June 6, 2002
GREENLAND ICE SHEET FLOWS FASTER DURING SUMMER MELTING
New measurements show that the flow
of ice in the Greenland ice sheet has
been accelerating since 1996 during
the summer melt season. The results
suggest that the ice sheet may be
responding more quickly to the warming
climate than previously thought.
In an article published in Science
magazine's online Sciencexpress June
7, Jay Zwally, an ICESat Project
scientist at the NASA Goddard Space
Flight Center, Greenbelt, Md., Waleed
Abdalati, Polar Program scientist at
NASA Headquarters, Washington, and
colleagues report that increases in ice
velocity during the summer are
correlated with the timing and the
intensity of ice sheet surface melting.
Using periodic Global Positioning
Satellite measurements from 1996
through 1999, the researchers
discovered that the ice flow speeds up
from 31.3 cm (12.3 inches) per day in
winter to a peak of 40 cm (15.7 inches) per day in the summer when surface
melting is largest. "This study demonstrates that surface meltwater travels quickly
through the 1200 meter (approx. 3/4 mile) thick ice 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," Zwally
said.
The meltwater makes its way from the
surface to the bedrock by draining into
crevasses and large tunnels called
moulins that may be as large as 10
meters (approx. 33 feet) in diameter.
More meltwater underneath the ice
sheet provides lubrication to allow the
ice sheet to move faster toward the
coastline of Greenland.
Over time, as ice melts from the top of
the ice sheet, the ice thins and spreads
out toward lower elevations closer to the
coast. The meltwater also carries heat
(in the form of water) from the top of the ice to the base of the ice that sits on the
bedrock.
A separate study by Abdalati and
Konrad Steffen of the University of
Colorado showed that the melting of the
ice sheet surface has been increasing
at a rate of nearly 20% over the last 21
years, while summer temperatures in
that same period have increased by
one quarter of a degree Celsius (.45°
Fahrenheit). The link between ice sheet
melt and ice flow suggest that the
increasing melt may be more significant
than previously believed.
The faster ice flow, ice thinning and
consequent lowering of the surface
elevation of the ice sheet can open a
feedback to more melting that has not been considered in computer models that
predict ice sheet response to climate change.
NASA's ICESat mission, which is planned for launch in December of this year, will
use a laser altimeter to monitor ice sheet elevations and show elevation changes
as small as 1 cm/yr. ICESat's measurements will tell us whether the ice sheets are
growing or shrinking overall, how much they are contributing to sea level change,
and will help predict future changes in ice volume and sea level.
Zwally and his colleagues speculate that increased movement of the ice sheet
due to more meltwater underneath the ice sheet may have contributed to the
demise of the Laurentide ice sheet approximately 10,000 years ago. During that
time, the Earth's axis of rotation was more tilted toward the Sun causing warmer
summers in the Northern Hemisphere.
Further, they suggest that the observed process may also have contributed to the
extensive melting of the Greenland Ice Sheet during the last Interglacial period,
some 125,000 years ago. According to the U.S. Geological Survey, previous
studies have shown that during the last Interglacial period, carbon dioxide (CO2)
concentrations in the atmosphere were relatively high, temperatures may have
been higher than the present, and sea level may have been approximately 6
meters (19.5 feet) higher. "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," Zwally said.
This research was funded under NASA's ICESat Project.
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Contacts:
Rob Gutro
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 301/286-4044)
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Meltwater Flowing Into a Moulin
Meltwater stream flowing into a large moulin in the ablation zone (area below the equilibrium line) of the Greenland ice sheet. Photo Courtesy: Roger J.
Braithwaite, The University of Manchester, UK
How Greenland's Ice Sheet Melts and Moves
This schematic highlights glaciological features of the ice sheet including surface lakes, crevasses, and large openings called moulins, that stretch up to 10 meters in diameter and drain to the bedrock. Meltwater descends through the moulins, down to the bedrock, contributing to the movement of the ice sheet. The Equilibrium Line (EQ LINE), similar to a snow line on the glacier, is at about 1200 meters elevation in west-central Greenland. In the ablation zone below the EQ LINE, all the winter snow plus some of the ice flowing from higher elevations melts each summer. "GPS" is the marker where the "Global PositioningSystem" was located to observe the movement of the ice sheet. The "V" indicates the velocity of the movement of the ice, some of which is from sliding over the bedrock.
A Satellite View of the Greenland Ice Sheet
These are elevation contours (50 m or 164
feet) overlain on a Landsat image taken on
June 22, 1990, which is typically about 1/3 of
the way through the melt season.
The greyer areas at lower elevations in the
image are bare ice, with some whiter patches
of remaining winter snow near the ice-snow
line. By the end of the melt season in late
August to early September the firn-ice (Firn is
snow that is more than one year old)
boundary usually retreats to around the
average location of the equilibrium line near
the Swiss Camp. The dark patches are melt
lakes, some of which show dark lines of inflow
channels. Later in the season, melt lakes also
form above the equilibrium line.
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