In the first direct, comprehensive mass survey of the entire Greenland
ice sheet, scientists using data from the NASA/German Aerospace Center
Gravity Recovery and Climate Experiment (GRACE) have measured a
significant decrease in the mass of the Greenland ice cap. GRACE is a
satellite mission that measures movement in Earth's mass.
In an update to findings published in the journal Geophysical Research
Letters, a team led by Dr. Isabella Velicogna of the University of
Colorado, Boulder, found that Greenland's ice sheet decreased by 162 (plus or
minus 22) cubic kilometers a year between 2002 and 2005. This is higher than
all previously published estimates, and it represents a change of about 0.4 millimeters
(.016 inches) per year to global sea level rise.
"Greenland hosts the largest reservoir of freshwater in the northern
hemisphere, and any substantial changes in the mass of its ice sheet
will affect global sea level, ocean circulation and climate," said Velicogna.
These results demonstrate GRACE's ability to measure monthly mass changes
for an entire ice sheet - a breakthrough in our ability to monitor such changes."
Other recent GRACE-related research includes measurements of seasonal
changes in the Antarctic Circumpolar Current, Earth's strongest ocean
current system and a very significant force in global climate change.
The GRACE science team borrowed techniques from meteorologists who use
atmospheric pressure to estimate winds. The team used GRACE to estimate
seasonal differences in ocean bottom pressure in order to estimate the
intensity of the deep currents that move dense, cold water away from
the Antarctic. This is the first study of seasonal variability along
the full length of the Antarctic Circumpolar Current, which links the
Atlantic, Pacific and Indian Oceans.
Dr. Victor Zlotnicki, an oceanographer at NASA's Jet Propulsion
Laboratory in Pasadena, Calif., called the technique a first step in
global satellite monitoring of deep ocean circulation, which moves heat
and salt between ocean basins. This exchange of heat and salt links sea
ice, sea surface temperature and other polar ocean properties with
weather and climate-related phenomena such as El Ninos. Some scientific
studies indicate that deep ocean circulation plays a significant role
in global climate change.
The identical twin GRACE satellites track minute changes in Earth's
gravity field resulting from regional changes in Earth's mass.
Masses of ice, air, water and solid Earth can be moved by weather patterns,
seasonal change, climate change and even tectonic events, such as this past
December's Sumatra earthquake. To track these changes, GRACE measures micron-scale
changes in the 220-kilometer (137-mile) separation between the two satellites,
which fly in formation. To limit degradation of GRACE's satellite
antennas due to atomic oxygen exposure and thereby preserve mission
life, a series of maneuvers was performed earlier this month to swap
the satellites' relative positions in orbit.
In a demonstration of the satellites' sensitivity to minute changes
in Earth's mass, the GRACE science team reported that the satellites
were able to measure the deformation of the Earth's crust caused by
the December 2004 Sumatra earthquake. That quake changed Earth's gravity
by one part in a billion.
Dr. Byron Tapley, GRACE principal investigator at The University of
Texas at Austin, said that the detection of the Sumatra earthquake
gravity signal illustrates GRACE's ability to measure changes on and
within Earth's surface. "GRACE's measurements will add a global
perspective to studies of large earthquakes and their impacts," said
Tapley.
GRACE is managed for NASA by JPL. The University of Texas Center for
Space Research has overall mission responsibility. GeoForschungsZentrum
Potsdam, or 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.
Imagery related to these latest GRACE findings may be viewed at:
http://www.nasa.gov/vision/earth/lookingatearth/grace-images-20051220.html.
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