VALENCIA, SPAIN - Global warming is "unequivocal," and carbon dioxide already in the atmosphere commits the world to an average 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."

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, and 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.

The panel portrays the Earth hurtling toward a warmer climate at a quickening pace and warns of inevitable human suffering. It says emissions of carbon, mainly from fossil fuels, must stabilize by 2015 and go down after that.

In the best-case scenario, temperatures will keep rising from carbon already in the atmosphere, the report said. Even if factories were shut down today and cars taken off the roads, the average sea level will reach as high as 4.6 feet above that in the preindustrial period, or about 1850.

"We have already committed the world to sea-level rise," said the panel's chairman, Rajendra Pachauri. But if the Greenland ice sheet melts, the scientists said, they could not predict by how many feet the seas would rise, drowning coastal cities.

Climate change is here, they said, as witnessed by melting snow and glaciers, higher average temperatures and rising sea levels.

Report sets stage

The report was adopted after five days of sometimes tense negotiations among 140 national delegations. It lays out blueprints for avoiding the worst catastrophes -- and possible outcomes, depending on how quickly and decisively action is taken.

"The world's scientists have spoken clearly and with one voice," Ban said, looking ahead to an important climate conference next month in Bali, Indonesia. "I expect the world's policymakers to do the same."

The report is intended to both set the stage and serve as a guide for the conference, at which world leaders will begin discussing a global climate change treaty to replace the 1997 Kyoto Protocol.

That treaty, which expires in 2012, required industrial nations to reduce greenhouse gases. A smooth transition to a new treaty is needed to avoid upsetting the fledgling carbon markets.

"This report will have an incredible political impact," said Yvo de Boer, the U.N.'s top climate change official. "It's a signal that politicians cannot afford to ignore."

The United States opted out of Kyoto in 2001, arguing that the science was unproven and that the burden of mandatory emission cuts was unfair because it excluded fast-growing China and India.

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