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Polar ice experts once thought Antarctica's
ice sheets were mostly immune to climate change. Research findings
of the past decade have started to melt away their confidence.
Satellites have revealed that the ice sheets are thinning and their
glacial slide into the sea is speeding up. Ice cores show that at times in
the geologic past, Antarctica was ice free. Complicating matters, the
West Antarctic Ice Sheet (WAIS), a mass of ice the size of Texas storing
enough water to raise global sea level by 5 meters (about 17 feet),
is resting on rock below sea level.
“Not just a bit below sea level, it's 2,000 meters below sea level,”
said David Vaughan, a principal investigator with the British Antarctic
Survey. “If there was no ice sheet there, this would be deep ocean,
deep like the middle of the Atlantic.”
Some scientists have theorized that this makes the WAIS inherently
unstable. If the ice sheet retreats beyond a certain point, a positive
feedback mechanism should, they say, lead to runaway retreat
that would not stop until most of the ice sheet disappears.
Antarctic Ice Flow and Sea Level Rise
The recent series of reports from the Intergovernmental Panel on
Climate Change (IPCC) did not include such bold predictions for the
possible loss of Antarctic ice. The IPCC's estimate was that Antarctic
ice flow would continue at the same rate it did from 1993 to 2003,
despite an observed acceleration since then.
The IPCC's restrained estimate about the ice flow, and its possible
contribution to sea level rise, was not, however, a heartening sign.
Rather, it reflected the consensus view that changes in the Antarctic
have been so rapid, science can not yet account for them.
“Models used to date do not include . . . the full effects of changes
in ice sheet flow, because a basis in published literature is lacking,”
stated the reports. “[U]nderstanding of these effects is too limited to
assess their likelihood or provide a best estimate or an upper bound
for sea level rise.”
The IPCC's mid-range projection is that seas will rise 44 centimeters
(17 inches) by the year 2050. That would put 100 million people
each year at risk from being displaced from their homes by coastal
flooding. If the WAIS were to entirely melt—which most experts
doubt will happen in our lifetimes—seas would rise ten times higher.
Will the WAIS sit on the global warming sidelines? Will it gradually
drip away, speeding up the slow motion flooding of our coasts?
Or will it collapse in front of our eyes? And when will scientists know
for sure?
These questions inspired an international conference held at
The University of Texas at Austin last March, co-sponsored by the
U.K. Department of Food and Rural Affairs and the Jackson School
of Geosciences. While not arriving at definitive answers, the participants
in the West Antarctic Links to Sea-level Estimation (WALSE)
Workshop developed a new hypothesis to explain recent observations
of ice sheet thinning and charted a course for future research that
might be incorporated into a new National Science Foundation polar
research initiative.
The Amundsen Sea Embayment
Don Blankenship and Jack Holt, polar researchers at the Jackson
School's Institute for Geophysics, are especially concerned about the
Amundsen Sea Embayment, a vast block of ice that makes up one third
of the WAIS. Recent satellite observations show the embayment
is the most rapidly changing portion of the WAIS. It's also thought to
contribute as much to sea level rise as the entire Greenland Ice Sheet.
Blankenship and Holt led the American half of a joint project
between the Institute for Geophysics and the British Antarctic Survey
in 2004 to reveal what lies below the Amundsen Sea Embayment.
Using airplanes with radar antennas strapped under the wings and
logging tens of thousands of air miles in a couple of months, the two
teams were able to create detailed topographic maps of the rocks and
sediment that form the bed on which miles of ice sits. The researchers
even identified lakes of liquid water which remain unfrozen due to
the enormous pressures of the ice above.
One alarming result of that work was the discovery that part of
the embayment known as Thwaites Glacier is not only experiencing
accelerated thinning, but it also acts as a sort of plug in the bath tub.
“Thwaites Glacier has access to the rest of the ice
sheet,” said Blankenship. “So changes there can propagate
to the interior and indeed we have an avenue
for draining all of the ice from West Antarctica into
the ocean via Thwaites Glacier.”
The topography of the bed underneath doesn't
provide any additional protection to hold the ice back.
“The bedrock goes very deep a long way inland
and even provides a mechanism for ice to connect
through from the other side of the ice divide,” said
Holt. “There's no big impediment there. This was
somewhat of a surprise.”
Antarctica is Melting Below Sea Level
A lay person hearing that the melting of the West
Antarctic Ice Sheet is speeding up might not be all
that surprised given the routine nature over the past
few years of news reports describing how the greenhouse
effect is warming our atmosphere, speeding
the arrival of spring, melting glaciers, and altering
plant and animal ranges.
For Antarctica, the emerging picture is far more
complex than the headlines. If the hypotheses of
polar experts like Blankenship and Holt are correct,
Antarctica might resemble less a block of ice liquefying
in a sunny greenhouse than a cog in an intricate
Rube Goldberg machine.
The surface of Antarctica is so cold and the ice so thick that raising
the region's air temperature a few degrees is not enough to cause
significant melting. Instead, scientists have long suspected that warm
water in the Amundsen Sea is flowing up under ice shelves—platforms
of floating ice attached to the grounded ice sheet—and melting
them from below. This increased melting speeds the flow of grounded
ice sheet into the water.
But it's unlikely these warmer waters result directly from recent
climate change. By measuring oxygen content, oceanographers have
discovered that the warm water welling up below the glaciers has
not been near the sea surface in the past few centuries. In oceanographer's
terms, the water is “old.” It is part of a mass known as Circumpolar
Deep Water connected to the North Atlantic through the
globetrotting ocean conveyor belt. This water has been at depth for
too long, scientists believe, for its temperature to reflect recent global
warming.
Polar scientists meeting at the three-day WALSE Workshop knew
that explaining this upwelling could go a long way towards predicting
the future of the WAIS. Fortunately, the workshop brought
together experts in atmosphere, oceans, and ice—all critical players
in this story.
New Hypothesis on Atmospheric Currents
Adrian Jenkins, a polar researcher from the British Antarctic Survey
and WALSE participant, developed a computer model that showed a
possible solution.
Antarctica is encircled by atmospheric currents that largely insulate
it from the rest of Earth's climate and keep it colder than it otherwise
would be. Jenkins' model showed that these circumpolar currents,
sometimes called “Westerlies,” “the Screaming 50s,” or “the
Roaring 40s,” actually push surface waters out away from the continent.
This results from the Coriolis Force, the byproduct of Earth's
rotation that causes cyclonic systems to turn counterclockwise in the
northern hemisphere and clockwise in the southern hemisphere. As
surface water is pushed away, warm deep water rises to replace it.
If the atmospheric currents speed up, more water is pulled up.
Indeed, observations indicate these atmospheric currents have sped
up in recent decades in response to global warming. So increased
upwelling seems likely.
There isn't enough observational data to validate this hypothesis
yet. For one thing, sea ice makes it difficult to get there to do the work.
Polar experts say repeated missions over several years are necessary to
correlate wind speeds with the temperature structure of the water.
Links Between Atmosphere, Ocean & Ice
Blankenship said when the workshop began, fewer than five attendees
suspected this link between atmosphere, ocean, and ice; by the
end, all 25 agreed it was the most plausible explanation. He said each
person was an expert in one, maybe two areas.
“But to say that atmospheric changes are causing the ocean
changes that are causing ice sheet changes, that requires more self
confidence than most of the people had,” he said. “That could only
happen by bringing together so many people with overlapping skill
sets. The result was a surprise and a significant moment. We all
agreed that was the most likely answer.”
Where to Now?
On the final day of the WALSE workshop, the attendees locked themselves
in a conference room and hashed out a consensus statement
including the state of knowledge in their field, the new hypothesis on
the cause of upwelling, and a list of challenges that lie ahead in
answering the outstanding questions.
In a draft article for EOS magazine, the participants wrote an
ambitious to-do list for their community: collect baseline oceanographic
data from the Amundsen Sea to begin charting changes that
might relate to ice sheet melting; create a better history of deglaciation
by dating marine sediment cores and rock exposure ages; create
more realistic ice sheet models; couple climate models with ice sheet
models; develop better tools for measuring ice sheet mass balance
with satellites; and restore satellite capability that was lost in 2000 for
measuring grounding-line retreat rates. (The grounding line is where
an ice sheet goes afloat. Behind the line is grounded ice sheet, beyond
that is floating ice shelf.)
The new IPCC reports on climate change had essentially sidestepped
the issue of Antarctica's potential contribution to sea level
rise. The authors pointed out, rightly, that there was just too much
uncertainty to make predictions. The workshop participants were able
to say, Okay, now what are we going to do about it?
Blankenship said the timing of the workshop was perfect.
“Two months later, we were sitting on the 12th floor of NSF presenting
the WALSE conclusions to 30 polar scientists on what to do
for the next decade in polar science,” he said.
The National Science Foundation (NSF) had organized the meeting
to start charting the course of a new interdisciplinary program
called Antarctic Integrated and Systems Science. The program is part
of the International Polar Year (2007-2008), a global campaign of
polar research.
“There was no forum to work on problems that were that complex
and interdisciplinary,” said Blankenship. “And chances are, that
will show up in next year's NSF budget. That's what WALSE did.
That's what it was intended to do.”
by Marc Airhart
For more information about the Jackson School contact J.B. Bird at jbird@jsg.utexas.edu,
512-232-9623 - or visit their website.
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