http://news.bbc.co.uk/2/hi/science/nature/2030473.stm
Friday, 7 June, 2002,
00:01 GMT 01:01 UK
Greenland's
warming ice flows faster
By Alex Kirby
BBC News Online environment correspondent
[ See
photograph from original article... ]
CAPTION:
"Meltwater flows into a large moulin and down to the bedrock."
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.
Coastal
slide
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.
More significant
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.
Previous work
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.
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