A
Disintegrating Glacier
Recent satellite images reveal two
new icebergs floating off the Antarctic coast. The icy
behemoths are fragments of the Ninnis Glacier.
NASA
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 December
6, 2000 -- Many processes that shape the Earth's
landscape happen too slowly to be witnessed in a human
lifetime. But recent analysis of satellite imagery shows
that a large glacier tongue on the coast of East
Antarctica has disintegrated, changing the shape of the
coastline almost overnight.
Rob
Massom, polar research scientist at the Antarctic
Cooperative Research Centre at the University of
Tasmania, Australia, discovered the recent breakout of
the entire Ninnis Glacier Tongue purely by chance.
"I was looking at sea ice
distribution and noticed something unusual in the
satellite images," said Massom. "The coastline
looked very different than it had in previous
images."
The Ninnis is one of two prominent
floating glacier tongues that extend into the Southern
Ocean from the coast of King George V Land. Situated
along the eastern coast of the Antarctic Ice Sheet south
of Tasmania, the King George V Land coast is
characterized by embayments and large floating ice
sheets that jut out from the coast, including the Ninnis
and Mertz Glacier Tongues.
Right: By February 20,
2000, Bergs A and B had almost totally separated,
rotated counterclockwise, and drifted to the north. Note
that both sections are now well away from the Ninnis
Glacier. (Images courtesy of Dr.
Rob Massom, Antarctic CRC © 2000 Canadian Space
Agency)
Large-scale breakouts of entire
floating glacier tongues are rare, according to Massom.
More commonly, iceberg calving events occur from the
outer edges of glacier tongues and produce only small
icebergs.
"The breakup of the Ninnis
Glacier Tongue has important implications," said
Massom. "To better understand the Antarctic Ice
Sheet's potential response to global climate change and
its effect on global sea level, it is important to
detect and monitor the calving of large icebergs."
Perhaps the most dramatic breakup of
Antarctic coastal ice in recent years was the
disintegration of the Larsen Ice Shelf on the eastern
side of the Antarctic Peninsula. Researchers at the
National Snow and Ice Data Center and the British
Antarctic Survey attributed this event to regional
warming trends, reporting an increase in mean annual
temperature of about 2.5 degrees Celsius (4.5 degrees
Fahrenheit) since the 1940s.
But Massom said that, so far, there is
no evidence linking the demise of the Ninnis Glacier
Tongue to warming in the region.
"The disintegration is likely to
be the consequence of a natural progression of events
that periodically occur in floating glacier tongues
around the margin of the Antarctic Ice Sheet," he
said. "What remains a mystery is why these
breakouts occur."
Watching ice melt
An ice sheet is a dome-shaped mass of
glacier ice with an area greater than 50,000 square
kilometers (19,300 square miles). Under the influence of
gravity, ice flows downhill from high points on the ice
sheet towards the coast in the form of glaciers. At or
near the coast, some of these glaciers flow directly
into the ocean and develop floating extensions, called
"glacier tongues." Blocks of ice periodically
break off, or calve, from the glacier tongues and float
free as icebergs.
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Above: The left image, taken
January 22, 2000, captures the Ninnis Glacier Tongue
region soon after the initial calving. The resultant
iceberg (sections A and B) has an area of approximately
900 square kilometers (560 square miles). The right
image, taken February 5, 2000, captures the iceberg
after it split into two sections (A and B). Berg A had
drifted 20 kilometers (about 12.5 miles) to the west,
Berg B had drifted to the northeast, and a smaller
section (C) remained grounded in front of the Ninnis
Glacier.
Massom, who has been studying the
distribution of polar sea ice for 20 years, came across
the Ninnis disintegration during a routine review of Advanced
Very High Resolution Radiometer (AVHRR) satellite
images, obtained from the Australian Bureau of
Meteorology. To get a closer look at the coastline,
Massom contacted the Alaska
Synthetic Aperture Radar (SAR) Facility, housed
within the Geophysical Institute at the University of
Alaska Fairbanks, to request Radarsat SAR imagery of the
area.
SAR is an ideal tool for looking at
polar regions during winter and in periods of heavy
cloud cover. Unlike the AVHRR sensor, which detects
radiation in the visible or thermal infrared range of
the electromagnetic spectrum, SAR provides surface
information regardless of weather and/or solar
illumination conditions. SAR images also provide greater
detail at a higher spatial resolution, which permitted
Massom to get a more accurate picture of changes in the
Antarctic coastline.
 Left:
A large portion of the Larsen B ice shelf breaks off the
West Antarctic Peninsula, February 13, 1995.
The George V Land coast was first
explored and mapped by members of the "eastern
coastal party" of the Australasian Antarctic
Expedition of 1911 to 1914, under the leadership of Sir
Douglas Mawson. A 1996 study of the George V Land coast,
which employed data acquired by the Japanese SAR
satellite JERS-1, indicated that the Ninnis Glacier
Tongue lost approximately two-thirds of its size between
1912 and 1993. But a more recent study (1998) of the
Oates and George V Land coasts casts doubt on the
expedition's mapping accuracy, suggesting that the most
significant retreat of the Ninnis Glacier Tongue took
place after 1980, with a significant calving event also
occurring in the early 1950s.
The disintegration marks the final
phase in a progressive retreat of the Ninnis Glacier
Tongue. In 1989 Landsat imagery captured a split in the
glacier tongue caused by shearing forces in the glacial
ice. By 1993, the split had extended laterally across 60
percent of the glacier.
Although the exact mechanism or set of
processes responsible for the split is unknown, analysis
of satellite imagery indicates that the detached tongue
did not immediately drift away, but was initially held
in place by perennial "fast ice." Fast ice is
thick sea ice that remains attached to the shore where
it may be anchored by grounded icebergs or coastal
promontories. In January 2000, a breakout of fast ice
occurred, forming a channel between the Ninnis Glacier
Tongue and the open ocean. Shortly thereafter, the
iceberg began to move and subsequently split into two
sections.
Below: More than a decade ago a
large iceberg (over a thousand square miles in area, and
a quarter of a mile thick) broke off an Antarctic
glacier. The National Iceberg Center, which monitors sea
ice in shipping lanes, christened the giant B10 in 1992.
In 1995 B10 split into two pieces. Amazingly, the larger
piece (B10A) was still the size of Rhode Island. This
true color Landsat 7 image shows relatively small
icebergs "calving" off the edge of B10A. The
new icebergs are drifting into international shipping
lanes, posing a threat. Remote sensing satellites such
as Landsat 7, SeaWinds, and Radarsat are being used to
monitor B10A and its child icebergs. [more]
According to Massom, the coastal
changes resulting from the Ninnis disintegration are
likely to have a significant impact on the behavior of
regional sea ice. Floating glacier tongues act as
obstacles to sea ice drift, provide anchor points for
fast ice, and contribute to the formation of open-water
areas surrounded by sea ice.
"This disintegration represents a
major change in the configuration of the East Antarctic
coastline, and the icebergs themselves will have an
impact on surrounding sea ice," he said.
Changes in Antarctic sea ice
distribution can also have a significant impact on
marine wildlife that depend on sea ice as a platform for
breeding, foraging, and social interaction. For example,
fast ice formation is thought to influence the survival
rate of penguins and seals.
Massom's study is one of the first to
observe the disintegration of a glacier tongue virtually
as it was happening.
"It's very rare to witness the
breakout of an entire glacier tongue," he said,
"and it caused a large adrenaline rush."
Right: An Antarctic penguin
toboggans across the ice. (Image courtesy of NOAA)
 He
is currently using satellite imagery to track the
movement of the two sections of the iceberg, which will
provide additional information on regional ocean current
patterns and indirectly on the measurement of ocean
water depths.
"The twin icebergs have behaved
quite differently in terms of their drift patterns since
they split apart," said Massom. "This
highlights the complexity of ocean currents in the
region, which are poorly understood."
Massom plans to continue using
satellite data to monitor long-term sea ice response to
the change in coastal configuration and the presence of
the two large icebergs. In situ data collection during a
field expedition aboard the Australian icebreaker Aurora
Australis, planned for 2002, will play a key role in
Massom's satellite data validation.
"Members of the Australasian
Antarctic Expedition didn't have satellite remote
sensing at their disposal," said Massom. "We
are at a distinct advantage compared to the early
explorers — we now have a superb tool that enables us
to detect and monitor changes in the Antarctic ice sheet
margin."
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Related Links:
Disintegration
of the Ninnis Glacier Tongue -- Original NASA Earth
Observatory article
Sudden
Disintegration of the Ninnis Glacier Tongue, East
Antarctica -- Brief paper by Massom outlining the
discovery of the tongue's calving
Earth's
Fidgeting Climate -- Science@NASA article about the
vexing ambiguities of global climate change research,
such as the difficulty of proving scientifically that
observed ice sheet melting is due to a human-induced
"greenhouse effect."
Antarctic
Cooperative Research Centre -- Home page
Dr.
Rob Massom -- Home page, including a link to his
e-mail address
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