*Clues to Rising Seas Are Hidden in Polar Ice*

By Juliet Eilperin
Washington Post Staff Writer
Monday, July 16, 2007; A06

Few consequences of global warming pose as severe a threat to human society
as sea-level rise. But scientists have yet to figure out how to predict it.

And not knowing what to expect, policymakers and others are hamstrung in
considering how to try to prevent it or prepare for it.

To calculate sea-level rise, the key thing researchers need to understand is
the behavior of the major ice sheets that cover
The disintegration of one would dramatically raise the ocean. But while
computer models now yield an increasingly sophisticated understanding of how
a warming atmosphere would behave, such models have yet to fully encapsulate
the complex processes that regulate ice sheet behavior.

"The question is: Can we predict sea level? And the answer is no," said
David Holland, who directs New York University's Center for Atmosphere Ocean
Science. Holland<>,
an oceanographer, added that this may mean researchers will just have to
watch the oceans to see what happens: "We may observe the change much more
than we ever predict it."

In its executive summary report for policymakers in February, the
Panel on Climate
composed of hundreds of leading climate scientists, barely hazarded a guess
on sea level, predicting that it would rise between 7.8 inches and two feet
by the end of the century. However, the United
panel -- which operated under the assumption that, by 2100, the Greenland
ice sheet would lose some mass but that the Antarctic ice sheet would gain
some -- did not venture a best estimate or an upper limit for possible
sea-level rise.

The panel could agree to say only there is a 50-50 chance that a global
temperature increase of between 1.8 and 7.2 degrees Fahrenheit would lead to
a partial melting of the ice sheets over a period of several hundred to
several thousand years.

Because so much is at stake -- a three-foot increase in sea level could turn
at least 60 million people into refugees, the World
-- ice sheet modelers are working furiously to try to unravel the
mystery of how these sheets accumulate and lose mass.

Michael Oppenheimer, a Princeton
of geosciences and international affairs, does make a prediction:
He figures that if the Greenland ice sheet disintegrates, sea level would
rise about 23 feet. If the West Antarctic sheet melts, as well, it would add
an additional 17 feet or so.

"If either of these ice sheets were to disintegrate, it would destroy
coastal civilization as we know it,"

One of the biggest challenges facing researchers is that ice sheets are
under "attack from the edges," in the words of Richard B. Alley, a Pennsylvania
State University<>geosciences
professor. Each sheet amounts to a pile of snow compressed over
time into a two-mile thick, continent-spanning sheet of ice, which spreads
out under its own weight, Alley said.

Near the coast, the pile develops quick-moving "ice streams," which flow
between slower-moving sections of ice and float out onto the ocean in an
"ice shelf." While recent satellite data have indicated that these ice
streams are flowing faster and delivering more water to the oceans, many
uncertainties remain.

David Vaughan, a glaciologist with the British Antarctic Survey in
said the terrain beneath the ice streams helps determine how they move, but
the contours of the land are largely unknown because it is buried so far
under the ice. The streams may run aground on elevated bedrock, slow down as
they move past rocky fjord walls or speed up as they move over mud.

"There's a continent of topography sitting under Antarctica," Vaughn said.
"Everything there has an impact on how the ice sheet flows, and very little
of that has been mapped."

Researchers are also trying to measure the layer of water that lies under
the ice sheets, as that also helps regulate ice stream flows.

"They're essentially afloat on their own sub-glacial water, even if there's
not much water there," said Garry Clarke, a glaciology professor at
the University
of British Columbia<>.
"We don't know very much about how water flows underneath ice sheets."

Another uncertainty is how much the oceans surrounding the ice sheets are
warming, something that is difficult to measure because the areas are
remote. Vaughan and his colleagues suspect that warmer waters around
Antarctica have contributed to melting the Western Antarctic ice sheet, but
there is little good data because few ships venture there.

Researchers are now going to extraordinary lengths to collect the data they
need. Holland at
returned from a trip to Greenland, where he was collecting
information about the Ilulissat glacier, which has doubled its speed over
the past decade as it flows toward the ocean and melts. To test the
temperature and salinity of the water surrounding the glacier, Holland and
other researchers had to hover in a helicopter and lower their instruments
into an opening in the ice.

"It's kind of beautiful, and scary and fun," he said.

Even with better data, scientists find it difficult to enter the information
into computer models. Most models do not attempt to calculate what could
happen to ice sheets at their edges.

Adding to the challenge, Oppenheimer said, is that models "are only good at
explaining things that happen at a large scale. Ice sheets are very complex
beasts, and the water moves at a very small scale."

Ice streams move along narrow channels, and plugging such detail into a
computer model takes a long time. But without that level of detail, the
results are incomplete.

Researchers have made some progress in ice sheet science over the past
decade by using satellites to measure the sheets' changing mass.

Last month, for example, a team of
university scientists used readings from NASA's QuikScat satellite to
measure snow accumulation and melt in Antarctica from July 1999 through July
2005. They discovered that broad areas of snow had melted in west Antarctica
in January 2005 in response to warmer temperatures. The finding was
surprising because Antarctica had shown relatively little warming in the
recent past.

Konrad Steffen, director of the Cooperative Institute for Research in
Environmental Sciences at the University of Colorado at
who led the study, said increases in snowmelt "definitely could have an
impact on larger-scale melting of Antarctica's ice sheets if they were
severe or sustained over time."

Because ice sheet modeling has not ranked as a high priority for government
laboratories and has not been integrated into large-scale climate models,
scientists from around the world are now collaborating to develop more
sophisticated models to inform policymakers about potential sea-level rise.
The researchers have convened two major meetings this year, one at the NOAA
Geophysical Fluid Dynamics Laboratory at Princeton University and one at the
University of Texas at
in an effort to generate a new generation of ice sheet models.

Vaughan, who attended both conferences, said he is hopeful that he and
others will solve the question of ice sheet modeling by the time he ends his
career: "It will be 15 years before I retire, and I want it nailed by then."

But other researchers are less optimistic. Holland, who like Vaughan is in
his mid-40s, doubts that scientists will master the problem before
greenhouse gas emissions trigger significant melting of the ice sheets that
he studies.

"We will get there eventually, but it won't be for a long time. It won't be
in my lifetime," Holland said. "There's no plan; there's no program. There's
no one responsible for sea-level rise."


Michael Balter
Contributing Correspondent, Science
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