Friday, February 1, 2013

Melting Ice Sheets...Cyclical or Unprecedented?

We know, thanks to the findings from numerous recent reports, that Antarctica’s Western Antarctic Ice Sheet (WAIS) is currently melting faster than previously thought, and may eventually lead to a three to five meter rise in sea level.   This is largely because the Pine Island Glacier, which acts to regulate much of the flow ice from the WAIS to the ocean, is simultaneously retreating, i.e.melting.  Dr. Claus-Dieter Hillenbrand, of the British Antarctic Survey, and his team, have raised the question, is the melting in this region completely unprecedented, or a cyclical phenomenon?   Hillenbrand and his team may have found a new way to answer this question, and predict the rate of future changes for the WAIS, by analyzing the area’s geological past.

What’s going on?  
The ice masses that we are concerned with are the Pine Island Glacier and the Western Antarctic Ice Sheet (WAIS).  Ice flows out from Antarctica to the sea.  The Pine Island Glacier acts as a bottleneck, slowing down the flow of ice that would drain into the Amundsen Sea.   As the Pine Island Glacier blocked the flow of ice out of Antarctica, this lead to a buildup of the huge mass of ice that is the WAIS.  

Lately though, the Pine Island Glacier has been showing signs of consistent rapid retreat. This retreat is accelerating the flow of ice from the WAIS out into the Amundsen Sea, at a rate that contributes an estimated 0.15 to 0.30 mm to global sea level rise, per year.   

What did the researchers do?
Hillenbrand and his team, extracted and examined three marine sediment cores which they pulled from Amundsen Sea Embayment, where the Pine Island Glacier lies.  Looking at microfossils in the mud from those cores, the researchers wanted to pinpoint when and where ice covered the bay.  Using data from the cores, the researchers studied the average rate of glacial retreat since the end of the last ice age, around 12,000 years ago.   The researchers chose core-extraction sites based on how ice shelves in that area had been positioned in the past.  

What did they find out?
By looking at the distance from the core locations to the current position of the glacier, as well as the age and depth of the extracted sediments, the researchers were able to get a clearer picture of whether and to what extent the area had shown signs of glacial retreat, in the last 12,000 years.  

Essentially, what's happening is abnormal, but not quite unprecedented. In the last 12,000 years the researchers show that glaciers had retreated about 96 kilometers, but in the last 20 years the glaciers retreated 25 kilometers.  That is a tremendous acceleration in glacial retreat.  However, their findings also showed other similar but rare sprints in glacier retreat over the last 12,000 years.  Whether we are in the middle of one of those sprints is up for debate.

What’s next?
Now we want to figure out exactly why this is happening.  Climate change has led to shifts in wind patterns and ocean currents.  We want to know if these or some other oceanographic or atmospheric shifts have lead to this accelerated glacial melt.  

So what?
This work will help us to make far more accurate models for predicting future ice loss with the Western Antarctic Ice Shelf.  Thus we will have a better idea of how to predict the rate of global sea level rise, into the coming decades.

You can find Hillenbrand et al’s original paper here

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