Greenland Ice Sheet Slams the Brakes On

By MICHAEL REILLY

July 5, 2008

Much noise has been made about how water lubricates the base of

Greenland's ice sheet, accelerating its slide into the oceans. In a

rare "good news" announcement, climatologists now say the ice may not

be in such a hurry to throw itself into the water after all. Mother

Nature, it seems, has given it brakes.

Since 1991, the western edge of Greenland's ice sheet has actually

slowed its ocean-bound progress by 10%, say the team, who have studied

the longest available record of ice and water flow in the region.

Greenland's mighty ice sheet has enough water locked away to raise

global sea level 6.5 metres were it to melt. Each summer, vast lakes

of meltwater form on its surface. The water seeps through cracks in

the kilometer-thick ice to bedrock, where it acts as a lubricant. The

sheet can move up to twice as fast in the summer, when meltwater is

flowing, as when it is not.

Many fear a positive feedback loop, whereby the accelerating flow will

bring more ice down out of the mountains and toward warmer

temperatures near sea level. Roderik Van De Waal and colleagues at

Utrecht University in the Netherlands now say there is no evidence

this will happen.

(more at link)

http://www.abcnews.go.com/Technology/GlobalWarming/story?id=5311299&page=1

The Science Magazine study:

Large and Rapid Melt-Induced Velocity Changes in the Ablation Zone of

the Greenland Ice Sheet

R. S. W. van de Wal,* W. Boot, M. R. van den Broeke, C. J. P. P.

Smeets, C. H. Reijmer, J. J. A. Donker, J. Oerlemans

Continuous Global Positioning System observations reveal rapid and

large ice velocity fluctuations in the western ablation zone of the

Greenland Ice Sheet. Within days, ice velocity reacts to increased

meltwater production and increases by a factor of 4. Such a response

is much stronger and much faster than previously reported. Over a

longer period of 17 years, annual ice velocities have decreased

slightly, which suggests that the englacial hydraulic system adjusts

constantly to the variable meltwater input, which results in a more or

less constant ice flux over the years. The positive-feedback mechanism

between melt rate and ice velocity appears to be a seasonal process

that may have only a limited effect on the response of the ice sheet

to climate warming over the next decades.

Institute for Marine and Atmospheric research Utrecht, Utrecht

University, Netherlands.

• To whom correspondence should be addressed. E-mail: r.s.w.vandewal@uu.nl

http://www.sciencemag.org/cgi/content/abstract/sci;321/5885/111?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=+Greenland%27s+ice+sheet&searchid=1&FIRSTINDEX=0&issue=5885&resourcetyp

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