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Cracking Up
Why Is the Ross Ice Shelf
Shedding So Many Icebergs?
Since March, the Ross Ice Shelf, as seen
in this satellite photo, has shed five
massive icebergs. The latest, B-20, was
knocked from Antarctica by a piece,
B-15A, of an ice chunk known as
“Godzilla,” B-15. (National Ice Center,
National Oceanic and Atmospheric
Administration)
By Amanda Onion
Oct. 5 — Imagine if the state of Connecticut somehow
splintered from the continental United States, then
floated south and rammed into Long Island,
knocking a chunk of the New York appendage into
the Atlantic.
Granted, that’s not likely to happen, but convert those
land masses to ice and you have the approximate scenario of
what most likely occurred last week on the western coast of
Antarctica.
A Connecticut-sized iceberg, known as B-15, peeled off
the Ross Ice Shelf in
March and has been
knocking around the
continent’s coastline ever
since. Although clouds
prevented satellites from
capturing the moment,
Doug MacAyeal, an iceberg
specialist at the University
of Chicago, is almost
positive B-15 was
responsible for knocking a
smaller iceberg, known as
B-20, from the shelf.
“B-20 was ready to calve and it needed a little shove
from some outside stimulus,” he says, “like the spank a
doctor gives to a newborn to get it to begin breathing.”
Trigger Uncertain
All this banging around by the iceberg known as “Godzilla”
(B-15 weighed about 2 billion tons and measured about
4,250 square miles before breaking up into five pieces) has
led to cluttered waters around the Ross Ice Shelf. In addition
to B-20, which extends for about 350 square miles, B-15
triggered another large iceberg, B-17, to calve late last
March. MacAyeal predicts there are more to come.
Is global warming also a factor? Scientists aren’t yet
sure.
“There has definitely been an increased amount of
calving,” says Bob Bindschadler of NASA’s Goddard Space
Flight Center. “Whether that is something that is part of a
natural variability or due to rising water temperatures we
can’t really say. But it does beg the question: What is
causing the increase?”
Throughout history, people have documented intimidating
encounters with icebergs. There was the Titanic’s fateful
collision with a 60-foot iceberg in 1912. That chunk of ice
from the north tore through the ship’s hull and doomed
1,503 passengers to a chilling death. And in 1956, crewmen
aboard a U.S. Navy icebreaker near the Ross Ice Shelf
reported citing a floating chunk that supposedly measured an
incredible 208 by 60 miles.
But it wasn’t until the early 1970s that scientists gained
access to pictures of icebergs forming through images
snapped by military and research satellites. These bird’s-eye
images are what captured the near disappearance of
Antarctica’s Wordie Ice Shelf between 1974 and 1989 and
the even more dramatic collapse of the Larsen Ice Shelf in
just 50 days in 1994.
Although satellites haven’t captured much iceberg activity
at the Ross Ice Shelf before this year, Stan Jacobs of
Columbia University’s Lamont-Doherty Earth Observatory has
pieced together early descriptions of the shelf since it was
first discovered in 1841. These records, he says, suggest it
may have been long due to shed some icebergs.
“Early records at the beginning of the century show the
region to be much further south than it is now,” Jacobs says.
“It may have gone 75 to 90 years without a large calving
event.”
Continent in Motion
Antarctica is a slow-moving continent that produces icebergs
as part of a natural process. At the interior of the continent,
the weight of piles of ice — sometimes a mile thick —
continually push downward and outward. At the continent’s
edges, the ice forms shelves and eventually falls off into the
sea.
Jacobs and MacAyeal suspect simple ice pile-up at the
Ross Ice Shelf might be the reason behind the recent giant
bergs like Godzilla. But others say that global warming can’t
be ruled out.
A recent study by the National Oceanic and Atmospheric
Administration showed ocean temperatures have increased
by an average of half a degree Fahrenheit near the surface
and a tenth of a degree at deep levels since the 1950s.
Bindschadler suspects this warmer ocean water sloshing at
the undersides of Antarctica’s ice shelves might increase
their melting.
“How effectively the water can get underneath, we don’t
know,” he says. “But the more it does, the more it will form
icebergs.”
Other scientists worry an increase in temperature could
speed the flow of ice streams on the continent. Ice streams
are the vast channels of slow-moving ice traveling from the
continent’s center to its edges.
Evidence of warming is more certain at the Antarctic
Peninsula where temperatures have risen by about 2 ½
degrees over the past 50 years. Following that warming, a
spate of icebergs split off in the 1980s and 1990s.
“The ice flows feeding the shelves aren’t speeding up, so
if the shelves continue disintegrating, they could become sort
of an endangered species, if you will,” says Bindschadler.
But, as Jacobs points out, the local warming at the
peninsula might be part of a natural fluctuation in
temperature.
Blocking Traffic
Whether or not global warming is making Antarctica’s ice
shelves less stable may remain uncertain for now. What is
more certain is the potential trouble the recent pile-up of
icebergs holds for resupply ships traveling to the McMurdo
Station, the logistics hub for the U.S. Antarctic programs.
Winter is now easing from the southern continent and
McMurdo hosts only a skeletal crew of about 250 through the
austral winter. But in coming months, more than 1,000
researchers, construction workers and other personnel are
due to arrive for the coming summer season.
The resupply ships, which carry everything from food to
vehicles to construction materials, are vital for keeping not
only McMurdo, but every U.S. station, including the South
Pole station, in operation.
Although the icebergs, themselves, are unlikely to block
the ships, they could cause ice buildup known as ice plates,
to form between them and the coast during the next winter
season. And, like large icebergs, ice plates are known to
stick around for many seasons. That could cause an icy
lockout of McMurdo by the station’s next delivery date in
2002.
“You could still access the station by air,” says Scambos.
“But trying to run McMurdo with only air support is a different
proposition. You’d probably have to scale back the entire
program and almost mothball the bases in the meantime.” |
