Secretary General Ban Ki-moon, describing climate change as "the defining challenge of our age," released the final report of a U.N. panel on climate change in Valencia on Saturday 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 by the U.N. Intergovernmental Panel on Climate Change (IPCC), which was awarded the Nobel Peace Prize last month. "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 Ban has no power to force members of the United Nations to act, his statements on Saturday increased the pressure on the United States and China, participants 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.
"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."