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,” 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.”