BLOGGER'S NOTE: I am updating this post because I found a more lucid explanation on the NewScientist site, which discusses the importance of wave action on the ice shelf.
Antarctic ice shelf 'hanging by a thread'
by Catherine Brahic, 25 March 2008, NewScientist news serviceA thin strip of ice, just 6 kilometres wide, is all that is holding back the collapse of a huge ice shelf in Antarctica, according to glaciologists.
The Wilkins ice shelf – previously some 16,000 square kilometres in area – has been disintegrating fast. On 28 February, an iceberg 41 km long and 2.5 km wide broke off the ice shelf. This triggered the runaway disintegration of a further 570 square kilometres of ice.
"I would be very surprised if it survives more than a couple more melt seasons," says Ted Scambos of the University of Colorado, US.
Other researchers, including David Vaughan of the British Antarctic Survey, believe it could be gone within weeks. "The ice shelf is hanging by a thread – we'll know in the next few days or weeks what its fate will be," he says.
'Narrow brace'
Although researchers agree the disintegration of the Wilkins ice shelf will not contribute to rising sea levels, they say it may help them understand what triggers such events in order to predict when they are likely to happen again.
"There now remains just a narrow beam of ice, roughly 6 km wide, which is bracing the rest of the shelf," says Scambos. "If that beam breaks up, about half of the ice shelf will probably very quickly break apart."
However Scambos says this is not likely to happen this year, as the southern winter is settling in.
The break up is already helping researchers understand what triggers the disintegration of ice sheets. They now know that waves generated by storms over the open ocean are just as important as air and water temperature.
No sea-level rise
"The waves travel thousands of kilometres and are long and gentle. They are able to flex and break an ice shelf," says Scambos.
Normally, a slurry of sea ice floating in front of the ice shelf would act as a wave breaker, keeping the surrounding waters still. But this year – as in 2002 when the Larsen B shelf dramatically disappeared in just 30 days – warm temperatures melted away much of the sea ice. The glaciologists believe this is what has caused the break up of the Wilkins ice shelf in recent weeks.
The Wilkins ice sheet is not connected to inland glaciers in the same way as Larsen B was. As a result, its collapse will not accelerate the flow of glaciers into the ocean, and so will not immediately cause a sea-level rise.
Link to article (you might need to be a subscriber to open this page): http://environment.newscientist.com/article/dn13530-antarctic-ice-shelf-hanging-by-a-thread.html
(Link to a different article on the BBC News website, with an embedded video: http://news.bbc.co.uk/2/hi/science/nature/7313264.stm.)
Huge Iceberg Breaks Away
ScienceDaily (Mar. 25, 2008) — British Antarctic Survey has captured dramatic satellite images of an Antarctic ice shelf that looks set to be the latest to break out from the Antarctic Peninsula. A large part of the Wilkins Ice Shelf on the Antarctic Peninsula is now supported only by a thin strip of ice hanging between two islands. It is another identifiable impact of climate change on the Antarctic environment.
Scientists monitoring satellite images of the Wilkins Ice Shelf spotted that a huge (41 by 2.5 km) km2 berg the size of the Isle of Man appears to have broken away in recent days -- it is still on the move.
Glaciologist Ted Scambos from the University of Colorado alerted colleagues Professor David Vaughan and Andrew Fleming of the British Antarctic Survey (BAS) that the ice shelf looked at risk. After checking daily satellite pictures, BAS sent a Twin Otter aircraft on a reconnaissance mission to check out the extent of the breakout.
Professor Vaughan, who in 1993 predicted that the northern part of Wilkins Ice Shelf was likely to be lost within 30 years if climate warming on the Peninsula were to continue at the same rate, says, "Wilkins is the largest ice shelf on the Antarctic Peninsula yet to be threatened. I didn't expect to see things happen this quickly. The ice shelf is hanging by a thread -- we'll know in the next few days or weeks what its fate will be."
Jim Elliott was onboard the BAS Twin Otter to capture video of the breakout for Vaughan and colleagues. He says, "I've never seen anything like this before -- it was awesome. We flew along the main crack and observed the sheer scale of movement from the breakage. Big hefty chunks of ice, the size of small houses, look as though they've been thrown around like rubble -- it's like an explosion."
The breakout is the latest drama in a region of Antarctica that has experienced unprecedented warming over the last 50 years. Several ice shelves have retreated in the past 30 years - six of them collapsing completely (Prince Gustav Channel, Larsen Inlet, Larsen A, Larsen B, Wordie, Muller and the Jones Ice Shelf.)
Professor Vaughan continues, "Climate warming in the Antarctic Peninsula has pushed the limit of viability for ice shelves further south -- setting some of them that used to be stable on a course of retreat and eventual loss. The Wilkins breakout won't have any effect on sea-level because it is floating already, but it is another indication of the impact that climate change is having on the region." Ted Scambos of the University of Colorado says,
"We believe the Wilkins has been in place for at least a few hundred years. But warm air and exposure to ocean waves are causing a break-up."
The Wilkins Ice Shelf covered an area of 16,000km2 (the size of Northern Ireland). Having been stable for most of the last century it began retreating in the 1990s. A major breakout occurred in 1998 when 1000km2 of ice was lost in a few months.
Satellite images processed at the US National Snow and Ice Data Center revealed that the retreat began on February 28 when a large (41 by 2.5 km) iceberg calved away from the ice shelf's south-western front. The edge of the shelf proceeded to crumble and disintegrate in a pattern that has become characteristic of climate-caused ice shelf retreats throughout the northern Peninsula, leaving a sky-blue patch spreading across the ocean surface compose of hundreds of large blocks of exposed old glacier ice. By 8 March, the ice shelf had lost just over 570 km2, and the patch of disintegrated Antarctic ice had spread over 1400km2. As of mid-March, only a narrow strip of shelf ice was protecting several thousand kilometres of potential further break-up.
The recent break out leaves a thin strip of ice between Charcot and Latady islands on the Antarctic Peninsula.
Climate warming has increased the volume of summer meltwater on glaciers, which has weakened ice shelves. Sea ice, which protects ice shelves from ocean swell, has reduced also as a result of warming temperatures.
The collapse of the 3250 km2 Larsen B Ice Shelf took place in 2002. During the past 40 years the average summer temperatures in this region of the north-east Peninsula has been 2.2°C. The western Antarctic Peninsula has showed the biggest increase in temperatures (primarily in winter) observed anywhere on Earth over the past half-century.
The Antarctic Peninsula is an area of rapid climate change and has warmed faster than anywhere else in the Southern Hemisphere over the past half century. Climate records from the west coast of the Antarctic Peninsula show that temperatures in this region have risen by nearly 3°C during the last 50 years -- several times the global average and only matched in Alaska.
Background info
Ice sheet -- is the huge mass of ice, up to 4 km thick, that covers Antarctica's bedrock. It flows from the centre of the continent towards the coast where it feeds ice shelves.
Ice shelf -- is the floating extension of the grounded ice sheet. It is composed of freshwater ice that originally fell as snow, either in situ or inland and brought to the ice shelf by glaciers. As they are already floating any disintegration (like Larsen B) will have no impact on sea level. Sea level will rise only if the ice held back by the ice shelf flows more quickly into the sea.
This discovery follows the recent UNEP report that the world's glaciers are continuing to melt away. Data from 30 reference glaciers in nine mountain ranges show that between the years 2004-2005 and 2005-2006 the average rate of melting and thinning has more than doubled.
Adapted from materials provided by British Antarctic Survey.
Link to article: http://www.sciencedaily.com/releases/2008/03/080325120714.htm
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