Clues to Rising Seas Are Hidden in Polar Ice
Washington Post Staff Writer
Monday, July 16, 2007; Page 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 Greenland and Antarctica.
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 Intergovernmental Panel on Climate Change,
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 Nations-sponsored
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.
“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.
Graphic Melting and Running Greenland's Jakobshavn Isbrae glacier has retreated more than two miles since this satellite photo was taken in 2003. Graphic Anticipating a New Coastline Sea levels are slowly rising as global warming frees up fresh water locked in polar ice. The melting of ice sheets in Greenland and West Antarctica could raise sea levels by about 39 feet, a process that could take hundreds or thousands of years. |
“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 NYU
recently 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 NASA
and 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 Boulder,
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 Austin, 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.”