Friday, July 20, 2012

Glaciers calving : cause for worry ?

The Petermann Glacier in northern Greenland has calved an iceberg twice the size of Manhattan, scientists say.
Images from a
NASA satellite show the island breaking off a tongue of ice that extends at the end of the glacier.

From BBC

The increasingly detailed and immediate pictures that we get of vast movements of ice at the Earth's poles make for a dramatic sight.

 Polar explorer Eric Philips looks down into one of the 'cracks' in the Petermann Glacier last year

But whether that drama is cause for worry is an open question.

Floating "tongues" of ice, like the one that has broken off the Petermann glacier reported on Thursday, extend beyond the glaciers, and are constantly fed by ice pouring off the ice sheet.
Eventually parts of these tongues break off under the forces surrounding them and become free-floating icebergs.

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Thousands of these "calving events" happen in Greenland each year, ranging from the unremarkable to the striking.

On August 5, 2010, an enormous chunk of ice, roughly 97 square miles in size, broke off the Petermann Glacier, along the northwestern coast of Greenland. The glacier lost about one-quarter of its 40-mile long floating ice shelf, the Northern Hemisphere's largest. It's not unusual for large icebergs to calve off the Petermann Glacier, but this new one is the largest to form in the Arctic since 1962. 

This most recent calving is neither of the two in terms of size; with an area of roughly 120 sq km (46 sq mi), it is about half the size of the iceberg that broke off the same glacier in 2010.
Looming larger in the future is the breakup of the Pine Island Glacier in Antarctica - which already shows a 30km-long crack - which could happen any day.
And they get far, far bigger than that.

Northern exposure

It is important to keep in mind that this is a natural and periodic process that has been going on since long before we were here to snap satellite photos.
What is at issue is whether or not the frequency of the events is changing, and why.

In the debate surrounding those questions, there are facts, educated guesses, and worrying trends.
The truth is that the questions are devilishly difficult to answer - the stability of ice sheets at both the Earth's poles depends on a wide range of factors: atmospheric temperatures, the temperatures of the surface of the sea, and the degree of sea ice cover, to name just a few.

The Pine Island glacier will eventually cut loose an 800sq km iceberg

One fact is that the Petermann glacier's margins have now retreated to a point not seen in the last 150 years.

Another is that Greenland has over the last two decades experienced a significantly higher atmospheric warming than the global average.
Yet another is that over that same period, the south of Greenland has seen "mass loss" - both the kind of calving seen at Petermann and simple melting of surface ice - increasing year on year.
And levels of Arctic sea ice cover - which can literally shore up some of Greenland's ice sheet - are on track to be among the lowest ever recorded.

But from there, it becomes educated guesses.

Estimates of the speed of mass loss, for example, have gone back and forth among groups of scientists (and the 2011 edition of the revered Times Atlas got it wrong altogether).

And it remains guesswork to determine whether that worrying mass-loss trend is making its way northward.
"To date, we've not really seen such a strong signal in northern Greenland - and Petermann is right at the northern limit of the ice sheet," said Jonathan Bamber, director of the Bristol Glaciology Centre at the University of Bristol.
"I think it's too early to say that this is the start of increased mass loss from northern Greenland - but it's certainly not good news," Prof Bamber told BBC News.
"Whether it's news that we should be particularly concerned about I think is difficult to answer at this stage."

'Not surprising'

In truth, the buildup and breakup of glaciers can change over timescales ranging from months to millennia, and scientists simply haven't been watching this closely for very long.

As is so often the case in science, what is needed are more observations.
"If we start seeing patterns of the calving front retreating over a range of quite a few glaciers in northern Greenland in the same sector, then we would be quite concerned - and that would be a strong indicator that there's a change taking place, related to either atmospheric warming or ocean warming," Prof Bamber said.

For now, glaciologists - working with Earth and climate scientists of every stripe - are loath to make an explicit connection between iceberg formation and the broader debate around climate change, and this is where worrying trends come in again.
"I can't say that yes, this calving event is a consequence of the marked atmospheric warming that's been taking place over Greenland in the recent decades, but it is certainly more likely to take place as a consequence of that warming," Prof Bamber said.
"You don't have to be a rocket scientist to realise that in a warming world, ice is going to melt - so it's not surprising that Greenland is responding."

Links :
  • TheGuardian : Greenland glacier calves iceberg twice the size of Manhattan
  • ESA : Earth from Space: Pine Island cracked
  • UDEL : Greenland glacier loses ice
  • Icy Seas :  New Petermann Ice Island forming July-16, 2012

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