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In
the most comprehensive survey ever undertaken of the massive
ice sheets covering both Greenland and Antarctica, NASA
scientists confirm climate warming is changing how much
water remains locked in Earth's largest storehouses of
ice and snow.
"If the trends we're seeing continue and climate
warming continues as predicted, the polar ice sheets could
change dramatically," said survey lead author Jay
Zwally of NASA's Goddard Space Flight Center, Greenbelt,
Md. "The Greenland ice sheet could be facing an irreversible
decline by the end of the century."
Other recent studies have shown increasing losses of
ice in parts of these sheets. This new survey is the first
to inventory the losses of ice and the addition of new
snow on both continents in a consistent and comprehensive
way throughout an entire decade.
The survey shows there was a net loss of ice from the
combined polar ice sheets between 1992 and 2002 and a
corresponding rise in sea level. The survey documented
for the first time extensive thinning of the West Antarctic
ice shelves, an increase in snowfall in the interior of
Greenland and thinning at the edges. All are signs of
a warming climate predicted by computer models.
The survey combines new satellite mapping of the height
of the ice sheets from two European Space Agency satellites.
It also used previous NASA airborne mapping of the edges
of the Greenland ice sheets to determine how fast the
thickness is changing. Researchers used nine years of
elevation mapping over much of Antarctica and 10.5 years
of data over Greenland from the European Remote-sensing
Satellites 1 and 2. The survey pinpointed where the ice
sheets were thinning and where they were growing.
In Greenland, the survey saw large ice losses along the
southeastern coast and a large increase in ice thickness
at higher elevations in the interior due to relatively
high rates of snowfall. This study suggests there was
a slight gain in the total mass of frozen water in the
ice sheet over the decade studied, contrary to previous
assessments.
According to Zwally, this situation may have changed
in just the past few years. Last month NASA scientists
at the Jet Propulsion Laboratory, Pasadena, Calif., reported
a speed up of ice flow into the sea from several Greenland
glaciers. That study included observations through 2005;
Zwally's survey concluded with 2002 data.
"The melting of ice at the edges of the ice sheet
is also increasing, which causes the ice to flow faster,"
Zwally said. "A race is going on in Greenland between
these competing forces of snow build-up in the interior
and ice loss on the edges. But we don't know how long
they will be approximately in balance with each other
or if that balance has already tipped in favor of the
recently accelerating outflow from glaciers."
The situation was very different in Antarctica. The ice
sheets had a major net loss of ice due to outflow from
West Antarctica. These loses, which may have been going
on for decades, outweighed the gains in snow and ice seen
in the East Antarctic ice sheet and parts of West Antarctica.
Also thinning were the ice shelves around West Antarctica,
where temperatures have been increasing. The floating
ice shelves are vulnerable to climate change. Some ice
shelves in the Antarctic Peninsula have totally disintegrated
in recent years, allowing the ice from the land to move
into the ocean faster.
When the scientists added up the gains and loses of ice
from the Greenland and Antarctic sheets, there was a net
loss of ice to the sea. The Greenland ice sheet annually
gained approximately 11 billion tons of water, while Antarctica
lost about 31 billion tons per year. The 20 billion net
tons added to the oceans is equivalent to the amount of
fresh water annually used in homes, businesses and farming
in New York, New Jersey and Virginia.
"The study indicates that the contribution of the
ice sheets to sea-level rise during the decade studied
was much smaller than expected, just two percent of the
recent increase of nearly three millimeters (0.12 inches)
a year," Zwally said. "Current estimates of
the other major sources of sea-level rise - expansion
of the ocean by warming temperatures and runoff from low-latitude
glaciers - do not make up the difference, so we have a
mystery on our hands as to where the water is coming from.
Continuing research using NASA satellites and other data
will narrow the uncertainties in this important issue
and help solve the mystery."
The survey was published this week in the Journal of
Glaciology (www.igsoc.org). For more information about
the research and images on the Web, visit:
http://www.nasa.gov/vision/earth/environment/ice_sheets.html
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NASA
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