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Thursday, 3 February 2011

The demise of an ice sheet?

The expansion of the melt zone in SW Greenland is speeding up every year, as the number of days with above average temperatures goes up.  Since 2003 this melt zone has increased even more.  Look closely at the ice surface -- it is pockmarked with meltwater lakes.  Fifty years ago meltwater lakes of this type were virtually unknown.
In addition to the increasing density of surface lakes, meltwater is now starting to flow on the surface of the ice sheet in open channels -- as in the satellite image below.  Some of these open channels coincide with moraine bands -- possibly the reduced albedo and locally increased temperature has something to do with it.  But many channels also flow on clean ice, possibly following fracture patterns in the ice.
Surface meltwater finds its way into the interior of the ice sheet via open holes called moulins.  Some of the meltwater rivers flowing into these moulins are so huge that they are comparable with Niagara Falls in the volume of water discharged each day.
This is truly catastrophic melting -- nothing like it has ever been seen before.  Note from the above map that the summer melt zone now stretches right across the ice sheet summit in southern Greenland.  Some researchers think that this part of the ice sheet is about to disintegrate catastrophically, leaving a remnant in north Greenland which will be about half the size of the ice sheet of the 1950's.

The massive amounts of meltwater now pouring off the ice sheet have a number of effects.  One is increased lubrication on the ice sheet bed, which means accelerated ice flow out towards the coast.  Some of the big outlet glaciers are now flowing at eight times their previous speed.  As ice is evacuated there is another effect -- the lowering of the ice sheet surface.  This of course leads to increased melting on vast expanses of the ice sheet surface that have low gradients -- and melting rates are also enhanced by the reduced albedo of the ice sheet surface, as extensive spreads of water absorb atmospheric heat at a faster rate than clean white ice sheet surfaces.  This is called a positive feedback mechanism.
This is of course an appalling situation, confirming the worst fears of climate scientists on the subject of man-made climate change.  I also think we are seeing exactly how an ice sheet begins its catastrophic collapse -- and this will tell us a lot about how the British-Irish Ice Sheet ended its days at the end of the Devensian (and probably earlier) glaciations.

2 comments:

Constantinos Ragazas said...

Brian,

What do you think of the article you posted as a link under your “News” section in your blog? The claim made there is that this stone alignment oriented with the summer solstice sun was made by Australian aborigines some 10,000 years ago as a sundial marking the passing of seasons! That is more than five thousand years older than the dates for Stonehenge!

Still insist that these stone alignments are evidence of 'superior Culture' and not 'exceptional Nature' as I claim? That prehistoric Australian aborigines 10,000 years ago were even more advanced than Neolithic Britons 5,000 years ago?

The time given in the article, however, matches very well with my claim that around 10,000 years ago and following the Younger Dryas, the earth was experiencing very unusual geomorphology involving ice sheets. Brian, as a geomorphologist you should especially be excited by this prospect!

Think outside the glacier ice box, Brian, to see the picture I am proposing! To paraphrase Frank Sinatra, “if I can make it here, I can make it anywhere”.

Constantinos

BRIAN JOHN said...

I didn't post this --Blogger does it automatically, whenever it finds anything related to Stonehenge! Interesting report -- rather wild speculation regarding age, but if it's true I don't have a problem with it. Why should ancient societies not have wanted to mark the cardinal compass points and rising / setting sun positions on the longest / shortest days of the year? Not difficult to do -- that just requires long observations. And if they then used these marker stones in their rituals, that's quite expected too.

This has nothing whatsoever to do with Stonehenge, and does nothing to confirm that there was any "unusual geomorphology" (whatever that is) going on.....