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The western part of Antarctica is shedding ice much faster today than it was just ten years ago, according to new satellite measurements.
The
measurements, which surveyed the coasts of nearly the entire continent,
suggest that climate models underestimate how quickly Antarctica
responds to ongoing global warming, said study co-author Jonathan Bamber of the University of Bristol in England.
Many past studies have tried to estimate how much ice Antarctica is losing.
(Related: "Hundreds of Glaciers Melting Faster in Antarctica" [June 6, 2007].)
But
the new study is the first to show that this loss is accelerating, at
least in western Antarctica and the Antarctic Peninsula, the
researchers say.
"In all the ice sheet models we have at present for Antarctica, things happen very slowly," Bamber said.
"[But] we're seeing things happen rather quickly."
They
found that for Antarctica overall, the ice loss increased about 75
percent over the ten-year period, from 112 gigatons of ice per year in
1996 to 196 gigatons of ice per year in 2006.
As to whether
Antarctica will lose or gain ice as global warming proceeds, the
measurements disagree with existing climate models that suggest "[the
ice sheet] is going to get bigger because of increased snowfall with
warming temperatures," Bamber said.
"We don't see that. We see
the ice sheet losing mass," he said. "So there's a bit of a paradigm
shift in what the ice sheet has done recently and what it could do in
the future."
Scientists are concerned the melting ice will contribute to a dangerous sea level rise.
Ice Losses
The
rate of glaciers and basins dumping ice into the ocean have increased
over the past decade, according to the study published in yesterday's
issue of the journal Nature Geoscience.
(Related: "Small Melting Glaciers Will Speed Sea Level Rise, Study Says" [July 19, 2007].)
The
researchers used measurements from European, Canadian, and Japanese
satellites, which scanned about 85 percent of Antarctica's coasts from
1996 to 2006.
The results showed that most of the ice is being lost through a few fast-flowing glaciers and basins.
"We
can pinpoint with a lot of precision exactly where the losses are
taking place and the characteristics of those losses," Bamber said.
The new study also covers a longer period of time than past research efforts.
The
research method, called radar inferometry, measured how quickly ice was
flowing. It also captured how thick the ice was at the grounding point,
which is where the ocean causes ice to lift off the land and start
floating.
The "most likely explanation" for the increased ice
loss is that warming waters are melting away ice at the grounding
point, according to Bamber.
"That's causing the buttressing
effect of the ice shelves to be less [effective], and that's allowing
the glaciers to flow faster into the ocean," he said.
Eric Rignot of NASA's Jet Propulsion Laboratory in Pasadena, California, is the lead study author.
"The
climate models neglected changes in speeds of glaciers," Rignot said.
"It turns out that is the main control on the ice mass balance."
"The
ice loss we see is going to continue, and it's going to grow" because
the oceans around Antarctica are expected to warm, Rignot added.
Uncertain Future
The
new study "tells us that the glaciers are losing more mass, which is
one part of the total mass change," said Andrew Shepherd of the
University of Edinburgh in Scotland.
However Shepherd, who was
not involved in the new research, argued that it's not clear whether
Antarctica is actually losing more ice now than it did ten years ago,
and what the future will hold.
"What we still don't know is
the total mass change [over that period], because we don't know how
much extra snow has fallen there," Shepherd said.
This
snowfall in inland Antarctica is the main unknown in these estimates,
Bamber agrees. But they say their method has been able to get a better
estimate of the snowfall than earlier studies. (Related: "Antartica Snowfall Not Curbing Sea Level Rise, Study Says" [August 11, 2006].)
The problem is that people rarely go to the interior of Antarctica, and measurements of snowfall are sparse.
But
an upcoming project will take measurements on the ground and should
help settle the matter, Bamber said. Also, improving how climate models
treat the dynamic nature of glaciers and ice sheets is crucial, experts
say.
The head of the UN Intergovernmental Panel of Climate
Change recently called for improvements in estimates of ice loss in
Greenland and Antarctica.
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