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JPL: NASA study finds rapid changes in Earth's Polar Sheets
Aug. 31, 2002
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/images/earth/antarctica
JPL is a division of the California Institute of Technology
in Pasadena. | |
