Sunday, May 24, 2009

Ice Sheets Calve, Get Used To It

In early April 2009 a large portion of the Wilkins Ice Shelf in Antarctica broke away. And Jennifer Marohasy, at was moved to ask;

"Given it is still really cold at the Antarctic, is it really very scientific to blame global warming for the likely collapse of the Wilkins Ice Shelf? How good is the evidence supporting this hypothesis as the possible cause of the possible shelf collapse, as opposed to, say, ice growth generating internal stress or undersea volcanic activity?"
To which I responded, "Ice sheets calve, get used to it."

The reason they calve is that the annual precipitation on the Antarctic coast is over 200mm a year, or 2 metres per decade, or 10 metres every half century. If the ice sheet was not attached to land based ice, and water temperatures remained below zero C, then the floating sheet would drop by 9 metres every half century as the total ice mass adjusts equilibrium in water.

But when the ice sheet is attached to land ice it cannot adjust it's mass in water (sink) without severing the attachment to the land ice. Ten metres of ice weighs 9 tonnes per square metre, 90,000 tonnes per hectare and 9 million tonnes per square kilometre. And that makes for an absolute $hitload of weight to be defying gravity with.

And the wider the ice shelf is, the more powerful the leverage becomes at the interface between land and shelf ice.

Interestingly, ocean surface temperatures for this area in March 2009 were zero C. Below the surface, at the bottom of the floating ice, the temperature was lower still. And if the break had taken place in January there may have been a very slim case in support of warming as the cause of the calving. But Southern Hemisphere temperatures have declined over the past decade to a greater extent than Northern Hemisphere temperatures.

So how can the Wilkins Ice Shelf calving be caused by global warming when there has been no warming for the past ten years?"

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At July 01, 2009 9:46 pm, Blogger papertiger said...

I'm reading The Ends of the Earth by Francis Spufford. She recounts "the fractured white bands of ice shelf" in the chapter where she discusses the heroic age of exploration, which is shorthand for the first expeditions to the continent. Look for Apsley Cherry-Gerard's Worst Journey in the World or Nobu Shirase's Nankyokuki. One or the other has an eyewitness account of ice shelf cracking, such as is balleyhood by the media with the Wilkins ice shelf, from the 1910-20 period.

At July 03, 2009 10:17 am, Blogger Ian Mott said...

Thanks for the references, Papertiger, I'll try to look them up.


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