VALENCIA, Spain, November 17, 2007 - Global warming is "unequivocal" and carbon dioxide already in the atmosphere commits the world to an eventual rise in sea levels of up to 4.6 feet, the  world's top climate experts warned Saturday in their most authoritative report to date.

"Only urgent, global action will do," said U.N. Secretary-General Ban Ki-moon, calling on the United States and China — the world's two biggest polluters — to do more to slow global climate change.

"I look forward to seeing the U.S. and China playing a more constructive role," Ban told reporters. "Both countries can lead in their own way."

Ban, however, advised against assigning blame.

Climate change imperils "the most precious treasures of our planet," he said, and the effects are "so severe and so sweeping that only urgent global action will do. We are all in this together. We must work together."

According to the U.N. panel of scientists, whose latest report is a synthesis of three previous ones, enough carbon dioxide already has built up that it imperils islands, coastlines and a fifth to two-thirds of the world's species.

As early as 2020, 75 million to 250 million people in Africa will suffer water shortages, residents of Asia's large cities will be at great risk of river and coastal flooding, according to the report.

Europeans can expect extensive species loss, and North Americans will experience longer and hotter heat waves and greater competition for water, says the report from the U.N. Intergovernmental Panel on Climate Change, which shared the Nobel Prize with Al Gore this year.

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