At that time, the panel said, sea levels were 12 to 20 feet higher than they are now. Muych of that extra water is now trapped in the ice sheets of Greenland and Antarctica, which are eroding in some places.

The panel said there was no solid scientific understanding of how rapidly the vast stores of ice in polar regions will melt, so their estimates on new sea levels were based mainly on how much the warmed oceans will expand, and not on contributions from the melting of ice now on land.

Other scientists have recently reported evidence that the glaciers and ice sheets in the Arctic and Antarctic could flow seaward far more quickly than estimated in the past, and they have proposed that the risks to coastal areas could be much more imminent. But the I.P.C.C. is proscribed by its charter from entering into speculation, and so could not include such possible instabilities in its assessment.

Michel Jarraud, the secretary general of the United Nations World Meteorological Organization, said the lack of clarity should offer no one comfort. “The speed with which melting ice sheets are raising sea levels is uncertain, but the report makes clear that sea levels will rise inexorably over the coming centuries,” he said. “It is a question of when and how much, and not if,” he said, adding: “While the conclusions are disturbing, decision makers are now armed with the latest facts and will be better able to respond to these realities.”

Achim Steiner, the executive director of the United Nations Environment Program, which oversees the I.P.C.C. along with the meteorological group, said society now had plenty of information on which to act.

“The implications of global warming over the coming decades for our industrial economy, water supplies, agriculture, biological diversity and even geopolitics are massive,” he said. “This new report should spur policymakers to get off the fence and put strong and effective policies in place to tackle greenhouse gas emissions.”

The warming and other climate shifts will be highly variable around the world, with the Arctic particularly seeing much higher temperatures, said Susan Solomon, the co-leader of the team writing the summary and the section of the I.P.C.C. report on basic science. She is an atmospheric scientist for the National Oceanic and Atmospheric Administration.