Even in the best-case scenario, temperatures will continue to rise from carbon already in the atmosphere, the report said.
Even if factories were shut down today and cars were taken off the roads, the average sea level would gradually rise over the next thousand years to reach as high as 4.6 feet (1.4 meters) above that in the preindustrial period, or about 1850.
"We have already committed the world to sea level rise," IPCC chairperson Rajendra Pachauri said.
If the Greenland ice sheet melts, the scientists said, the seas would rise further, 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. "I expect the world's policy makers to do the same."
After Kyoto
The report is intended to both set the stage and serve as a guide for a climate conference next month in Bali, Indonesia.There, 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. A smooth transition to a new treaty is needed, experts say, to avoid upsetting fledgling carbon markets established by the Kyoto agreement.
"This report will have an incredible political impact," Yvo de Boer, the UN'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.
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," Ban said. "But they are even more terrifying because they are real."
Copyright 2007 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.
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