By Randolph E Schmid / AP 
 
Washington, Mar 24: The rumblings of global warming are echoing across Greenland. Groups of scientists studying ancient climate, tweaking computer models of future climate and even listening to earthquakes add to the evidence that global warming is melting polar ice, according to a series of papers in this week's issue of the journal Science.

At the current rate of rising temperatures, by the year 2100 Arctic summers could be as warm as they were 130,000 years ago. Back then, in a time known as the last interglacial, the oceans were 20 feet higher than they are now.

That does not mean the researchers are predicting a 20-foot ocean rise by the end of this century; more like a couple of feet, they think. But such a warming is expected to accelerate melting of the polar ice and could lead to considerable additional sea-level rise, they said.

For example:

At current rate that Earth's temperature is rising, by 2100 it will probably be 4 degrees warmer than it is 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, according to researchers led by Bette Otto-Bliesner of the National Center for Atmospheric Research. Melting could raise sea levels by 1 foot to 3 feet over the next 100 years to 150 years, she said.

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 — lurch suddenly when lubricated by water from melted ice and snow, causing the ground to tremble.

Otto-Bliesner and Overpeck worked together on looking at ancient climate and assessing whether modern computer models correctly reflect those earlier times.

That allowed them to use the models to look at possible future climate and the potential for rapid polar melting. 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 degrees to 8 degrees.

The warming could raise global sea levels by up to 3 feet this century through a combination of thermal expansion of the water and melting of polar ice, Overpeck and Otto-Bliesner said.

Greenhouse gases such as carbon dioxide are chemicals that have been increasing in the atmosphere since the Industrial Revolution, raising fears of altering the planet's climate by trapping heat from the sun.

Michael Oppenheimer of Princeton University, who was not part of the research teams, said one point that stands out 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 in a statement. "Some of Greenland's glaciers, as large as Manhattan and as tall as the Empire State Building, can move 10 meters 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 reported.

"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.

Between January 1993 and October 2005 seismometers detected 182 quakes in Greenland. All were located in valleys draining the Greenland ice sheet and they ranged in magnitude from 4.6 to 5.1.