'Glacial earthquakes' warn of global warming

Dramatic new evidence has emerged of the speed of climate change in the polar regions which scientists fear is causing huge volumes of ice to melt far faster than predicted.

Scientists have recorded a significant and unexpected increase in the number of "glacial earthquakes" caused by the sudden movement of Manhattan-sized blocks of ice in Greenland.

A second study has found that higher temperatures caused by global warming could melt the Arctic and Antarctic ice sheets much sooner than previously thought, with a corresponding rise in sea levels.

Both studies - along with a series of findings from other scientists over the past year - point to a disturbing change in the polar climate which is causing the disappearance of glaciers, ice sheets and floating sea ice.

The rise in the number of glacial earthquakes over the past four years lends further weight to the idea that Greenland's glaciers and its ice sheet are beginning to move and melt on a scale not seen for perhaps thousands of years.

The annual number of glacial earthquakes recorded in Greenland between 1993 and 2002 was between six and 15. In 2003 seismologists recorded 20 glacial earthquakes. In 2004 they monitored 24 and for the first 10 months of 2005 they recorded 32.

The latest seismic study, published today in the journal Science, found that in a single area of north-western Greenland scientists recorded just one quake between 1993 and 1999. But they monitored more than two dozen quakes between 2000 and 2005.

"People often think of glaciers as inert and slow-moving, but in fact they can also move rather quickly," said Goran Ekstrom, professor of geology and geophysics at Harvard University, who led the study.

"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," Professor Ekstrom said.

Average temperatures in the Arctic have risen far faster than in other parts of the world over the past few decades, resulting in the rapid acceleration in polar melting.

As the glacial meltwater seeps down it lubricates the bases of the "outlet" glaciers of the Greenland ice sheet, causing them to slip down surrounding valleys towards the sea, explained Meredith Nettles of Columbia University.

"Our results suggest that these major outlet glaciers can respond to changes in climate conditions much more quickly than we had thought," Dr Nettles said.

"Greenland's glaciers deliver large quantities of freshwater to the oceans, so the implications for climate change are serious. We believe that further warming of the climate is likely to accelerate the behaviour we've documented," she said.

The seismologists also found that the glacial earthquakes of Greenland occurred mainly during the summer months, indicating that the movements were indeed associated with rapidly melting ice - normal "tectonic" earthquakes show no such seasonality. Of the 136 glacial quakes analysed by the scientists, more than a third occurred during July and August.