations can embrace.”

Although Mr. Ban has no power to enforce members of the United Nations to act, his statements on Saturday increased the pressure on the United States and China, participants here said.

Members of the panel said their review of the data led them to conclude as a group and individually that reductions in greenhouse gases had to start immediately to avert a global climate disaster, which could leave island nations submerged and abandoned, reduce African crop yields by 50 percent, and cause a 5 percent decrease in global gross domestic product.

The panel’s fourth and final report summarized and integrated the most significant findings of three sections of a climate-science review that were released between January and April. Because the data had not previously been reviewed as a whole, scientists said the synthesized report was more explicit, creating new emphasis and alarm.

The first section of the review had covered climate trends; the second, the world’s ability to adapt to a warming planet; the third, strategies for reducing carbon emissions. With their mission concluded, the hundreds of IPCC scientists spoke more freely than they had previously.

“The sense of urgency when you put these pieces together is new and striking,” said Martin Parry, a British climate expert who was co-chairman of the delegation that wrote the second report. “I’ve come out of this process more pessimistic about the possibilities than I thought I would.”

The panel, which was awarded the Nobel Peace Prize last month, said the world would have to reverse the growth of greenhouse gas emissions by 2015 to prevent serious climate disruptions.

“If there’s no action before 2012, that’s too late,” said Rajendra Pachauri, a scientist and economist who heads the IPCC. “What we do in the next two to three years will determine our future. This is the defining moment.”

He said that since the IPCC began its work five years ago, scientists had recorded “much stronger trends in climate change,” like a recent melting of Arctic ice that had not been predicted. “That means you better start with intervention much earlier.”

Saturday’s synthesis report was reviewed and approved by delegates from 130 nations gathered here this week. But unlike the earlier reviews, in which governments had insisted on changes that diluted the reports’ impact, this time scientists and environmental groups said there had been no major dilution of the data.

For example, this report’s summary was the first to acknowledge that the melting of the Greenland ice sheet from rising temperatures could result in a substantive sea-level rise over centuries rather than millennia.

“Many of my colleagues would consider that kind of melt a catastrophe” so rapid that mankind would not be able to adapt, said Michael Oppenheimer, a climate scientist at Princeton University who contributed to the IPCC.

“It’s extremely clear and is very explicit that the cost of inaction will be huge compared to the cost of action,” said Jeffrey D. Sachs, director of Columbia University’s Earth Institute. “We can’t afford to wait for some perfect accord to replace Kyoto, for some grand agreement. We can’t afford to spend years bickering about it. We need to start acting now.”

He said that delegates in Bali should take action immediately where they agree, for example, on public financing for new technologies like capturing emissions of the main greenhouse gas, carbon dioxide, and pumping it underground. He said energy ministers should start a global fund to help poor countries avoid deforestation, which releases greenhouse gases and reduces the uptake of carbon dioxide through photosynthesis.

United Nations officials pointed out that strong policies were needed, like increasing the energy efficiency of cars and setting up carbon markets, a system that essentially forces companies and countries to pay for the cost of the greenhouse gases they emit.

The European Union already has such a carbon trading system in place for many industries, and is fighting to bring airlines into the plan.

“Stabilization of emissions can be achieved by deployment of a portfolio of technologies that exist or are already under development,” said Achim Steiner, head of the United Nations Environment Program.

But he noted that developed countries would have to help poorer ones adapt to climate shifts and adopt cleaner energy choices, which are often expensive.

Mr. Steiner emphasized that the report sent a message to individuals as well as world leaders: “What we need is a new ethic in which every person changes lifestyle, attitude and behavior.”

Meanwhile, the Bush administration’s reaction to the report was muted. At a news conference Friday night after the report was approved, James L. Connaughton, the chairman of the president’s Council on Environmental Quality, said President Bush had agreed with leaders of the other major industrialized nations that “the issue warrants urgent action, and we need to bring forward in a more accelerated way the technologies that will make a lasting solution possible.”

He declined to say how much warming the administration considered acceptable, saying, “We don’t have a view on that.”

Mr. Connaughton acknowledged that the United States, like other nations, had tried to make some changes to the draft. Dr. Sharon L. Hays, the leader of the American delegation here, said the goal was not political but “to make sure the final report matches the science.” She noted that the United States had invested $12 billion in climate research since 2001.

Stephanie Tunmore, a member of Greenpeace International who had observer status as the countries debated the text, questioned that explanation.

She said, for example, that the United States had tried to remove a section of the report titled “Reasons for Concern,” which listed consequences of climate change that are either likely or possible. One was the melting of ice sheets, which the panel said could take place more rapidly than previously thought.

The Americans argued that there was no reason to include the section, because all of it was contained somewhere in the previous IPCC technical documents, she said. But the section remained in the report.