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ANTARCTIC
ICE SHELF COLLAPSE IS TRIGGERED BY WARMER SUMMERS, MELT WATER
Warmer surface temperatures over
just a few months in the Antarctic can splinter an ice shelf
and prime it for a major collapse, NASA and university scientists
report in the latest issue of the Journal
of Glaciology.
Using satellite images of tell-tale
melt water on the ice surface and a sophisticated computer
simulation of the motions and forces within an ice shelf,
the scientists demonstrated that added pressure from surface
water filling crevasses can crack the ice entirely through.
The process can be expected to become more widespread if Antarctic
summer temperatures increase.
"The importance of melt
water implies that ice shelf stability may not be limited
by the mean annual temperature, as has long been thought,
but by the mean summer temperature," says co-author
Christina Hulbe of the University of Maryland and NASA’s Goddard
Earth Science & Technology Center. "As the mean summer
temperature exceeds 0 degrees Celsius, surface melting is
likely to promote ice-shelf retreat."
The team of scientists – Ted
Scambos and Jennifer Bohlander of the National Snow and Ice
Data Center at the University of Colorado, Mark Fahnestock
of the University of Maryland, and Hulbe – focused on the
Larsen Ice Shelf on the Antarctic Peninsula, which experienced
major retreats in 1995 and 1998. Over 775 square miles (2000
square kilometers) of the northern section of this ice shelf
disintegrated in January 1995 during a storm.
"This result implies that
other ice shelves are closer to the breaking point than we
previously thought," says Scambos. "The shelf retreats
that have occurred so far have had few consequences for sea-level
rise, but breakups in some other areas, such as the Ross Ice
Shelf, could lead to increases in ice flow off the Antarctic
and cause sea level to rise."
The floating ice shelves, which
account for about 2 percent of all Antarctic ice, typically
undergo cycles of advance and retreat over many decades. It
has long been recognized that melt water filling of crevasses
could enlarge the cracks, but this study is the first to connect
the basic physics of that process with significant effects
of surface melt ponds on ice-shelf viability. The extra outward
pressure of the water counteracts the internal pressure holding
the ice together.
Crevasses routinely form at the
landward side of the shelf as glacial ice pushes past coastal
features and flows into the floating ice. The crevasses slowly
travel seaward as the ice shelf grows.
Satellite observations of melted
water on the ice surface provided an important clue to the
water-pressure theory. Analyzing images of the Larsen Ice
Shelf over the past 20 years, Fahnestock found that the years
with the longest duration of surface melt water were also
the years of major shelf breakup events. The "melt season"
during the major retreat year of 1995, for example, was over
80 days long, about 20 days longer than average.
To find out if the accumulated
melt water "wedge" could split a crevasse to the
bottom of the ice, Hulbe used a computer model to simulate
the thermodynamics of a northern section of the Larsen Ice
Shelf before and after the major retreats of the 1990s. Depending
on the internal strength of the ice, a water-filled crevasse
just 15-50 feet (5-15 meters) deep could fracture through
the 220-yard (200-meter) thick ice shelf. The splintered remains
are probably held together by bridges between crevasses until
a combination of winds, tides, and another season of melting
lead to a breakup.
"We need to monitor the
summertime temperatures to see what the future holds for these
ice shelves," says Hulbe. While some areas of the Antarctic
have warmed by as much as 2.5 degrees Celsius in the last
50 years, few records have been kept of seasonal temperatures
over ice shelves.
"In the past, researchers
thought the Rose Ice Shelf area was up to 20 degrees below
the climatic limit and therefore very stable," says Scambos.
"Our initial look at summertime temperatures there shows
it is just a few degrees below what we think is the threshold
for surface ponding."
"The Link Between Climate
Warming and Break-up of Ice Shelves in the Antarctic Peninsula,"
Scambos, T.A., Hulbe, C., Fahnestock, M., and Bohlander, J.
Journal of Glaciology, vol. 46, no. 154, pp. 516-530.
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