- COLUMBUS, Ohio -- The interior of the West Antarctic ice sheet -- the
largest grounded repository of ice on the planet -- isnít melting
rapidly, is reasonably stable and has been so for more than a century.
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- Thatís the conclusion drawn by
an international team of scientists who analyzed five years of satellite
radar measurements covering a large part of the southernmost continent.
Their report was published in a recent issue of the journal Science.
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- The study is important in that it offers
one of the best investigations so far of possible mass balance changes
in the ice covering Antarctica. While global warming has been blamed for
possible reductions in the Antarctic ice sheet, scientists have argued
over whether there was definitive evidence of such ice loss. Itís
also unclear whether the west Antarctic ice sheet would be unstable in
a warmer world.
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- The new study suggests that the answer
is no, at least for the middle of the ice sheet.
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- ìBased on our short, five-year
period of observation of the interior of Antarctica, we do not seem to
detect that the ice is melting more than one centimeter per year,î
explained C.K. Shum, an associate professor of civil and environmental
engineering and geodetic science at Ohio State University.
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- ìThat would mean that the interior
Antarctic ice sheet does not seem to be contributing to sea level rise
more than 1 millimeter per year.î Shum said that a one-centimeter
(0.4 inch) decrease in Antarctic ice sheet volume roughly converts into
a one-millimeter (0.04 inch) rise in global sea level.
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- Shum, along with other scientists from
University College in London and the Delft University of Technology in
the Netherlands, analyzed radar altimetry data retrieved from two European
Space Agency remote sensing satellites -- ESA-1 and ESA-2 -- used to measure
ice altitudes from 1992 through 1996.
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- The orbits of the satellites reached
to 81.5 degrees N, allowing them to regularly monitor at least 60 percent
of the continentís grounded ice.
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- The majority of the West Antarctic ice
sheet sits atop dry land while the East Antarctic ice sheet is grounded
below sea level. Changes in the East Antarctic sheet would have little
effect on sea level since the ice displaces sea water. But a complete melt
of West Antarctic ice would pour new water into the oceans, raising sea
levels.
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- ìWe assume that global warming
is underway now,î Shum says, ìand it may be enhanced by human
activities but, until now, its effect on ice loss in Greenland and the
Antarctic has been mostly speculation. We wanted to look at ice sheets
to see how they contribute to sea level rise.î
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- Researchers understand some causes for
sea level rise, Shum said, but the role ice sheets may play ìremains
an uncertainty.î During the last ice age -- 18,000 years ago -- sea
levels were at least 100 meters lower than they are today.
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- Shum and his colleagues turned to the
ESA satellites to look for ice sheet growth. The two, circling the globe
in 800-kilometer (497-mile) orbits, were able to measure the height of
the ocean surface to an accuracy of about 5 centimeters (two inches).
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- ìBut over ice, the readings are
less accurate,î Shum said. ìThe satellites also have problems
accurately measuring coastal ice regions.î
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- The researchers had to devise new algorithms
to decipher the raw, ice sheet altimetry data and correct for several variables
such as radar penetration below the ice surface and snow accumulation.
Even with these limitations, the study represents the longest time series
for which data is available.
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- Shum said the National Aeronautics and
Space Administration is planning a new mission for the year 2001 called
ICESAT. It would position a new satellite in a near-polar orbit, increasing
the amount of ice sheet coverage, and use a more accurate laser altimeter
to take measurements.
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- These, combined with the radar altimetry
data, would give a much better assessment of mass balance changes, if any,
in the Antarctic ice sheets.
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- This project was supported in part by
the NASA Physical Oceanography Program and the United Kingdom Natural Environment
Research Council.
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