ICE TONGUES AND ICE SHELVES
|The Ross Ice Shelf Edge|
Photo: Michael Van Woert, NASA
The weight of Antarctica’s huge layers of ice pushes down forcing ice to slowly move away from the center of the continent, out to the surrounding ocean.
Glaciologists call this movement of ice, ‘internal deformation.’
Much of the movement of the interior of the Antarctic ice sheets is by internal deformation.
Ice also moves by basal sliding.
As an ice layer moves, it creates friction.
Heat from the friction melts an undersurface of water.
If ice is sitting on a soft sediment bed, the bed itself could become saturated with water and move also, carrying the ice layer with it.
The two ice sheets that cover Antarctica each have numerous domed ice centers that have accumulated on the surface over eons of time.
These ice domes have glaciers and ice streams that radiate from their central mass.
Glaciologists refer to moving ice as a glacier if it slides alongside a mountain or a rock valley, or slides over rock or a sediment base. Ice streams flow through ice, through adjacent more stable ice.
Smaller glaciers often join progressively larger glaciers, forming a network similar to a river tributary system.
As glaciers move they sculpt the land, removing rock and sediment material and depositing it as the ice continues on its way.
This leaves a mark on the landscape recording a glaciers presence.
With the moving ice creating friction there is an underlay of water that produces an upflow of rock and sediment debris into the ice above.
Freezing takes place with new ice sliding on the sediment.
Over time and the upflow of sediment there are many layers within the ice.
Ice streams slide on surfaces of sediment, flowing along as rivers inside the ice.
Like glaciers they might come together from different directions and meet, or they might flow as single streams.
Ice streams can be seen on satellite photos.
They are marked by deep crevasses, or fissures that separate the fast flowing ice streams from the adjacent ice.
Some streams move at speeds a hundred times faster then the surrounding ice.
Glaciers and ice streams can be tens of kilometers in width and hundreds of kilometers in length.
In their movement outward, both pull ice from the ice sheet interior much faster than the sheet itself is moving.
They act as huge conveyer belts taking the continent’s snow cover out to the surrounding seas.
|Wilkins Ice Shelf from British Antarctic Survey Twin Otter|
Study says ‘collapsing’ Thwaites Glacier in Antarctica melting from geothermal heat, not ‘climate change’ effects
June 9, 2014
Researchers find major West Antarctic glacier melting from geothermal sources.
Remember the wailing from Suzanne Goldenberg over the “collapse” of the Thwaites glacier blaming man-made CO2 effects and the smackdown given to the claim on WUWT?
AUSTIN, Texas — Thwaites Glacier, the large, rapidly changing outlet of the West Antarctic Ice Sheet, is not only being eroded by the ocean, it’s being melted from below by geothermal heat, researchers at the Institute for Geophysics at The University of Texas at Austin (UTIG) report in the current edition of the Proceedings of the National Academy of Sciences.
The findings significantly change the understanding of conditions beneath the West Antarctic Ice Sheet where accurate information has previously been unobtainable.
The Thwaites Glacier has been the focus of considerable attention in recent weeks as other groups of researchers found the glacier is on the way to collapse, but more data and computer modeling are needed to determine when the collapse will begin in earnest and at what rate the sea level will increase as it proceeds. The new observations by UTIG will greatly inform these ice sheet modeling efforts.
Using radar techniques to map how water flows under ice sheets, UTIG researchers were able to estimate ice melting rates and thus identify significant sources of geothermal heat under Thwaites Glacier. They found these sources are distributed over a wider area and are much hotter than previously assumed.
The geothermal heat contributed significantly to melting of the underside of the glacier, and it might be a key factor in allowing the ice sheet to slide, affecting the ice sheet’s stability and its contribution to future sea level rise.
The cause of the variable distribution of heat beneath the glacier is thought to be the movement of magma and associated volcanic activity arising from the rifting of the Earth’s crust beneath the West Antarctic Ice Sheet.
Knowledge of the heat distribution beneath Thwaites Glacier is crucial information that enables ice sheet modelers to more accurately predict the response of the glacier to the presence of a warming ocean.
Until now, scientists had been unable to measure the strength or location of heat flow under the glacier. Current ice sheet models have assumed that heat flow under the glacier is uniform like a pancake griddle with even heat distribution across the bottom of the ice.
The findings of lead author Dusty Schroeder and his colleagues show that the glacier sits on something more like a multi-burner stovetop with burners putting out heat at different levels at different locations.
“It’s the most complex thermal environment you might imagine,” said co-author Don Blankenship, a senior research scientist at UTIG and Schroeder’s Ph.D. adviser. “And then you plop the most critical dynamically unstable ice sheet on planet Earth in the middle of this thing, and then you try to model it. It’s virtually impossible.”
That’s why, he said, getting a handle on the distribution of geothermal heat flow under the ice sheet has been considered essential for understanding it.
Gathering knowledge about Thwaites Glacier is crucial to understanding what might happen to the West Antarctic Ice Sheet. An outlet glacier the size of Florida in the Amundsen Sea Embayment, it is up to 4,000 meters thick and is considered a key question mark in making projections of global sea level rise.
The glacier is retreating in the face of the warming ocean and is thought to be unstable because its interior lies more than two kilometers below sea level while, at the coast, the bottom of the glacier is quite shallow.
Because its interior connects to the vast portion of the West Antarctic Ice Sheet that lies deeply below sea level, the glacier is considered a gateway to the majority of West Antarctica’s potential sea level contribution.
The collapse of the Thwaites Glacier would cause an increase of global sea level of between 1 and 2 meters, with the potential for more than twice that from the entire West Antarctic Ice Sheet.
The UTIG researchers had previously used ice-penetrating airborne radar sounding data to image two vast interacting subglacial water systems under Thwaites Glacier. The results from this earlier work on water systems (also published in the Proceedings of the National Academy of Sciences) formed the foundation for the new work, which used the distribution of water beneath the glacier to determine the levels and locations of heat flow.
In each case, Schroeder, who received his Ph.D. in May, used techniques he had developed to pull information out of data collected by the radar developed at UTIG.
According to his findings, the minimum average geothermal heat flow beneath Thwaites Glacier is about 100 milliwatts per square meter, with hotspots over 200 milliwatts per square meter. For comparison, the average heat flow of the Earth’s continents is less than 65 milliwatts per square meter.
The presence of water and heat present researchers with significant challenges.
“The combination of variable subglacial geothermal heat flow and the interacting subglacial water system could threaten the stability of Thwaites Glacier in ways that we never before imagined,” Schroeder said.
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NASA worried by unusually big iceberg six times the size of Manhattan
April 25, 2014
Image from earthobservatory.nasa.gov
Dubbed “B31” the iceberg could pose some significant problems for ships if it continues to melt or break apart in the Southern Ocean, April 2014
At 255 square miles (660 sq. km) and 500 meters thick, B31 is one of the biggest icebergs on the planet and currently six times the size of Manhattan.
Although the process of icebergs breaking off from glaciers is typical “iceberg calving,” as its known, typically occurs at the Pine Island Glacier every six to 10 years NASA’s Earth Observatory is attempting to keep a special eye on B31.
“Iceberg calving is a very normal process,” NASA glaciologist Kelly Brunt said on the agency’s website.
“However, the detachment rift, or crack, that created this iceberg was well upstream of the 30-year average calving front of Pine Island Glacier (PIG), so this a region that warrants monitoring.”
Currently, B31 is not in the way of any Antarctic shipping lanes, but Brunt said its current trajectory means that's where the iceberg is headed.
NASA glaciologist Kelly Brunt to the Guardian:
"It's floating off into the sea and will get caught up in the current and flow around the Antarctica continent where there are ships."
NASA has been monitoring the Pine Island Glacier since 2011, when it first observed a crack that eventually got larger and resulted in B31 breaking off into the ocean.
The massive glacier has been highlighted by scientists over the last 20 years due to the fact that, as NASA put it, “it has been thinning and draining rapidly and may be one of the largest contributors to sea level rise.”
As noted by the Guardian, Kelly said this iceberg alone wouldn’t contribute significantly to rising ocean levels even if it melted completely.
However, should the Pine Island Glacier continue shrinking in size, it could raise global sea levels by 1.5 meters.
Despite the increased interest by NASA, the agency said keeping track of B31’s movement over the next six months will be difficult, since winter is descending on the region and it will be blanketed in darkness.
University of Sheffield researcher Grant Bigg told the Earth Observatory:
For article and comments on RT: click here
“We are doing some research on local ocean currents to try to explain the motion properly.
It has been surprising how there have been periods of almost no motion, interspersed with rapid flow.”
The iceberg is now well out of Pine Island Bay and will soon join the more general flow in the Southern Ocean, which could be east or west in this region.”
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CNN Goes Full Propaganda On CO2
Antarctic Ice Shelf ‘Hanging by Thread’: European Scientists
Images taken by its Envisat remote-sensing satellite show that Wilkins Ice Shelf is “hanging by its last thread” to Charcot Island, one of the plate’s key anchors to the Antarctic peninsula, ESA said in a press release.
|PARIS New evidence has emerged that a large plate of floating ice shelf attached to Antarctica is breaking up, in a troubling sign of global warming, the European Space Agency (ESA) said on Thursday.
“Since the connection to the island… helps stabilise the ice shelf, it is likely the breakup of the bridge will put the remainder of the ice shelf at risk,” it said.
Wilkins Ice Shelf had been stable for most of the last century, covering around 16,000 square kilometres (6,000 square miles), or about the size of Northern Ireland, before it began to retreat in the 1990s.
Since then several large areas have broken away, and two big breakoffs this year left only a narrow ice bridge about 2.7 kilometres (1.7 miles) wide to connect the shelf to Charcot and nearby Latady Island.
The latest images, taken by Envisat’s radar, say fractures have now opened up in this bridge and adjacent areas of the plate are disintegrating, creating large icebergs.
Scientists are puzzled and concerned by the event, ESA added.
The Antarctic peninsula the tongue of land that juts northward from the white continent towards South America has had one of the highest rates of warming anywhere in the world in recent decades.
But this latest stage of the breakup occurred during the Southern Hemisphere’s winter, when atmospheric temperatures are at their lowest.
One idea is that warmer water from the Southern Ocean is reaching the underside of the ice shelf and thinning it rapidly from underneath.
“Wilkins Ice Shelf is the most recent in a long, and growing, list of ice shelves on the Antarctic Peninsula that are responding to the rapid warming that has occurred in this area over the last fifty years,” researcher David Vaughan of the British Antarctic Survey (BAS) said.
“Current events are showing that we were being too conservative, when we made the prediction in the early 1990s that Wilkins Ice Shelf would be lost within 30 years. The truth is, it is going more quickly than we guessed.”
In the past three decades, six Antarctic ice shelves have collapsed completely Prince Gustav Channel, Larsen Inlet, Larsen A, Larsen B, Wordie, Muller and the Jones Ice Shelf.
© 2008 Agence France Presse
|Wilkins Ice Shelf 2007 and 2008
Image: British Antarctic Survey
Ice drains from the Antarctic ice sheets primarily through outlet glaciers, ice streams or valley glaciers. As much of the ice reaches the coast it becomes a part of an ice shelf extension floating into the sea.
Ice tongues form where glaciers flow out to sea, and where there is no ice shelf. They are long, narrow ice projections that may float or may be anchored to the surface of the sea, floating at the seaword end. They can build up over centuries and are considered geographically permanent features of the coastline.
Ice tongues can rapidly change their size and shape. As the tongue lengthens, tides, waves and storms slowly weaken the end and sides. Pieces of the tongue break off and float to sea as icebergs.
Drygalski Ice Tongue is a large ice tongue 70 kilometers long and 20 kilometers wide. It drains David Glacier in Southern Victoria Land. The ice tongue discovered by Robert Scott in 1902 and named after German explorer Erich von Drygalski has been growing seaward, measured for the past few decades at 150 to 900 meters per year.
Advanced high-resolution radiometer satellite imaging was able to detect the breakup of the Ninnis Ice Tongue. Estimates of the length of this tongue have been as much as 140 kilometers since it was first documented during the Australasian Antarctic Expedition of 1911. In January 2,000 a break occurred where much of the tongue split away from the glacier, changing the face of what had been known as the coastline of East Antarctica. The huge ice platform subsequently split into two sections. Additional calving followed with smaller icebergs drifting well away from the edge of the Ninnis Glacier.
The Mertz Ice Tongue also on the George V Coast drains the Mertz glacier, which in turn helps to drain Dome C of the East Antarctic Ice Sheet. This tongue has been estimated at various sizes from 80 kilometers to its 2001 size of 40 kilometers. Icebergs are continually being calved from the tongue, the icebergs looking like great icy grottoes. With their blue caves indented in the sides, their furrows and clefts carved by the sea and wind, they look like magnificent blue and white statues.
|Drygalski Ice Tongue.
February 21, 2005, saw the ice tongue calve a new baby.
The five-by-ten-kilometer iceberg was floating off the left side oof the ice tongue on February 22, when this image was acquired by NASA.
The event is a normal part of the evolution of the ice tongue—pieces regularly break from the tongue as the glacier pushes more ice out over the sea.
This image shows cracks, formed by time and ocean currents, which become more numerous towards the end of the tongue.
Ice shelves develop the same way as ice tongues, except the ice shelves are often much larger, connecting to two or many extended coastlines.
Pressure from the inner sheet forces the ice sheet itself, including glaciers and ice streams, away from the underlying rock bed into the surrounding sea.
As glacier ice is pushed outwards, rock debris remains near the grounding line.
The cleaned ice becomes a part of the protruding ice shelf, and internal ice streams, a platform that floats on the tidal movements.
Mass for ice shelves is gained three ways.
By inland ice continuing to be pushed further into the ocean.
By the freezing of seawater onto the underside of the shelf, especially near the shore.
By ice-laden winds providing additional ice coatings onto the surface.
Mass is lost by ice platforms, usually very large, and flat-topped, calving along the edges.
At the seaward side melting takes place underneath.
Ice shelves cover more than half of the Antarctic coast.
The largest shelf is the Ross Ice Shelf on the New Zealand side of Antarctica.
The second largest is the combined Ronne Filchner Ice Shelf covering a massive area that extends and is part of the Weddell Sea.
Other ice shelves are the Amery Ice Shelf, the Shakelton Ice Shelf, The George VI Ice Shelf, the Wilkins Ice Shelf, the Larsen Ice Shelf, and the Riiser-Larsen Ice Shelf.
All around the continent are narrow shelves that have formed along the coast.
The Amery Ice Shelf in East Antarctica drains an estimated 33 thousand million tonnes of ice per year from the East Antarctic Ice Sheet.
The ice movement rate at the front of the ice shelf is about 1,200 meters, or ¾ miles per year.
The Lambert Glacier that feeds the Amery Ice Shelf has flow lines hundreds of kilometers.
The long glacier, extending over an area of 900,000 square kilometers, has at its center a mosaic of ice lakes and troughs accumulated through past occurrences of surface meltwater.
Fascinating formations can also be seen by satellite along the Fimbul Ice Shelf. Ice rise formations off the coast of Queen Maud Land are believed to be rock covered by ice.
Glacier streams flowing over rocky outcroppings on the ice sheet has carried the rock onto the shelf.
The largest ice shelf is the Ross Ice Shelf a large frozen area between Byrd Land and the Transantarctic Mountains.
800 kilometers across, the Ross shelf receives ice from a number streams flowing into it from the West Antarctic Ice Sheet.
Islands of ice sit at points and act as buttresses for the ice streams to go around.
The Crary Ice Rise, nearly 50 meters high, rests in the downstream flow from two ice streams.
Another buttress on the ice sheet is the Steershead crevasses and ice rise.
Cracks in the ice or ‘rifts’ develop in ice sheets due to a straining and deformation of the ice.
Rifts can be present hundreds of kilometers prior to the ice becoming part of an ice shelf.
Many rifts on the Ross shelf have giant crevasses where dark horizontal bands can be seen.
These are ash deposits from old eruptions of volcanoes from nearby Ross Island from Mt. Erebus, Mt. Terra Nova and Mt. Terror.
New rifts can develop in the ice as well as rifts already formed.
|Wilkins Ice Shelf from British Antarctic Survey Twin Otter|
In satellite images it is possible to see rifts developing and extending kilometers inland from the seaward edge of the shelf.
These rifts grow until a calving of ice breaks off into the sea.
Ice shelves produce more than two thirds of all Antarctic icebergs.
They also produce some of the largest icebergs.
These hundreds of square kilometer clean blocks of ice break into smaller pieces as seawater works its way through the long, narrow openings of fissures and cracks.
Ice shelves warming.
|These four Moderate Resolution Imaging Spectroradiometer satellite images from 2002 show the progressive breaking apart of the northern section of the Larsen B ice shelf on the eastern side of the Antarctic Peninsula.|
March 22nd 2000 a super iceberg broke away from the Ross Ice Shelf.
It was 295 kilometers in length and 37 kilometers wide.
In 2000 the Ross Ice Shelf, shed five massive icebergs.
Icebergs named B-20, B-15, B-17, Godzilla, were ramming into each other, breaking into smaller bergs.
The super iceberg B-15, weighing 2 billion tons, broke into a number of smaller pieces.
A year afterwards B-15B had nearly cleared its way around Cape Adare, 950 kilometers from its original position.
B-15A was continuing to jostle back and forth in the waters near Ross Island.
Scientists believe the Ross ice shelf has a cycle of about 50 years of larger ice rifts breaking, and the shelf renewing itself, but they are not sure if global warming is becoming a factor in some ice shelf disintegration.
The last few decades has seen many smaller ice shelves around the northern portion of the Antarctic Peninsula change drastically.
Fracturing and rifting has been taking place in both the Larsen shelves on the east side and the Wilkins shelf on the west.
Scientists are saying these two shelves may disappear completely within the next few years.
The processes of fracturing and rifting and total breakup of the ice platforms are not yet fully understood, but a fatal weakening if these two shelves has probably already taken place.
February of 2001 saw areas of water in the King George VI ice shelf that have, since historical records began, always been frozen solid.
Some of these areas at the western base of the peninsula have no depth readings on maps because even the biggest icebreakers have not previously been able to penetrate the ice.
31, January 2002 saw the beginning of 3,250 km2
of Larsen B ice shelf disintegrate.
Taking place over a 35 day period, Larsen B, the floating ice mass on the east side of the Peninsula, has been reduced to 40% of its minimum stable extent measured over the previous five years.
|Map of Antarctic Peninsula showing Wilkins Island|
Antarctic Ice Shelf melting
Sunday, 24 February 2008
Antarctic glaciers surge to ocean
By Martin Redfern
Rothera Research Station, Antarctica
The UK work is discovering just how fast the ice is moving
UK scientists working in Antarctica have found some of the clearest evidence yet of instabilities in the ice of part of West Antarctica.
If the trend continues, they say, it could lead to a significant rise in global sea level.
The new evidence comes from a group of glaciers covering an area the size of Texas, in a remote and seldom visited part of West Antarctica.
The "rivers of ice" have surged sharply in speed towards the ocean.
David Vaughan, of the British Antarctic Survey, explained: "It has been called the weak underbelly of the West Antarctic Ice Sheet, and the reason for that is that this is the area where the bed beneath the ice sheet dips down steepest towards the interior.
"If there is a feedback mechanism to make the ice sheet unstable, it will be most unstable in this region."
There is good reason to be concerned.
Satellite measurements have shown that three huge glaciers here have been speeding up for more than a decade.
The biggest of the glaciers, the Pine Island Glacier, is causing the most concern.
Julian Scott has just returned from there.
He told the BBC: "This is a very important glacier; it's putting more ice into the sea than any other glacier in Antarctica.
"It's a couple of kilometres thick, its 30km wide and it's moving at 3.5km per year, so it's putting a lot of ice into the ocean."
The team drove its skidoos for thousands of km across the ice
It is a very remote and inhospitable region.
It was visited briefly in 1961 by American scientists but no one had returned until this season when Julian Scott and Rob Bingham and colleagues from the British Antarctic survey spent 97 days camping on the flat, white ice.
At times, the temperature got down to minus 30C and strong winds made work impossible.
At one point, the scientists were confined to their tent continuously for eight days.
"The wind really makes the way you feel incredibly colder, so just motivating yourself to go out in the wind is a really big deal," Rob Bingham told BBC News.
When the weather improved, the researchers spent most of their time driving skidoos across the flat, featureless ice.
"We drove skidoos over it for something like 2,500km each and we didn't see a single piece of topography."
Rob Bingham was towing a radar on a 100m-long line and detecting reflections from within the ice using a receiver another 100m behind that.
The signals are revealing ancient flow lines in the ice.
The hope is to reconstruct how it moved in the past.
Julian Scott was performing seismic studies, using pressurised hot water to drill holes 20m or so into the ice and place explosive charges in them.
He used arrays of geophones strung out across the ice to detect reflections, looking, among other things, for signs of soft sediments beneath the ice that might be lubricating its flow.
The Pig Pine Island Glacier is a major draining feature on the Wais
He also placed recorders linked to the global positioning system (GPS) satellites on the ice to track the glacier's motion, recording its position every 10 seconds.
Throughout the 1990s, according to satellite measurements, the glacier was accelerating by around 1% a year.
Julian Scott's sensational finding this season is that it now seems to have accelerated by 7% in a single season, sending more and more ice into the ocean.
"The measurements from last season seem to show an incredible acceleration, a rate of up to 7%. That is far greater than the accelerations they were getting excited about in the 1990s."
The reason does not seem to be warming in the surrounding air.
One possible culprit could be a deep ocean current that is channelled onto the continental shelf close to the mouth of the glacier.
There is not much sea ice to protect it from the warm water, which seems to be undercutting the ice and lubricating its flow.
Julian Scott, however, thinks there may be other forces at work as well.
Much higher up the course of the glacier there is evidence of a volcano that erupted through the ice about 2,000 years ago and the whole region could be volcanically active, releasing geothermal heat to melt the base of the ice and help its slide towards the sea.
Geothermal activity may be playing its part, says Julian Scott
David Vaughan believes that the risk of a major collapse of this section of the West Antarctic ice sheet should be taken seriously.
"There has been the expectation that this could be a vulnerable area," he said.
"Now we have the data to show that this is the area that is changing. So the two things coinciding are actually quite worrying."
The big question now is whether what has been recorded is an exceptional surge or whether it heralds a major collapse of the ice. Julian Scott hopes to find out.
"It is extraordinary and we've left a GPS there over winter to see if it is going to continue this trend."
If the glacier does continue to surge and discharge most of it ice into the sea, say the researchers, the Pine Island Glacier alone could raise global sea level by 25cm.
That might take decades or a century, but neighbouring glaciers are accelerating too and if the entire region were to lose its ice, the sea would rise by 1.5m worldwide.
|Humpback whaleTwo decades after leaky moratorium on whale hunting, most majestic of sea mammals have made little headway in recovering |
A humpback whale tail.
More than two decades after the start of a leaky moratorium on whale hunting, the most majestic of sea mammals have made little headway in recovering their once robust populations.
Photo: AFP/Rodrigo Buendia
||Collapse of Antarctic ice shelf could have global effects
03 Aug 2005
CBC News The unprecedented collapse of an ice-shelf in Antarctica could indirectly lead to a significant rise in global sea levels, researchers say.
The Larsen B ice shelf covered more than 3,000 square kilometres and was 200 metres thick until its northern part disintegrated in the 1990s. Three years ago, the central part also broke up.
An international team of researchers used data collected from six sediment cores near the former ice shelf to show the shelf had been relatively intact for at least 10,000 years or since the last ice age.
The collapse therefore goes beyond what would be expected naturally at the time. Rather, the demise is likely the result of long-term thinning due to melting from underneath, as well as short-term surface melting from global climate change, the researchers suggest.
Then in five years, the shelf shrunk by 5,700 square kilometres, say scientists who found the break up caused changes in currents and species in the area.
"As the ice shelves are disintegrating, the glaciers that are feeding them from the land are surging forward," said Robert Gilbert, a geography professor at Queen's University in Kingston, Ont.
Glaciers are no longer being held back from the ice shelf, and are pushing ice bergs into the sea, said Gilbert, one of the co-authors of the study in Thursday's issue of the journal Nature.
As the glaciers melt, global sea levels could change more than predicted, he said. Flooding could result in low-lying areas.
Scientists are now watching to see if the most southern part of the Larsen ice shelf, the coldest part of Antarctica, is going to break up.
Larsen ice shelf|
(Courtesy: Queen's University)
Copyright © CBC 2005
|Ice blocks floating in|
Ice blocks are seen floating in the Weddell Sea near the Argentine Base Marambio in the Antarctic Peninsula March 8, 2008.
New evidence a large plate of floating ice shelf attached to Antarctica is breaking up.
Photo: REUTERS/Enrique Marcarian
Published on Monday, October 16, 2006 by Reuters
Antarctic Ice Collapse Linked to Greenhouse Gases
Scientists said on Monday that they had found the first direct evidence linking the collapse of an ice shelf in Antarctica to global warming widely blamed on human activities.
by Alister Doyle
Shifts in winds whipping around the southern Ocean, tied to human emissions of greenhouse gases, had warmed the Antarctic peninsula jutting up toward South America and contributed to the break-up of the Larsen B ice shelf in 2002, they said.
"This is the first time that anyone has been able to demonstrate a physical process directly linking the break-up of the Larsen Ice Shelf to human activity," said Gareth Marshall, lead author of the study at the British Antarctic Survey.
The chunk that collapsed into the Weddell Sea in 2002 was 3,250 sq kms (1,255 sq miles), bigger than Luxembourg or the U.S. state of Rhode Island.
Most climate experts say greenhouse gases, mainly from fossil fuels burned in power plants, factories and cars, are warming the globe and could bring more erosion, floods or rising seas. They are wary of linking individual events such as a heatwave or a storm to warming.
But the British and Belgian scientists, writing in the Journal of Climate, said there was evidence that global warming and a thinning of the ozone layer over Antarctica, caused by human chemicals, had strengthened winds blowing clockwise around Antarctica.
The Antarctic peninsula's chain of mountains, about 2,000 meters (6,500 ft) high, used to shield the Larsen ice shelf on its eastern side from the warmer winds.
"If the westerlies strengthen the number of times that the warm air gets over the mountain barrier increases quite dramatically," John King, a co-author of the study at the British Antarctic Survey, told Reuters.
The Antarctic peninsula's chain of mountains, about 2,000 meters (6,500 ft) high, used to shield the Larsen ice shelf on its eastern side from the warmer winds.
If the westerlies strengthen the number of times that the warm air gets over the mountain barrier increases quite dramatically.
John King, British Antarctic Survey
The average summer temperatures on the north-east of the Antarctic peninsula had been about 2.2 Celsius (35.96F) over the past 40 years.
But on summer days when winds swept over the mountains into the area the air could warm by 5.5 C (9.9 F). And on the warmest days, temperatures could reach about 10 C (50.00F).
King said temperature records in Antarctica went back only about 50 years but that there was evidence from sediments on the seabed which differ if covered by ice or open water that the Larsen ice shelf had been in place for 5,000 years.
"Further south on the main Antarctic continent temperatures are pretty stable," he said. "There is no clear direct evidence of human activity affecting the main area."
The collapse of the Larsen B ice shelf did not raise world sea levels because the ice was floating. A brimful glass of water with an ice cube jutting out will not spill if it melts because ice contracts as it melts.
But King said the removal of the floating ice barrier could accelerate the flow of land-based glaciers toward the sea, at least in the short term. That extra ice could raise sea levels.
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