U.S., Kiwi scientists team up to look at stability of ice shelf
Posted September 3, 2010
It’s been about a dozen years since Howard Conway and colleagues used a snowmobile to pull a radar system on a sled across Antarctica’s Roosevelt Island, a 7,500-square-kilometer dome of ice in the northeastern corner of the Ross Ice Shelf.
Their measurements of ice thickness, stratigraphy, accumulation rate and other parameters allowed them to estimate the age of the ice and the thinning history of the island. That information, when combined with other data from the Ross Sea Embayment, created a rough timeline for the deglaciation of the West Antarctic Ice Sheet (WAIS).
Their work, published in the journal Science in 1999, suggested the grounding line where the ice sheet meets the sea floor retreated rather rapidly since the Last Glacial Maximum about 20,000 years ago when ice sheets draped both hemispheres.
In addition, results of the study implied that the retreat of the WAIS was part of a natural event triggered well before anthropogenic warming got involved in the climate picture over the last couple of centuries. The paper concluded that if the present rate of natural deglaciation continues, the WAIS would disintegrate in about 7,000 years, which would raise sea level by 5 to 6 meters.
In 2010, Conway will return to Roosevelt Island with an international team of scientists to refine those measurements of past melting to understand how the region may respond to future warming.
“We need a model to look at the future, so we need ground-truth data to make sure our models are right,” explained Conway, a soft-spoken glaciologist and research professor at the University of Washington. They will use radar and GPS to measure thinning and accumulation rates across the Island.
Conway’s three-member team will be based at a field camp set up by Antarctica New Zealand, with air transportation to and from the site provided by the U.S. Antarctic Program. The Kiwi team is drilling a 750-meter-deep ice core from the top of the island’s southern summit.
The ice contains properties that scientists can analyze to recreate past climatic and atmospheric conditions. In the case of Roosevelt Island, researchers expect to retrieve an annual history for the last 30,000 years, from the end of the last glacial period through the abrupt warming of the Holocene.
We want to “evaluate the stability of the Ross Ice Shelf in a warming world, and with that, help constrain future Antarctic contribution to sea-level rise,” said Nancy Bertler, a researcher from the Joint Antarctic Research Centre at Victoria University and GNS Science, who leads the New Zealand team, via e-mail.
“This information is particularly important for the next IPCC report,” she added, referring to the Intergovernmental Panel on Climate Change, a scientific body that assesses the current state of the climate.
The fourth IPCC assessment in 2007 concluded that most of the observed increases in globally averaged temperatures since the mid-20th century were “very likely” due to anthropogenic, or human-induced, increases in greenhouse gas concentrations. The reported estimate sea levels would probably rise by 18 to 59 centimeters.
However, that calculation didn’t factor in the accelerated loss from polar regions. Both Greenland and West Antarctica are losing mass to the ocean much more rapidly than expected, Bertler said. The Antarctic Peninsula, in particular, is one of the fastest-warming regions on the planet, with several ice shelves having collapsed in the last several decades.
The disappearance of an ice shelf allows the glaciers and ice streams that flow into them to move more rapidly, speeding the draining of the ice sheet.
“As we see these smaller ice shelves disappear, we are concerned at what temperature threshold and over what time period the world’s largest ice shelf, the Ross Ice Shelf, could disintegrate and allow West Antarctic ice to accelerate into the ocean, exacerbating global sea level rise,” Bertler said.
Hence the need for the international Roosevelt Island Climate Evolution (RICE) project, which also includes Danish, British, German, Italian and Australian collaborators.
In fact, the Ross Ice Shelf has disappeared in the past based on previous research, most recently by the ANDRILL program, which drilled sediment cores in the Ross Sea Embayment during the 2006-07 and 2007-08 field seasons. The data from one core suggested that during the mid-Pliocene, about 3.5 million years ago, the WAIS periodically collapsed.
Global temperatures were higher during that time, and atmospheric carbon dioxide levels were about 400 parts per million (ppm). CO2 levels are about 393 ppm today.
“There are no annually resolved climate records from the Pliocene period,” Bertler noted. “To provide constraints on the rate of change, the RICE project attempts the next best thing — it seeks to reconstruct the behavior of the Ross Ice Shelf retreat during a time of rapid change.”
That would be the past 30,000 years, when global temperatures increased by 6 degrees centigrade and global sea level rose by about 120 meters. However, the Ross Ice Shelf retreat occurred predominantly during the past 8,000 years, when global temperatures stabilized.
“We will correlate reconstructed local climate conditions with the precisely dated retreat history,” Bertler said. “The RICE record will provide important knowledge to improve predictions on the future behavior of the Ross Ice Shelf and will help to advance models to improve estimates of future change.”
Conway said scientists believe that melting under the ice shelf helps drive the changes in the ice sheet. In addition, rising sea level caused by deglaciation in the Northern Hemisphere also causes the grounding line to retreat.
“However, it takes some time for the ice sheet to respond to these changes. We are trying to improve our understanding of the response time to different changes,” he said.
New Zealand designed and built a new drill for the project based on a Danish design, called the Hans Tausen Drill. Alex Pyne, projects manager at the Antarctic Research Centre’s science drilling office, was behind the effort. Pyne also served as the drilling science coordinator and drill site manager for ANDRILL.
The drill has a reach of 1,000 meters, which makes it an intermediate drill in the world of ice-core drilling. In comparison, the Deep Ice Sheet Core (DISC) drill being used for a project at WAIS Divide by the USAP is drilling an ice core almost 3.5 kilometers deep.
However, the Kiwi drill doesn’t have the luxury of being transported in large cargo aircraft. All the equipment for the camp must fit into small ski-equipped Twin Otter or Basler aircraft.
“The technological challenge was to keep the system lightweight and individual pieces small enough [so] that this was possible,” Bertler said.
Noted Conway, “It is a lighter camp than an American camp.”
Conway’s team will spend about a month on the island, named after President Franklin D. Roosevelt by the famous polar explorer Rear Adm. Richard E. Byrd in 1934 after its discovery. He hopes for better weather than what they had in 1997-98.
“We’re hoping it won’t be too stormy. It’s not as stormy as WAIS Divide,” he said. “It’s very exciting. I’m thrilled and feel very privileged to be going back there and collaborating with [New Zealand].”
NSF-funded research in this story: Howard Conway, University of Washington, Award No. 0944307.
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