by Catherine Brahic, New Scientist, October 16, 2008
Dramatic images taken at least every six hours over an entire year reveal how the world's fastest-flowing glacier is draining Greenland's ice sheet and contributing to sea-level rise world-wide.
Jason Amundson of the University of Alaska Fairbanks and colleagues set up a complete "life monitoring" system around the end of the Jakobshavn glacier in Greenland, at the point where it dumps its ice into a narrow fjord and out to sea, during the summer of 2007. The system remained in place until May 2008.
The Jakobshavn glacier, known as Sermeq Kujalleq in Greenlandic, is the world's fastest flowing ice sheet (watch a NASA animation of the Jakobshavn Glacier) and a major contributor to the demise of Greenland's ice.
Each year, 7% of the ice lost from Greenland passes through the fjord. The incredible rate at which it sends ice crashing out to sea doubled to 12 kilometres per year between 1997 and 2003. This increased sea-level rise by 0.06 millimetres per year – roughly 4% of the 20th century rate of sea-level increase.
Three cameras took pictures every 10 minutes from 13 May to 8 June 2007, then every hour for the next month, every six hours over the winter and once more every 10 minutes from 7 to 14 May 2008.
Satellite pictures (see right) show that the ice sheet is not homogenous – two huge rivers of ice flow out into the fjord. Stitched together, Amundson's images reveal how this happens.
During the summer, huge chunks of ice broke off from the tip of the ice river about every 75 hours – a process known as calving.
Berg birth
In the winter, the ice river grew out over the water at the top of the fjord, creating an ice tongue several kilometres long. Amundson's pictures show that calving events stopped until the ice tongue disintegrated in four separate events between mid-April and mid-May 2008. In total, the team recorded 32 calvings.
At each calving event, the ice sheared through its entire 900-metre thickness and the new icebergs would flip over, drag up sediment from the bottom, and shove off into the fjord pushing floating ice ahead of it at a speedy one to two kilometres per hour. The ice river normally moves downstream 35 metres per day.
In addition to photographing the events, the team measured seismic waves generated by the break-ups and monitored the movement of the ice using GPS stations posted on the ice.
"It's been known for over 30 years that calving events release low-frequency seismic energy," says Amundson. The slow rumblings, known as glacial earthquakes, can be detected around the world and are happening more and more frequently.
Previously, seismologist Göran Ekström at Harvard University had proposed that when ice breaks off the tip of Greenland ice rivers, the rest of the glacier suddenly jolts forwards, triggering the low rumblings. But Amundson's life-monitoring stations show no such jolts.
"It is still possible that the glacial earthquakes are generated by a slow acceleration of the glacier," he says. "More likely, I think, is that they are caused by icebergs bouncing around in the fjord."
Journal reference: Geophysical Research Letters (DOI: 10.1029/2008GL035281, in press)
Link to article: http://environment.newscientist.com/article/dn14956-timelapse-footage-reveals-ice-sheet-in-crisis.html
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