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Larsen B Ice
Shelf Collapses in Antarctica
Posted: 18 March 2002
Updated: 21 March 2002 14:40 MST National Snow and Ice Data Center
Photo
from the deck of the British Research vessel, "James Clark Ross" on or
about 8 March 2002. The bergs in the background are about 25 m high. Photo
courtesy of Keith Nicholls, British Antarctic Survey.
Recent Moderate Resolution
Imaging Spectroradiometer (MODIS) satellite imagery analyzed at the University
of Colorado's National Snow and Ice Data Center revealed that the northern
section of the Larsen B ice shelf, a large floating ice mass on the eastern side
of the Antarctic Peninsula, has shattered and separated from the continent. The
shattered ice formed a plume of thousands of icebergs adrift in the Weddell Sea.
A total of about 3,250 km2 of shelf area disintegrated in a 35-day
period beginning on 31 January 2002. Over the last five years, the shelf has
lost a total of 5,700 km2, and is now about 40 percent the size of
its previous minimum stable extent.
Ice shelves are thick plates of
ice, fed by glaciers, that float on the ocean around much of Antarctica. The
Larsen B shelf was about 220 m thick. Based on studies of ice flow and sediment
thickness beneath the ice shelf, scientists believe that it existed for at least
400 years prior to this event, and likely existed since the end of the last
major glaciation 12,000 years ago .
For reference, the area lost in
this most recent event dwarfs Rhode Island (2717 km2) in size. In
terms of volume, the amount of ice released in this short time is 720 billion
tons, enough ice for about 12 trillion 10 kg bags.
This is the largest single event
in a series of retreats by ice shelves in the Peninsula over the last 30 years.
The retreats are attributed to a strong climate warming in the region. The rate
of warming is approximately 0.5 degrees Celsius per decade, and the trend has
been present since at least the late 1940s. Overall in the Peninsula, extent of
seven ice shelves has declined by a total of about 13,500 km2 since
1974. This value excludes areas that would be expected to calve under stable
conditions.
Ted Scambos, a researcher with
the National Snow and Ice Data Center (NSIDC) at University of Colorado, and a
team of collaborating investigators, developed a theory of how the ice
disintegrates. The theory is based on the presence of ponded melt water on the
surface in late summer as the climate has warmed in the area. Meltwater acts to
enhance fracturing of the shelf by filling smaller cracks and forcing them
through the thickness of the ice due to the weight of the water. The idea was
suggested in model form by other researchers in the past (Weertman, 1973;
Hughes, 1983); satellite images have provided substantial observational proof
that it is in fact the main process responsible for the Peninsula shelf
disintegrations. Christina Hulbe of Portland State University and Mark
Fahnestock of University of Maryland collaborated with Scambos on the research.
A number of international
scientists have also cooperated in the general study of the demise of the
shelves and the climatic trend in the Antarctic Peninsula. As early as November
of last year, Pedro Skvarca, Head of the Glaciological Division of the Instituto
Antartico Argentino, warned of a possible impending breakup, due to very warm
spring temperatures and a dramatic 20 percent increase in the rate of flow of
the ice shelf. He and his team were the last people to set foot on the northern
portion of the shelf. Later in the summer, the Argentine group returned to their
base at Marambio, near the shelf, to await what they anticipated would be the
final disintegration event. They flew over the shelf repeatedly, measuring its
extent with GPS during the course of the breakup event.
A British research vessel, the
RRS James Clark Ross, was in the area just as the event was occurring and
provided images from the ocean surface in the region of the event. Keith
Nicholls of British Antarctic Survey (BAS) provided the images.
In prior studies, Dr. David
Vaughan and Chris Doake of BAS have reported extensively on the climate warming
in the area, and have modelled shelf stresses and possible causes of breakup.
They collaborated with Skvarca and with Austrian and German scientists, Dr.
Helmut Rott and Dr. Wolfgang Rack, who conducted detailed satellite radar image
studies and field studies in the area. The radar study also showed ice flow
increase in the years leading to breakup and an increased velocity of the
glaciers as the shelves disappeared. Radar images have provided very detalied
views of the events of past ice shelf collapses. Dr. Rott is a professor at the
Department of Meteorology and Geophysics at Innsbruck University; Dr. Rack is
now at Alfred Wegener Institute in Bremerhaven, Germany.
The melt water fracturing theory
fared well in this last event . Sequential images from the MODIS sensor, a new
satellite imager flying on NASA's Terra platform, showed extensive melt ponding
over the Larsen B in late January, consistent with an unusually warm summer and
extended melt season. In a series of images taken in February, several of the
melt ponds disappeared, presumably as they drained through opening fractures in
the ice. By 23 February, 790 km2 had shattered from the front. The
next image from 5 March showed another 1960 km2 of ice gone. The
event continued to 7 March with an additional loss of 525 km2. The
area lost by the shelf was was almost solely the region covered by melt ponds in
late January. The timing of the event, at the end of a particularly warm summer,
is consistent with the theory.
31 January 2002
17 February 2002
23 February 2002
5 March 02
MODIS images from
NASA's Terra satellite, National Snow and Ice Data Center, University of
Colorado, Boulder.
Other scientists, and Scambos,
continue to look for additional mechanisms that may contribute to the breakups.
One idea is that meltwater seeping between ice crystals and warming of the shelf
as a whole, reduces the fracture toughness of the ice so that the shelf shatters
under the same stresses imposed by local geography and the flow it used to
tolerate. Another idea is that meltwater seeps into shallow cracks and expands
the cracks as it refreezes during the winter. Ocean warming and sub-ice currents
dragging on the underside of the ice have also been cited as possible
contributors.
While the breakup of the ice
shelves in the Peninsula has little consequence for sea level rise, the breakup
of other shelves in the Antarctic could have a major effect on the rate of ice
flow off the continent. Ice shelves act as a buttress, or braking system, for
glaciers. Further, the shelves keep warmer marine air at a distance from the
glaciers; therefore, they moderate the amount of melting that occurs on the
glaciers' surfaces. Once their ice shelves are removed, the glaciers increase in
speed due to meltwater percolation and/or a reduction of braking forces, and
they may begin to dump more ice into the ocean than they gather as snow in their
catchments. Glacier ice speed increases are already observed in Peninsula areas
where ice shelves disintegrated in prior years.
With the Peninsula shelf breakups
as a guide, we can now reassess the stability of ice shelves around the rest of
the Antarctic continent. Past assessments of stability were based primarily on
mean annual temperature; with this guideline, most shelves outside the Peninsula
were considered well within their climate limit. Given the success of the melt
pond theory, we use the climate conditions and physical parameters of ice
shelves at the point of ponding as a guide in this assessment. In particular,
the next shelf to the south, the Larsen C, is very near the stability limit, and
may start to recede in the coming decade if the warming trend continues. Melt
ponds are occasionally observed in limited regions of the Larsen C shelf. More
importantly, the warmest part of the giant Ross Ice Shelf is in fact only a few
degrees too cool in summer presently to undergo the same kind of retreat
process. The Ross Ice Shelf is the main outlet for several major glaciers
draining the West Antarctic Ice Sheet, which contains the equivalent of 5 m of
sea level rise in its above-sea-level ice.
Although several recent large
iceberg calving events have been observed on the Ross and elsewhere in
Antarctica, none of these are thought to be related to ice shelf instability.
Animation of Larsen B breakup, 31
January to 7 March 2002
MODIS images from NASA's Terra
satellite, National Snow and Ice Data Center, University of Colorado, Boulder.
This is a true color animation of
the events of January, February, and March 2002 as recorded by NASA's MODIS
satellite sensor. (MODIS stands for Moderate Resolution Imaging Spectrometer, a
sensor flying on NASA's Terra satellite.) The images show the Larsen B ice shelf
and parts of the Antarctic Peninsula (on left). The first scene from 31 January
2002 shows the shelf in late austral summer with dark bluish melt ponds dotting
its surface. In the next two scenes minor retreat takes place, amounting to
about 800 km2, during which time several of the melt ponds well away
from the ice front drain through new cracks within the shelf. The main collapse
is seen in the last two scenes, on 5 March and 7 March, with thousands of sliver
icebergs and a large area of very finely divided 'bergy bits' where the shelf
formerly lay. Brownish streaks within the floating chunks mark areas where rocks
and morainal debris are exposed from the former underside and interior of the
shelf. The last phases of the retreat totalled ~2600 km2.Resolution
of the original images is 500 m.
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