Antarctic and Greenland ice sheet thinning spreads
24 September 2009, by Tamera Jones
The most comprehensive study of Antarctic and Greenland glaciers yet shows that ice-sheet thinning has reached the far north of Greenland, has intensified in Antarctica and is spreading and penetrating far into the interior of both ice sheets.
A glacier meets the ocean in the Antarctic.
The researchers report in Nature that thinning has continued decades after the collapse of large ice shelves and say that a combination of warmer summers and warm ocean currents are most likely to blame.
This is the first time scientists have been able to see just how much ice sheets are thinning as a result of accelerating glaciers.
'Glaciers can lose ice because less snow falls on them, because summer melting increases, or because the glaciers start to flow faster, which puts the glacier 'out of balance'. What we've shown is that many glaciers across both regions are considerably out of balance, because they are flowing faster,' says Dr Hamish Pritchard of British Antarctic Survey, who led the study.
Pritchard and his team, which included scientists from the University of Bristol used data from NASA's ICESat satellite - Ice, Cloud and land Elevation Satellite - to compare how fast ice in fast-moving glaciers moved compared with ice in slow-moving ice next to the glaciers. The data the team analysed covered a five-year period, from 2003 to 2008.
The team's results show 'the glaciers have lost ice, because they're accelerating towards the sea'.
'It seems that when ice shelves collapse it's like a shock, like taking a cork out of a bottle and the effect propagates rapidly inland and persists for decades.'
Dr Hamish Pritchard, British Antarctic Survey
'Ice loss from accelerating glaciers, called dynamic thinning, is a much faster way of losing ice from an ice sheet than melting alone. We think this is what happened to some of the great ice sheets at the end of the last Ice Age,' explains Pritchard.
'Our results show that ice loss is happening in many parts of Antarctica and Greenland. We're surprised at just how widespread this is,' he adds.
Some researchers have suggested that changing wind patterns have re-directed ocean currents south and brought warm water into direct contact with ice in Antarctica, a view supported by the authors.
Some of the fastest thinning glaciers such as Pine Island Glacier and neighbouring Smith and Thwaites Glacier in West Antarctica are thinning by as much as 9 metres per year.
Ice loss from dynamic thinning is poorly understood. So much so that the Intergovernmental Panel on Climate Change (IPCC) decided not to include it in its latest predictions of sea level rise.
'We don't have a very good understanding of sudden glacier acceleration and how the changes we see along the coast spread inland into the ice sheet interior. That's partly because we know so little about the beds of glaciers hidden by up to thousands of metres of ice, or about the oceans surrounding the ice sheets. These places are difficult to explore.'
'But dynamic thinning of Antarctic and Greenland glaciers could become by far the biggest contribution to sea level rise, so the IPCC's prediction of an 18 to 59 centimetre rise over the next century could be an underestimate,' adds Pritchard.
Ice sheet thinning in Antarctica and Greenland. Red areas indicate regions of strongest thinning.
'What we do see is that the strongest thinning is where glaciers speed up as a result of ice shelf collapse. It seems the collapse is like a shock, like taking a cork out of a bottle and the effect propagates rapidly inland and persists for decades,' says Pritchard.
Scientists have known for a few years that glaciers are accelerating in Antarctica and Greenland, but because both regions are so difficult to access, it's been a challenge to get a complete picture of the severity of the situation.
Up until now, the standard approach scientists have used to understand how the regions are changing is to mount radars on polar-orbiting satellites to measure ice height. Radar satellites have let scientists get a broad picture of ice loss in both regions. In an effort to get a more detailed picture, NASA launched ICESat in January 2003.
ICESat uses lasers to measure ice height. Lasers give scientists much higher resolution images and the satellite's orbit also means the whole of the Antarctic coastline can be imaged. 'Its cover is more extensive and the resolution is better. Comparing radar and laser is like comparing a torch beam with the point of a laser,' explains Pritchard.
Technology has moved on since the first radar satellites however. Cryosat-2, a European Space Agency Satellite due for launch at the end of February 2010 uses the next generation radars and has been specifically designed to deliver high resolution images of the coastal areas in both polar regions which will help researchers monitor changes.
Extensive dynamic thinning on the margins of the Greenland and Antarctic ice sheets
Hamish D Pritchard, Robert J Arthern, David G Vaughan & Laura A Edwards
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