- Recent NASA airborne measurements and a new review of
space-based measurements of the thickness of Earth's polar ice sheets
concludes
they are changing much more rapidly than previously believed, with unknown
consequences for global sea levels and Earth's climate.
-
- Large sectors of ice in southeast Greenland, the Amundsen
Sea Embayment in West Antarctica and the Antarctic Peninsula are changing
rapidly by processes not yet well understood, said researchers Dr. Eric
Rignot of NASA's Jet Propulsion Laboratory, Pasadena, Calif., and Dr.
Robert
Thomas of EG&G Services at NASA's Wallops Flight Facility, Wallops
Island, Va. Their study, published this week in the journal Science,
reviews
progress in measuring changes in ice sheet thickness based upon technical
advances and observations made over the past decade.
-
- "Earth's polar ice sheets are changing over
relatively
short time scales, that is, decades versus thousands of years," Rignot
said. Thomas added that today's more precise, widespread measurements tell
us rapid changes are common. "These observations run counter to much
accepted wisdom about ice sheets, which, lacking modern observational
capabilities,
was largely based on 'steady-state' assumptions," Thomas said.
-
- "Remote sensing is allowing researchers to look
at polar processes on continental scales and in greater detail than
before,"
said Dr. Waleed Abdalati, Cryospheric Program manager, NASA Headquarters,
Washington D.C. "Closer examination using even broader advanced remote
sensing techniques, including NASA's upcoming Ice, Cloud and Land Elevation
Satellite, the Gravity Recovery and Climate Experiment and Europe's planned
Cryosat mission--combined with widespread interferometric synthetic
aperture
radar (InSAR) data, ice thickness surveys and ground-based
measurements--will
enable us to estimate ice sheet mass balance for Greenland and Antarctica
even more precisely."
-
- Rignot said understanding how polar ice sheets evolve
is vital to society. "The Antarctic and Greenland ice sheets together
hold enough ice to raise sea level by 70 meters (230 feet)," he said.
"Even a small imbalance between snowfall and discharge of ice and
melt water from ice sheets into the ocean could be a major contributor
to the current sea level rise rate of 1.8 millimeters (0.07 inches) a year
and impact ocean circulation and climate. During past periods of rapid
deglaciation, ice sheet melting raised sea level orders of magnitude faster
than today. This is the real threat of the ice sheets."
-
- Rignot and Thomas' review summarizes current progress
for two methods of measuring changes in ice sheet thickness: the mass
budget
method, which compares losses by melting and ice discharge with total net
input from snow accumulation; and measuring elevation changes over time.
These methods use various space remote sensing resources, such as laser
and radar altimetry, the Global Positioning System and InSAR.
-
- The review reports Greenland's ice sheet is losing 50
cubic kilometers (12 cubic miles) of mass a year due to rapid thinning
near its coasts. That's enough to raise sea level 0.13 millimeters (0.005
inches) annually. "Rapid coastal thinning cannot be explained by a
few warm summers and is attributed to a dynamic ice sheet response,"
Rignot said. "A possible contributor to the observed trend is
increased
lubrication from additional surface melt water reaching glacier beds
through
crevasses and moulins."
-
- Rignot says the mass balance in Antarctica is much harder
to calculate because the ice sheet is far larger, more remote and not well
covered by existing key satellites. The researchers calculated net ice
gains or losses for 33 Antarctic glaciers, including 25 of the 30 largest
ice producers.
-
- The West Antarctic ice sheet was found to be thickening
in the west, thinning rapidly in the north, and probably losing mass
overall
by roughly 65 cubic kilometers (roughly 15.5 cubic miles) a year, enough
to raise sea level by about 0.16 millimeters (0.006 inches) a year. InSAR
observations show several major glaciers that are accelerating and
contributing
to sea level rise. Radar altimetry shows ice shelves in the Amundsen Sea
Embayment are rapidly thinning, possibly in reaction to a warmer ocean,
as suggested by recent oceanographic data. Melting of ice shelf bottoms
is far larger than expected here due to intrusion of warm water on the
continental shelf, implying a larger interplay of ice and ocean in ice
sheet evolution.
-
- Rignot said little is known about the mass balance of
Antarctic Peninsula mountain glaciers, which receive a quarter of
Antarctica's
snow accumulation. The peninsula has warmed 2 to 3 degrees Celsius (3.6
to 5.4 degrees Fahrenheit) over the past 50 years, causing rapid thinning,
enhanced melting and rapid disintegration of its ice shelves. The peninsula
is a unique laboratory to determine whether retreating ice shelves can
induce faster ice sheet flow and raise global sea level, a hypothesis
formulated
decades ago but still disputed. Recent results show large glacier
acceleration
in response to ice shelf collapse. If ice shelves do buttress glaciers,
the Antarctic ice sheet's contribution to sea level rise could be much
larger in the future than previously believed.
-
- Illustrations related to this study may be viewed at:
http://www.jpl.nasa.gov/i
mages/earth/antarctica
-
- JPL is a division of the California Institute of
Technology
in Pasadena.
-
- http://www.jpl.nasa.gov/releases/2002/release_2002_168.html
-
- Note: This story has been
adapted from a news release issued by NASA/Jet Propulsion Laboratory for
journalists and other members of the public. If you wish to quote from
any part of this story, please credit NASA/Jet Propulsion Laboratory as
the original source. You may also wish to include the following link in
any citation:
-
- http://www.sciencedaily.com/releases/2002/09/020902072155.htm
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