VALENCIA, Spain - Secretary General Ban Ki-moon, describing climate change as "the defining challenge of our age," released the final report of a UN panel on climate change here yesterday and called on the United States and China to play "a more constructive role."
His challenge to the world's two greatest greenhouse gas emitters came just two weeks before the world's energy ministers meet in Bali, Indonesia, to begin talks on creating a global climate treaty to replace the Kyoto Protocol, which expires in 2012.
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," Ban said of the report, the Synthesis Report of the UN Intergovernmental Panel on Climate Change. "In Bali, I expect the world's policy makers to do the same."
He added, "The breakthrough needed in Bali is for a comprehensive climate change deal that all nations can embrace."
Although Ban has no power to force members of the United Nations to act, his statements yesterday 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 states submerged and abandoned, African crop yields down by 50 percent, and cause a 5 percent decrease in global gross domestic product.
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 avert those problems and others.
"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."
He said that since the IPCC began its work five years ago, scientists have recorded "much stronger trends in climate change," like a recent melting of polar ice that had not been predicted. "That means you better start with intervention much earlier."
The panel's fourth and final report summarized and integrated the most significant findings of three sections of the panel's exhaustive climate-science review that were released from January through April.
The first 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 now 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 scientist and co-chairman of the delegation that wrote the second report.
The Bush administration's reaction to the report was muted.
At a news conference Friday night, 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."
Yesterday's synthesis report was more explicit than the earlier reports, which have been ratified by governments that insisted on changes that diluted their impact.
The fourth report had to be reviewed and approved by delegates from 130 nations gathered here last week. But this time, both 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.
UN officials pointed out that strong policies were needed, such as increasing the energy efficiency of cars and setting up carbon markets, a market-based system that effectively forces companies and countries to pay for the cost of the greenhouse gasses they emit.
"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, head 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 do agree.