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Antarctic ice shelves 'tearing apart', says study

By Matthew Knight, CNN
March 29, 2012 -- Updated 0248 GMT (1048 HKT)
The birth of an iceberg on Pine Island Glacier in West Antarctica in October 2011.
The birth of an iceberg on Pine Island Glacier in West Antarctica in October 2011.
STORY HIGHLIGHTS
  • New satellite survey of West Antarctica ice shelves has revealed fracturing away from land
  • Nearly 40 years of satellite data give most comprehensive snapshot of ice shelf evolution
  • West Antarctic ice shelf melting adds 2mm to sea levels each year

(CNN) -- A new satellite study of ice shelves in West Antarctica has revealed they are steadily losing their grip with adjacent land and could intensify the acceleration of ice loss in the area.

The ice shelves (floating extensions of land-based ice sheets) in the eastern Amundsen Sea Embayment are fracturing at their margins on rocky bay walls, according to glaciologists from the University of Texas at Austin's Institute for Geophysics (UTIG).

Lead author of the study Joseph MacGregor said in a statement: "Typically, the leading edge of an ice shelf moves forward steadily over time, retreating episodically when an iceberg calves off (breaks off and floats out to sea), but that is not what happened along the shear margins."

"Anyone can examine this region in Google Earth and see a snapshot of the same satellite data we used, but only through examination of the whole satellite record is it possible to distinguish long-term change from cyclical calving," MacGregor added.

The study, which examined satellite data from 1972 to the end of 2011, is the most comprehensive yet of ice shelf evolution say the scientists, and reveal substantial changes which were "especially rapid" during the past decade.

An accelerating glacier can tear away from its margins, creating rifts that negate the margins' resistance to ice flow and causing additional acceleration
Ginny Catania, University of Texas at Austin

The shear margins which bind the ice shelves laterally are now heavily rifted they say, resembling cracks in a mirror when observed in satellite images.

"As a glacier goes afloat, becoming an ice shelf, its flow is resisted partly by the margins, which are the bay walls or the seams where two glaciers merge," Ginny Catania, assistant professor at UTIG said in a statement.

"An accelerating glacier can tear away from its margins, creating rifts that negate the margins' resistance to ice flow and causing additional acceleration," she added.

Interactive: Sea-level change map

The ice shelves in West Antarctica, which include the floating extensions of the Thwaites and Pine Island Glaciers, are already losing volume say scientists.

As they get smaller they become less able to hold back grounded ice upstream, according to scientists whose study is published in the Journal of Glaciology.

Satellite imaging in what is a particularly inhospitable, heavily crevassed part of the continent is vital for ice shelf research, says Hamish Pritchard of the British Antarctic Survey.

"We've suspected for quite a long time that the glaciers are fairly sensitive to what happens with ice shelves. Big changes are happening along the coast," Pritchard said.

"There are certain large glaciers (like Thwaites and Pine Island) that have been accelerating for the past 20 years or so -- they are losing a lot of ice into the sea and they are thinning," he added.

West Antarctica is particularly interesting because of what could happen in the future, Pritchard says.

"The ice sheet is actually sitting on the sea floor -- 2,000-3,000 meters deep in some places," he said.

"That means if you thin the ice enough by draining the ice away from the edges, what could happen is the ice could just start to float off and the whole ice sheet could collapse, and quite quickly."

Melting in West Antarctica is contributing about 1-2 millimeters a year to sea levels but that could rise if the acceleration trend continues, Pritchard says.

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