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Scientists
were able to conduct the first-ever gravity survey of
the entire Antarctic ice sheet using data from the joint
NASA/German Aerospace Center Gravity Recovery and Climate
Experiment (GRACE). This comprehensive study found the
ice sheet's mass has decreased significantly from 2002
to 2005.
Isabella Velicogna and John Wahr, both from the University
of Colorado, Boulder, conducted the study. They demonstrated
for the first time that Antarctica's ice sheet lost a
significant amount of mass since the launch of GRACE in
2002. The estimated mass loss was enough to raise global
sea level about 1.2 millimeters (0.05 inches) during the
survey period; about 13 percent of the overall observed
sea level rise for the same period. The researchers found
Antarctica's ice sheet decreased by 152 (plus or minus
80) cubic kilometers of ice annually between April 2002
and August 2005.
That is about how much water the United States consumes
in three months (a cubic kilometer is one trillion liters;
approximately 264 billion gallons of water). This represents
a change of about 0.4 millimeters (.016 inches) per year
to global sea level rise. Most of the mass loss came from
the West Antarctic ice sheet.
"Antarctica is Earth's largest reservoir of fresh
water," Velicogna said. "The GRACE mission is
unique in its ability to measure mass changes directly
for entire ice sheets and can determine how Earth's mass
distribution changes over time. Because ice sheets are
a large source of uncertainties in projections of sea
level change, this represents a very important step toward
more accurate prediction, and has important societal and
economic impacts. As more GRACE data become available,
it will become feasible to search for longer-term changes
in the rate of Antarctic mass loss," she said.
Measuring variations in Antarctica's ice sheet mass is
difficult because of its size and complexity. GRACE is
able to overcome these issues, surveying the entire ice
sheet, and tracking the balance between mass changes in
the interior and coastal areas.
Previous estimates have used various techniques, each
with limitations and uncertainties and an inherent inability
to monitor the entire ice sheet mass as a whole. Even
studies that synthesized results from several techniques,
such as the assessment by the Intergovernmental Panel
on Climate Change, suffered from a lack of data in critical
regions.
"Combining GRACE data with data from other instruments
such as NASA's Ice Cloud and land Elevation Satellite,
radar and altimeters that are more effective for studying
individual glaciers is expected to substantially improve
our understanding of the processes controlling ice sheet
mass variations," Velicogna said.
The Antarctic mass loss findings were enabled by the
ability of the identical twin GRACE satellites to track
minute changes in Earth's gravity field resulting from
regional changes in planet mass distribution. Mass movement
of ice, air, water and solid earth reflect weather patterns,
climate change and even earthquakes. To track these changes,
GRACE measures micron-scale variations in the 220 kilometer
(137 mile) separation between the two satellites, which
fly in formation.
GRACE is managed for NASA by the Jet Propulsion Laboratory
(JPL), Pasadena, Calif. The University of Texas Center
for Space Research has overall mission responsibility.
GeoForschungsZentrum Potsdam (GFZ), Potsdam, Germany,
is responsible for German mission elements. Science data
processing, distribution, archiving and product verification
are managed jointly by JPL, the University of Texas and
GFZ. The results will appear in this week's issue of Science.
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