Greenland's warming ice flows faster
BBCNews.com, June 7, 2002
New measurements by US scientists show that since 1996 the Greenland ice sheet has been moving faster during the summer melting season.
The rate is accelerating because more melted water is trickling down from the surface of the sheet to the bedrock. There it lubricates the sheet, which moves faster towards the coast.
The scientists say this suggests the ice may be responding more quickly than thought to a warming climate. Their research, reported in Science magazine's online Sciencexpress, was funded under the US space agency Nasa's ICESat project.
Using periodic Global Positioning Satellite measurements from 1996 to 1999, the team found that the ice flow accelerated from 31.3 centimetres (12.3 inches) a day in winter to a peak of 40 cm (15.7 in) in the summer, when surface melting was greatest.
Jay Zwally is an ICESat project scientist at the Nasa Goddard Space Flight Center. He said: "This study demonstrates that surface meltwater travels quickly through the 1,200-metre-thick ice (1,310 yards) to the bedrock to make the ice slide faster.
"This process was known for decades to enhance the flow of small mountain glaciers, but was not known to occur in the large ice sheets."
The meltwater drains into crevasses and large tunnels, or moulins, which can be 10 metres (33 feet) wide. The melting also means a gradual thinning of the ice as well as its downhill coastward slide.
In a further twist, the water carries heat from the sheet's surface to its base. Waleed Abdelati, a polar programme scientist at Nasa HQ, and Konrad Steffen, of the University of Colorado, undertook a separate study.
This showed that the melting of the ice sheet surface has increased by almost 20% over the last 21 years.
In that period summer temperatures have risen by 0.25 Celsius. The researchers say the link between melting and ice flow suggests that the increasing rate of melting may be more significant than had been believed.
They warn that the combination of faster flow, ice thinning and lowering of the sheet's surface elevation could trigger a feedback process.
This would lead to more melting, something they say has not been considered in computer models predicting ice sheet response to climate change.
In December Nasa is due to launch an ICESat mission which will use a laser altimeter to monitor ice sheet elevations and show changes as small as one centimetre a year.
The scientists say it is possible that increased movement of the ice sheet because of more meltwater may have contributed to the disappearance of the Laurentide ice sheet about 10,000 years ago.
They think the process they have identified could also help to explain the extensive melting of the Greenland ice sheet during the last interglacial period about 125,000 years ago.
Jay Zwally said: "During this time, when the climate was warmer, the ice sheet was less extensive. With the predicted greenhouse warming, we may be returning to similar conditions."
In July 2000, Nasa scientists said the ice around the coast of Greenland was thinning fast, by more than one metre (three feet) annually.
In March 1999, Nasa reported that the Greenland ice sheet was thinning by up to one metre a year.
The scientists said then they thought the probable explanation was that water was filtering down to the bedrock, making the ice likelier to slip off. They did not then suggest a link with climate change.