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 n immense expanse of Antarctic ice that has been receding
steadily for 10,000 years poses the most immediate threat of
a large sea level rise because of its potential instability,
a new study indicates.
 The
West Antarctic Ice Sheet -- about 360,000 square miles, or roughly
the size of Texas and Colorado combined -- rests on the Antarctic
land mass below sea level, which makes it particularly susceptible
to rising sea level. Its complete collapse would raise global
sea level 15 to 20 feet, enough to flood many low-lying coastal
regions.
The new study
shows that the ice sheet's complete disintegration in the next
7,000 years could be inevitable, said Howard Conway, a University
of Washington research associate professor of geophysics, who
is the lead author for a paper describing the research in the
Oct. 8 issue of Science.
While human-caused
climate change could hasten the ice sheet's demise, it might
be that there is nothing humans can do to slow or reverse the
trend, Conway said.
"Collapse
appears to be part of an ongoing natural cycle, probably caused
by rising sea level initiated by the melting of the Northern
Hemisphere ice sheets at the end of the last ice age," he
said. "But the process could easily speed up if we continue
to contribute to warming the atmosphere and oceans."
UW geophysics
professor Edwin Wad-dington; Anthony Gades, a UW geophysics research
associate; University of Maine geological sciences professor
George Denton; and Brenda Hall, a UM post-doctoral researcher
in geological sciences, also took part in the study.
Using evidence
gathered from raised beaches and radar imaging of subsurface
ice structures to reconstruct historic changes, the scientists
found the ice sheet has both thinned and decreased in area since
the last glacial maximum 20,000 years ago. Ice covering the region
once was as much as a half-mile thick in places. Land previously
weighed down by the dense ice has elevated since being freed
from its burden. The timing of deglaciation was determined by
carbon-14 dating of samples found on raised beaches that are
now up to 90 feet above present sea level.
Other evidence
comes from Roosevelt Island, an ice island in the Ross Sea. Floating
ice now surrounds it, but reconstructions suggest that ice in
the area of Roosevelt Island was about 1,600 feet thicker and
was grounded during the last ice age.
The researchers
found that the grounding line (the boundary between floating
ice and grounded ice) has receded about 800 miles since the ice
age and has withdrawn an average of about 400 feet per year for
the last 7,600 years. That average is similar to the current
rate, and there is no indication the retreat is slowing, Conway
said. If the grounding line continues to withdraw at that rate,
complete disintegration of the ice sheet will take about 7,000
years.
Other scientists
have found evidence suggesting the West Antarctic Ice Sheet might
have disintegrated in the past. Fragments of tiny algae called
diatoms have been recovered from cores drilled through the ice
and into the land beneath. It is believed diatoms require open
water to build their colonies, which suggests the region once
was free of ice, perhaps as recently as 130,000 years ago between
the last two ice ages, Conway said. 
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geophys.washington.edu; Waddington at (206) 543-4585 or edw