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By Peter Rejcek
Konrad Steffen is one of the world’s leading experts
on climate change in Greenland, having traveled to the
ice-covered island every year since 1990. He oversees
a network of 22 weather stations that collect data about
the ice sheet, and returns with his graduate students
each year to the same field camp north of the Arctic Circle
to study how the ice sheet and its glaciers are responding
to a warming climate.
Greenland is the poster child for global warming, the
canary in the coalmine, and every other cliché
used to describe the phenomenal amount of ice loss under
way — a discharge causing sea level to rise steadily,
millimeter by millimeter, like grains of sand passing
through an hourglass.
Far away, in another hemisphere, the West Antarctic Ice
Sheet and its glaciers are pouring ice into the ocean
just as quickly. And at the northern tip of the Antarctic
Peninsula, which juts away from the continent like a rhinoceros
horn, temperatures are rising faster than just about anywhere
on the planet. The disintegration of large ice shelves,
formerly thought a once-in-a-lifetime occurrence, has
happened twice already in this decade.
“Our focus is to measure the Larsen C from the
glaciological viewpoint and climatological viewpoint …
To find out what is happening in a warming climate,”
explained Steffen during an interview in his office at
the Cooperative Institute for Research in Environmental
Sciences (CIRES) on the University of Colorado-Boulder
campus, where he serves as director of the institute.
The Larsen Ice Shelf was a series of three ice shelves
with distinct embayments in the northwest Weddell Sea.
The Larsen A sloughed away in 1995, and the Larsen B followed
in a spectacularly rapid fashion in February 2002, though
a relatively small chunk remains. Farther south and larger
than the states of New Hampshire and Vermont combined,
the Larsen C may disintegrate within the next decade.
Like physicians visiting a terminally ill patient, Steffen
and Rignot want to monitor the ice shelf before it disappears
— an opportunity that has eluded scientists in the
past. Earlier this year, the Wilkins Ice Shelf on the
west side of the peninsula began its fitful breakup, a
process that continued through the winter. [See related
story: Last thread]
“We want to study the state of the health of the
[Larsen] ice shelf and predict when it might collapse,”
said Rignot by phone from his office at NASA’s Jet
Propulsion Laboratory (JPL) in Pasadena, Calif., where
he is a senior research scientist. “We want to gain
insight into the collapse of an ice shelf.”
Ice shelves themselves don’t contribute to sea level
rise, but their disappearance allows the glaciers that
feed them to flow freely and more quickly into the ocean.
Understanding the processes that contribute to destroying
these floating blocks of ice, which can be several hundred
meters thick, will help the scientists develop better
computer models for ultimately predicting how high the
seas may rise in the future.
“The modeling is only as good as we understand the
process. Currently, we know very little about the process
of ice shelf disintegration,” Steffen explained.
What the scientists do know from satellite data and some
past studies is that the ice shelf is definitely warming
and changing. Rignot said Larsen C is thinning at the
north but possibly thickening to the south. The region
receives some of the heaviest precipitation on the entire
continent, most of which is a polar desert.
Satellites have observed melting on the shelf surface,
Steffen noted. “We can see these melt waves going
across the entire Larsen C that did not happen earlier
on, like 10 years ago,” he said.
Steffen, his graduate student Dan McGrath, and Ala Khazendar,
a post-doc from Rignot’s group, will head south in
October for the field component of the project, which
includes collaboration with Chilean and British scientists.
The logistics of the operation — flying and landing
on the ice shelf, and then establishing a camp and setting
up weather stations — have proven to be challenging.
The U.S. Antarctic Program has limited assets in the
region, with one small research base, Palmer Station,
on the other side of the peninsula. The research vessel
Laurence M. Gould supports the station and science out
of Punta Arenas, Chile, across the often rough Drake Passage.
To jump to the other side of the thin but mountainous
peninsula, Steffen and his team will need air support.
Originally, the plan was to work with the Chileans to
fly to the British Antarctic Survey (BAS) research station
Rothera aboard a military plane, and then onward to the
ice shelf on a Twin Otter aircraft. But contaminated jet
fuel at the BAS station would have delayed the trip until
January.
“We cannot wait that long because it’s beginning
to melt on the ice shelf,” Steffen said. “You
can’t land your Twin Otter on an ice shelf that starts
to get wet; you get too much friction and can’t take
off.”
Instead, about two months ago, BAS volunteered to coordinate
the entire operation, taking Steffen’s team and two
British groups to the ice shelf, where they will share
the same field camp and make complementary measurements.
“In Greenland, I’m used to organizing my own
logistics. … I just charter a plane and go where
I need to go. In Antarctica, it’s not that easy.”
The big task in this first of three field seasons will
be the deployment of several 10-meter-tall automatic weather
towers like the ones in Greenland, which record climate
data such as precipitation and temperature. A GPS will
measure the horizontal and vertical motions of the shelf.
“We want to measure the tidal motion, so we go to
millimeter accuracy,” Steffen said.
In addition, the scientists will scout the underbelly
of the ice shelf using ground-penetrating radar. The thought
is that warmer ambient temperature — about 2.5 degrees
centigrade in the last 50 years — alone isn’t
breaking up the ice shelf. Steffen and other glaciologists
believe a slightly warmer ocean is assaulting the ice
from underneath, carving out huge cavities and channels
that thin and weaken the shelf.
“We have a warming climate, but the temperature
doesn’t really penetrate all the way through an ice
shelf,” Steffen explained. He has already observed
the phenomenon in Greenland’s glaciers and floating
ice tongues, similar to ice shelves but bounded by mountains.
There he observed the ocean whittle away a half-kilometer-thick
glacier down to 100 meters in places. In addition, melt
pools form on top, changing the structure of the ice as
it seeps down and refreezes.
“If you do that over several seasons, you can warm
up the ice body to a temperature not where it’s breaking
but where the water can penetrate farther down. There
must be weakening from underneath and from the top,”
Steffen explained. “You only need a very small temperature
change in the ocean, or a current change underneath the
ice shelf, that’s able to carve out these bigger
channels.”
In the air
NASA is funding the airborne component of the project,
according to Rignot. He and Robert Thomas of EG&G
Services at NASA’s Wallops Flight Facility in Virginia
are leading that part of the project in collaboration
with Chilean colleagues from Centro de Estudios Cientificos
.
A P3 aircraft from the Chilean Navy will carry a radar
sounder from the Applied Physics Laboratory and NASA’s
laser altimeter system, which is flown every year in Greenland.
The radar can determine the thickness of the ice in some
areas. The laser altimeter with centimeter accuracy will
map the topography.
NASA-funded airborne studies made similar radar passes
in the region in 2002 and 2004, the latter part of a larger
three-week expedition that included sites in Central and
South America. Still, there are little data on the ice
thickness, particularly of the 10 to 15 glaciers that
feed into Larsen C. That’s because they’re deeply
entrenched in narrow valleys, where the ice contains more
cracks and possibly many water inclusions that absorb
the radar signal
“It makes it difficult when you want to model a
system like this, and you’re limited from the start
by the definition of its basic geometry,” Rignot
said.
NASA scientists will use the results of the project to
develop models for the processes they measure. Rignot
said he could not speculate as to when they might be able
to create reliable models that will predict ice shelf
breakup, glacier speedup and the subsequent rise in sea
level.
“I think we’re at the stage where the uncertainty
is larger than before, because we realize that the existing
models are not able to explain the current evolution of
ice shelves and ice sheets. They are too simplistic,”
he explained. “Hopefully, the Larsen C project will
provide critical observation of that particular ice shelf.”
Still, Rignot remains optimistic about filling in some
vital data gaps over the next few years before Larsen
C cracks apart and slips away into a thousand icebergs.
“It’s going to be an exciting experiment because
we have a lot of tools in hand to make some major advances
in our understanding of what’s happening at Larsen
C right now,” Rignot observed. “Hopefully, this
will help us refine how these ice shelves are collapsing
with time. Once they collapse, there is no return; our
natural laboratory is gone.”
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