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Worldwide sea levels may rise by 80 centimetres to two metres by 2100 thanks to global warming, but dire predictions of larger increases seem unrealistic, U.S. scientists said this week.

They examined scenarios for loss of ice from Greenland, Antarctica and the world's smaller glaciers and ice caps into the world's oceans, as well as ocean expansion simply because of rising water temperatures.

Their calculations yielded estimates for global sea level increases by the end of the century that are lower than many existing projections, but alarming nonetheless.

"If you look at the actual mechanics of how glaciers work, there doesn't seem to be a realistic way that we know about to get more than about two metres of sea-level rise in the next century," Tad Pfeffer of the University of Colorado's Institute of Arctic and Alpine Research, whose study was published in the journal Science, said in a telephone interview.

"The real unknown right now is what we call the dynamic effect of ice not melting but just being pushed straight into the ocean," Dr. Pfeffer added, referring to pieces of ice breaking off from huge masses of ice such as glaciers and ice sheets and floating in the sea.

Scientists have been working to predict the global effects in coming decades of rising temperatures attributed to human activities that have fueled a "greenhouse effect" on Earth.

Rising sea levels are one of the threats. Scientists have debated how much the seas will rise this century, and some have predicted increases far higher than what this study predicts.

Previous projections of six metres or more of sea level rise by the end of the century do not seem to be supported by solid evidence, Dr. Pfeffer said.

Dr. Pfeffer and scientists at the University of Montana and the University of California at San Diego came up with an estimate of a sea level rise of about 80 centimetres. But their calculations using a "realistic worst-case scenario" produced a predicted rise of two metres, Dr. Pfeffer said.

Even an increase in that range would likely cause major problems in low-lying coastal areas that are home to untold millions of people, he said.

For example, regions of Bangladesh, China and elsewhere could be devastated, while coastal cities such as New Orleans, Amsterdam or Venice could be swamped.

"These places throughout the world where Third World populations live within a metre of sea level and grow a lot of their food within a meter of sea level, they're the ones who are really going to be hurt," Dr. Pfeffer said.