By Marc Airhart, Jackson School of Geosciences, The University of Texas at Austin
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.
© iStockphoto / Giorgio Fochesato / NASA
“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.
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.
Pulling the Plug?
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.”
It came from the deep
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.
A new hypothesis
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.
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.
This article is from this site: http://geology.com/research/west-antarctic-ice-sheet.shtml
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