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Giant rivers of ice thread their way across the Antarctic Ice Sheet to the sea. The rivers, termed by scientists "streams", are the largest flows of ice in the world. Some streams are over 2 kilometers thick, 30 km wide and travel at speeds up to 1 km per year. Radar observations by polar orbiting, European ERS satellites have been used by CPOM scientists to map in detail the geography of the streams, many of which were hitherto unknown.

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The ice velocity observed is mapped in the image below"




Pine Island Glacier

A particularly interesting stream of ice is named the 'Pine Island Glacier' (PIG). Pine Island Glacier is one of several glaciers draining the West Antarctic Ice Sheet. The Pine Island Glacier transports an enormous volume of ice from the West Antarctic Ice Sheet to the sea. It is both the largest transporter of ice in Antarctica and also the Antarctic's fastest moving glacier. It is currently thinning and retreating which is of concern since it's flow velocity and ice volume is accelerating.

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The West Antarctic Ice Sheet (WAIS) contains enough water to raise global sea level by ~5 m (1) if the ice were to melt into the ocean. The Pine Island Glacier (PIG) (Fig. 1) has the largest discharge (75 Gt year-1) of all WAIS ice streams .. Satellite radar interferometry observations (12) have shown that the PIG grounding line retreated 5 km inland between 1992 and 1996.

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The observations can be found in the paper here: Acceleration of Pine Island Glacierflow. The image below shows the glacier grounding lines and flow velocity in 1996.




Calving of B21 in 2001

In 2001 the Pine Island Glacier calved an enormous iceberg dubbed B21 42 km by 17 km. The calving has been captured by the NASA/Goddard Space Flight Center Scientific Visualization Studio

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The Pine Island Glacier is the largest discharger of ice in Antarctica and the continent's fastest moving glacier. Even so, when a large crack formed across the glacier in mid 2000, it was surprising how fast the crack expanded, 15 meters per day, and how soon the resulting iceberg broke off, mid-November, 2001. This iceberg, called B-21, is 42 kilometers by 17 kilometers and contains seven years of glacier outflow released to the sea in a single event. This series of images from the MISR instrument on the Terra satellite not only shows the crack expanding and the iceberg breakoff, but the seaward moving glacial flow in the parts of the Pine Island Glacier upstream of the crack.

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Calving of Pine Island in 2007
In 2007 Pine Island calved agai this time observed by the Envisat Advanced Synthetic Aperture Radar (ASAR) instrument

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Envisat captures the birth of a giant iceberg that has broken off from the Pine Island Glacier in West Antarctica. Spanning 34 km in length by 20 km in width, the new iceberg covers an area nearly half the size of Greater London.

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The attached .kmz file includes the animation as a time series. A large remnant of B21 is may still visible in the Google Earth imagery.


An enormous iceberg (right) breaks off the Knox Coast in the Australian Antarctic Territory - Photograph: Torsten Blackwood/Getty Image

Antarctic Ice Loss

A recent 2007 article in Science by Shepard documents the loss of significant ice on the West Antarctic Ice Sheet as well as Greenland.

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"The thinning is 10 times greater than the rate of snowfall in the basin," said Shepherd. "The speed of the glacier means that much more mass is going out [through melting and breaking off of icebergs] than is coming in.'' ... Shepherd said if the present rate of change continues, the main stem of the Pine Island Glacier will be undercut by the sea and lifted up in about 600 years. When the glacier floats, it would cause a dramatic shift in sea level, he said.

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Ice loss in Antarctica increased by 75 percent in the last 10 years due to a speed-up in the flow of its glaciers and is now nearly as great as that observed in Greenland ...
Rignot said the losses, which were primarily concentrated in West Antarctica's Pine Island Bay sector and the northern tip of the Antarctic Peninsula, are caused by ongoing and past acceleration of glaciers into the sea. This is mostly a result of warmer ocean waters, which bathe the buttressing floating sections of glaciers, causing them to thin or collapse. "Changes in Antarctic glacier flow are having a significant, if not dominant, impact on the mass balance of the Antarctic ice sheet,"

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The Antarctic ice loss between 1996 and 2006 is shown in the graphic below. The colors indicate the speed of the ice loss. Purple/red is fast. Green is slow. From NASA/MODIS. The results of the study are published in February's issue of Nature Geoscience.

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The team found that the net loss of ice mass from Antarctica increased from 112 (plus or minus 91) gigatonnes a year in 1996 to 196 (plus or minus 92) gigatonnes a year in 2006. A gigatonne is one billion metric tons, or more than 2.2 trillion pounds. ...Rignot says the increased contribution of Antarctica to global sea level rise indicated by the study warrants closer monitoring.

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both the ice shelf and the glacier are thinning rapidly, and thinning is mostly of dynamic origin.

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Waterfall #2. Ice surface conditions on Larsen-B during the warmest recorded summer. A water-fall cascades over the ~ 30 m high Larsen B ice-front, draining meltwater off the ice shelf observed during February 2002. (Photo: Pedro Skvarca)

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Antarctic Ice Shelfs
The Antarctic ice shelf consists of glacial ice hundreds to thousands of feet thick floating on the sea. As the ice streams move outward, icebergs break off which may be hundreds of feet above water.








Ice shelf buttressing of Antarctic glaciers
The mechanism by which for global warming can influence the rate at which glaciers grain the Antarctic ice sheet is though to be related to the thinning of buttressing ice in ice shelves surrounding the ice sheet. This effect is shown in the graphic below



The West Antarctic Ice Sheet is surrounded by ices shelves hundreds of meters thick which may be be holding back the Antarctic ice sheet glaciers. In a 2004 Scicene article. Richard A. Kerr discusses the buttressing effect.

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These latest results from West Antarctica confirm an unsettling view of glacier behavior. For 30 years, glaciologists have debated whether one part of a glacier can "feel" what's happening in a distant part of the same glacier. At the coastal end of the Pine Island Glacier, for example, warmer water seems to be melting the underside of the glacier's floating ice shelf (Science, 24 July 1998, pp. 499 and 549), pushing landward the point at which the advancing glacier floats off the sea floor.
If an ice shelf pinned against an embayment's shore and floor helps slow a glacier's flow--as was hypothesized in the 1970s--and if changes at the coast could make themselves felt far up the glacier, then the Pine Island Glacier's so-called grounding line retreat would accelerate glacier flow well upstream. The researchers think that's what they're seeing. "I'm convinced the glacier feels what is happening a long way away," says Thomas. Similar accelerations struck after two other floating ice tongues recently broke up in West Antarctica and Greenland (Science, 30 August 2002, p. 1494).

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Potential breakup of the West Antarctic Ice Sheet
In the Scicene article, glaciologist Robert Thomas summarizes evidence for this 'buttressing effect' and it's potential to lead to the breakup of the West Antarctic Ice Sheet.

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Perhaps half the present increase in global sea level of ~1.8 mm/year is caused by melting of terrestrial ice (1). During the 1990s, nonpolar glaciers accounted for an estimated 0.4 mm/year (2) and Greenland for ~0.15 mm/year (3). Although data from Antarctica are still sparse, they suggest a net loss from West Antarctica equivalent to ~0.2 mm/year and approximate balance in East Antarctica, where uncertainty remains large (4). Substantial grounding line retreat (5, 6), thinning (7), and acceleration (8) have been observed on glaciers flowing into the Amundsen Sea, with small ice shelves now but larger ones in the past (9). These glaciers flow into ice shelves over beds well below sea level, and sustained thinning would allow them to float free from bedrock, potentially easing resistive forces acting on upstream ice and thereby leading to further glacier acceleration.

The extent to which ice shelves affect the dynamics of tributary glaciers remains an unresolved controversy within glaciology. Early suggestions that ice-shelf weakening would result in increased discharge from the ice sheet (10–12) require ice-shelf "back forces"to affect glacier dynamics over long distances. If correct, this implies that "marine ice sheets"with beds deep below sea level may be vulnerable to rapid collapse if their deep beds extend to the coast and if buttressing ice shelves are removed

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The danger here is that the West Antarctic Ice Sheet lies on bedrock well below sea level. Rapidly flowing glaciers such as the Pine Island Glacier could lead to the intrusion of the sea into the interior of the ice sheet by floating away ice at the periphery. Indeed the grounding line where the floating Glacial tounge contacts bedrock for Pine Island Glacier has retreated by roughly 5 km. The Image below shows the bedrock elevation of Antarctica. Any areas below sea level (green- blue) have the potential to 'float away' You can see that most of the West Antarctic Ice Sheet is below sea level. (from the Bedmap project



A graphic in an article in Nature shows the outline Antarctic as it built up the current icecap. Note the areas below sea level.




Edited by blt (02/18/08 12:52 AM)

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Why is the Pine Island Glacier flow volume increasing? A volcano under the ice could be could be part of the story.

Volcanic Activity
Near the Pine Island Glacier lies the. Takahe Volcano.



Mt Takahe is a large shield volcano which rises to over 11,000 feet or 3,460m with an 8 km wide caldera. The last eruption occurred in 5550 BC. Takahe is likely to be responsible for volcanic ash layers, broadly found in West Antarctica.

Recently evidence of a 'subglacial' volcano has been discovered near Pine Island Glacier.

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Using airborne ice-sounding radar, scientists from British Antarctic Survey (BAS) discovered a layer of ash produced by a ‘subglacial’ volcano It extends across an area larger than Wales.... This eruption occurred close to Pine Island Glacier on the West Antarctic Ice Sheet. ... The volcano on the West Antarctic Ice Sheet erupted 2000 years ago (325BC) and remains active.

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“This eruption occurred close to Pine Island Glacier on the West Antarctic Ice Sheet. The flow of this glacier towards the coast has speeded up in recent decades and it may be possible that heat from the volcano has caused some of that acceleration. However, it cannot explain the more widespread thinning of West Antarctic glaciers that together are contributing nearly 0.2mm per year to sea-level rise. This wider change most probably has its origin in warming ocean waters.”
"The flow of this glacier towards the coast has speeded up in recent decades and it may be possible that heat from the volcano has caused some of that acceleration. However, it cannot explain the more widespread thinning of West Antarctic glaciers that together are contributing nearly 0.2mm per year to sea-level rise. This wider change most probably has its origin in warming ocean waters.”

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