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Ice Bridge
Supporting Wilkins Ice Shelf Collapses
This before and
after image shows the collapse of the ice bridge connecting the remainder of
Wilkins Ice Shelf to Charcot Island. NSIDC processed these images from the NASA
Moderate Resolution Imaging Spectroradiometer (MODIS) sensor, which flies on
NASA's Earth Observing System Aqua and Terra satellites. - Credit: National
Snow and Ice Data Center
8 April 2009 The National Snow
and Ice Data Center-An ice bridge connecting the Wilkins Ice Shelf on the
Antarctic Peninsula to Charcot Island has disintegrated. The event continues a
series of breakups that began in March 2008 on the ice shelf, and highlights the
effect that climate change is having on the region.
Images from the NASA Moderate
Resolution Imaging Spectroradiometer (MODIS) sensors on the Terra and Aqua
satellites showed the shattering of the ice bridge between March 31, 2009 and
April 6, 2009. The loss of the ice bridge, which was bracing the remaining
portions of the Wilkins ice shelf, will now allow a mass of broken ice and
icebergs to drift into the Southern Ocean.
Scientists at NSIDC and around
the world have been watching the ice bridge since last March, anticipating its
collapse. Now that it has broken up, researchers are closely monitoring the
remaining portion of the Wilkins Ice Shelf to see if the loss of the ice bridge
allows the ice shelf to collapse further.
The Wilkins is following a
pattern of instability and rapid collapse that many Antarctic Peninsula ice
shelves have experienced in recent years. Scientists think that the dramatic
loss of these ice shelves, which have existed for hundreds to thousands of
years, is an important sign of climate change in the Southern Hemisphere. The
loss of an ice shelf can also allow the glaciers that feed into it to start
flowing ice into the ocean at an accelerated rate, contributing to a rise in
global sea levels.
The Wilkins Ice Shelf first began
to break up in the mid-1990s. Last March, the Wilkins lost another 400 square
kilometers (160 square miles) in a rapid retreat (March 2008 Press Release), and
the ice shelf continued to form new cracks over the winter.
The Wilkins Ice Shelf is located
on the southwestern Antarctic Peninsula, the fastest-warming region of the
Earth. In the past 50 years, the Antarctic Peninsula has warmed by 2.5 degrees
Celsius (4 degrees Fahrenheit). In the early 1990s, the Wilkins Ice Shelf had a
total area of 17,400 square kilometers (6,700 square miles). Events in 1998 and
the early years of this decade reduced that to roughly 13,680 square kilometers
(5,280 square miles). In 2008, a series of disintegrations (rapid repeated
calvings in which the ice shelf pieces are small enough to topple over) and
break-up events (rifting of large sections of the shelf, leading to large
tabular iceberg calvings) shrunk the area of stable shelf to roughly 10,300
square kilometers (4,000 square miles), a net loss within a year of
approximately 3,600 square kilometers (1,400 square miles).
April 8,2009 NASA Earth
Observatory-A narrow ice bridge connecting Charcot Island and Latady
Island - the last remnant of the northern part of Antarctica's Wilkins Ice
Shelf - broke apart in early April 2009. These photo-like images, from the
Moderate Resolution Imaging Spectroradiometer (MODIS),
show the break-up of the ice bridge.
In the lower image, taken by the
MODIS instrument on NASA's Terra
satellite on March 31, 2009, the ice bridge was still intact. The ice appears to
be smooth, an unbroken surface. Less than a week later, late on April 6, the
MODIS instrument on NASA's Aqua satellite
captured the top image. The smooth bridge is gone, replaced by chunks of ice.
The breakup was initially observed in radar imagery by the European
Space Agency.
The pieces of the former ice
bridge join multiple other chunks of ice formed as the northern portion of the
ice shelf broke apart throughout the previous decade. The broken pieces of the
shelf have remained frozen in place since 1998, but now that the ice bridge no
longer provides a barrier, the remnants of the ice shelf may flow out into the
Southern Ocean. A careful comparison of the two images reveals that some of the
ice nearest the bridge shifted between March 31 and April 6.
Cracks that formed in the ice
shelf below and right of the bridge in late 2008 expanded after the ice bridge
broke and the remnant ice nearest the shelf shifted away, says Ted Scambos of
the National Snow and Ice Data Center. These changes are emphasized by
differences in light between the two images. The Sun was low in the sky on April
6. The clouds cast long shadows on the ice beneath. By contrast, the Sun was
relatively high, and the light more direct on March 31. Fewer shadows outline
the topography on March 31. The low Sun angle highlights cracks in the ice in
the April 6 image. The cracks were first seen in radar images collected by the
European Space Agency, and were evident on November
26, 2008.
Many factors contributed to the
collapse of the northern portion of the ice shelf, including brine on the ice,
physical stresses on the shelf, and warming temperatures, says Scambos.
Throughout 2008, parts of the ice shelf (formerly to the left of the bridge)
broke away. The ice bridge had been the last intact portion of the northern edge
of the ice shelf. The southern portion of the Wilkins Ice Shelf (part of which
appears in the lower right corner of the images) is still intact, but may be
more vulnerable now that the northern edge has disintegrated.
What is the significance of the
disintegration of the northern portion of the Wilkins Ice Shelf? The collapse of
the ice shelf will not contribute to sea level rise, since the ice had already
been floating on the water. When other ice shelves such as the Larsen, have
collapsed, they allowed glaciers to pump more ice into the ocean at a faster
rate, which did contribute to sea level rise. The Wilkins Ice Shelf, however,
does not buttress any major glacier, says Scambos. The Wilkins Ice Shelf is the
tenth major ice shelf to collapse in recent times, another sign that warming
temperatures are impacting Earth's fragile cryosphere.
The island visible
in the upper left of the image is Charcot Island. The Wilkins Ice Shelf is
connected to these by an ice bridge which is approximately 100 km long and only
few km wide. Should the ice bridge break up due to increasing temperatures in
the Antarctic spring, this would remove the stabilising factor that has been
keeping the ice sheet grounded to the peninsula.
The above animation is comprised of images acquired by Envisat's Advanced
Synthetic Aperture Radar (ASAR). The ice bridge is visible as a narrow strip in
the image centre.
Credit: NASA, ESA,
NSIDC
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