The Earth is already shaking beneath melting ice as rising temperatures threaten to shrink polar glaciers and raise sea levels around the world.

By the end of this century, Arctic readings could rise to levels not seen in 130,000 years when the oceans were several metres higher than now, according to new research appearing in Friday's issue of the journal Science.

Even now, giant glaciers lubricated by melting water have begun causing earthquakes in Greenland as they lurch toward the ocean, other scientists report in the same journal.

In principal findings:

At the current warming rate, Earth's temperature by 2100 will probably be at least 4 C warmer than now, with the Arctic at least as warm as it was 130,000 years ago, reports a research group led by Jonathan T. Overpeck of the University of Arizona.

Computer models indicate that warming could raise the average temperature in parts of Greenland above freezing for multiple months and could have a substantial impact on melting of the polar ice sheets, says a second paper by researchers led by Bette Otto-Bliesner of the National Centre for Atmospheric Research. Melting could raise sea level one to three feet over the next 100 to 150 years, she said.

And a team led by Goeran Ekstroem of Harvard University reported an increase in "glacial earthquakes," which occur when giant rivers of ice some as big as Manhattan move suddenly as meltwater eases their path. That sudden movement causes the ground to tremble.

Otto-Bliesner and Overpeck wrote separate papers and also worked together, studying ancient climate and whether modern computer climate models correctly reflect those earlier times. That allowed them to use the models to look at possible future conditions. The researchers studied ancient coral reefs, ice cores and other natural climate records.

"Although the focus of our work is polar, the implications are global," Otto-Bliesner said. "These ice sheets have melted before and sea levels rose. The warmth needed isn't that much above present conditions."

According to the studies, increases in greenhouse gases in the atmosphere over the next century could raise Arctic temperatures as much as 5 to 8 C.

The warming could raise global sea levels by up to one metre this century through a combination of thermal expansion of the water and melting of polar ice, Overpeck and Otto-Bliesner said.

Michael Oppenheimer of Princeton University, who was not part of the research teams, said, "One point stands out above all others and that is that a modest global warming may put Earth in the danger zone for a major sea level rise due to deglaciation of one or both ice sheets."

Ekstroem and colleagues reported that glacial earthquakes in Greenland occur most often in July and August and have more than doubled since 2002.

"People often think of glaciers as inert and slow-moving, but in fact they can also move rather quickly," Ekstroem said. "Some of Greenland's glaciers, as large as Manhattan and as tall as the Empire State Building, can move 10 metres in less than a minute, a jolt that is sufficient to generate moderate seismic waves."

Melting water from the surface gradually seeps down, accumulating at the base of a glacier where it can serve as a lubricant allowing the ice to suddenly move downhill, the researchers said.

"Our results suggest that these major outlet glaciers can respond to changes in climate conditions much more quickly than we had thought," said team member Meredith Nettles of Columbia University's Lamont-Doherty Earth Observatory.

(China Daily 03/25/2006 page6)