VALENCIA, Spain — 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.
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 gradually rise over the next 1,000 years to 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," the panel's chairman, Rajendra Pachauri, said. But if the Greenland ice sheet melts, the scientists said, they could not predict by how many feet the seas will rise, drowning coastal cities.
Climate change is here, they said, as witnessed by melting snow and glaciers, higher average temperatures and rising sea levels. If unchecked, global warming will spread hunger and disease, put further stress on water resources, cause fiercer storms and more frequent droughts, and could drive up to 70 percent of plant and animal species to extinction, according to the panel's report.
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 various 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 in Bali, Indonesia, next month. "I expect the world's policy makers 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 succeed the 1997 Kyoto Protocol.
That treaty, which expires in 2012, required industrial nations to reduce greenhouse gases and a smooth transition to a new treaty is needed to avoid upsetting the fledgling carbon markets.
"This report will have an incredible political impact," Yvo de Boer, the U.N.'s top climate change official, told The Associated Press. "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 since it excluded fast-growing China and India.
Chief U.S. delegate Sharon Hays said doubts have been dispelled. "What's changed since 2001 is the scientific certainty that this is happening," she said in a conference call late Friday. She did not indicate that Washington would abandon its policy of voluntary emission cuts.
China and India have said any measures impinging on their development and efforts to lift their people from poverty were unacceptable _ a point likely to be heeded at the Bali talks.
The report offered dozens of measures for avoiding the worst catastrophes if taken together _ at a cost of less than 0.12 percent of the global economy annually until 2050. They ranged from switching to nuclear and gas-fired power stations, developing hybrid cars, using more efficient electrical appliances and managing cropland to store more carbon.
Ban said a new agreement should provide funding to help poor countries develop clean energy resources, adapt to climate conditions and give them the technology to help themselves.
He said he witnessed the devastation of climate change in disappearing glaciers of Antarctica, the deforested Amazon and under the ozone hole in Chile.
"These scenes are as frightening as a science fiction movie," said Ban. "But they are even more terrifying because they are real."