Friday, March 27, 2015

Antarctic ice shelves are melting dramatically, study finds

A new study published by Science and led by Scripps Institution of Oceanography at UC San Diego researchers has revealed that the thickness of Antarctica’s floating ice shelves has recently decreased by as much as 18 percent in certain areas over nearly two decades, providing new insights on how the Antarctic ice sheet is responding to climate change.
Data from nearly two decades of satellite missions have shown that the ice volume decline is accelerating.

From The Guardian by

The ice around the edge of Antarctica is melting faster than previously thought, potentially unlocking metres of sea-level rise in the long-term, researchers have warned.
A team of US scientists looked at 18 years’ worth of satellite data and found the floating ice shelves that skirt the continent are losing 310km3 of ice every year.
One shelf lost 18% of its thickness during the period.
The loss of ice shelves does not contribute much directly to sea level rise.
But they act like a cork in a bottle at the point where glaciers meet the sea – jamming the flow of ice from the massive ice sheets of east and west Antarctica.

 Satellite view of a large iceberg separating from Antarctica’s Pine Island Glacier, where ice loss has doubled in speed over the last 20 years.
Photograph: MODIS/Aqua/NASA

Professor Andrew Shepherd, director of the Centre for Polar Observation and Modelling at the University of Leeds, said the rates of ice loss were unsustainable and could cause a major collapse.
This is already occurring at the massive Pine Island glacier, where ice loss has doubled in speed over the last 20 years as its blocking ice shelf has melted.
“This is a real concern, because such high rates of thinning cannot be sustained for much longer, and because in the places where Antarctic ice shelves have already collapsed this has triggered rapid increases in the rate of ice loss from glaciers above ground, causing global sea levels to rise,” he said. 

 Changes to the thickness and volume of Antarctica's ice shelves between 1994 and 2012.
Credit: Paolo, et al./Science
The new research, published in the journal Science on Thursday, discovered for the first time that ice shelf melt is accelerating.
Dr Paul Holland, a climate scientist at the British Antarctic Survey (BAS), said the loss of the shelves would speed the complete collapse of the west Antarctic ice sheet, which would eventually cause up to 3.5m of sea level rise.
But he said it was highly unlikely this would occur this century.
He said the “worst case scenario” for 2100 was that ice sheets would contribute an additional 70cm to the sea level rise caused by the warming of the ocean.

 Antarctica's Brunt Ice Shelf.
Credit: Michael Studinger/NASA.

The UN’s climate science body has not previously included the ice sheets of Antarctica and Greenland in its predictions for future sea level rise because scientists are not certain how fast they will slide into the ocean.

 Pine Island Glacier on Sentinel-1A’s radar
This image combining two scans by Sentinel-1A’s radar shows that parts of the Pine Island glacier flowed about 100 m (in pink) between 3 March and 15 March 2015.
Light blue represents stable ice on either side of the stream.
Pine Island is the largest glacier in the West Antarctic Ice Sheet and one of the fastest ice streams on the continent, with an average of over 4 km per year.
About a tenth of the ice sheet drains out to the sea by way of this glacier.
With its all-weather, day and night radar vision, the Sentinel-1 mission is an important tool for monitoring polar regions and the effects that climate change has on ice.

Holland said: “What humanity needs to know is what’s the sea level rise in 2100 and the biggest source of uncertainty in that is what’s going to happen to the ice sheets.”
Over the past decade the loss of ice shelf volume in Antarctica increased from 25km3 to 310km3 every year.
It is unclear whether the loss of ice is directly related to man-made climate change or a cyclical change in ocean currents.
But the extra sea level rise from ice sheets will exacerbate the rise caused by the expansion of oceans as the world warms.
Professor David Vaughan, director of science at BAS, said the findings would help scientists to make more accurate predictions about future sea level rise.
“The rate of ice loss, especially when considered in terms of the percentage of ice lost in the last two decades, is dramatic. This research is a significant step towards improving our ability to predict the future of the Antarctic ice sheet and its contribution to global sea level rise.”

 Schematic diagram of an Antarctic ice shelf showing the processes causing the volume changes measured by satellites.
Ice is added to the ice shelf by glaciers flowing off the continent and by snowfall that compresses to form ice.
Ice is lost when icebergs break off the ice front, and by melting in some regions as warm water flows into the ocean cavity under the ice shelf.
Under some ice shelves, cold and fresh meltwater rises to a point where it refreezes onto the ice shelf.
Helen Amanda Fricker, Professor, Scripps Institution of Oceanography, UC San Diego, Author provided

The western coast ice shelves contributed the majority of the ice loss.
The rate of loss increased by 70% in the last decade.
Two shelves in this region could completely disappear within a century.
Conversely, there were some areas in east Antarctica where the shelves stayed stable or grew slightly. Vaughan said the regional variations were predicted by previous studies.
Holland said it was important not to confuse floating ice shelves, which can be up to 2km thick, with the much thinner sea ice.
The one metre thick layer of sea ice around Antarctica has been expanding in recent decades, which some scientists think is because of increasing polar winds, which push the ice further out.

Links :
  • Climate Central : Antarctica’s Icy ‘Doorstops’ Thin; Rising Seas At Risk
  • Scientific America : Antarctica's Ice Shelves Thin, Threaten Significant Sea Level Rise
  • Washington Post : Antarctica’s floating ice shelves, the doorstop of the continent, are melting away
  • NPR : Big Shelves Of Antarctic Ice Melting Faster Than Scientists Thought
  • The Conversation : Shrinking of Antarctic ice shelves is accelerating

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