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By Peter Rejcek
If the global climate change story has a lead protagonist
south of the Equator, it’s West Antarctica, a marine-based
ice sheet discharging ice into the ocean about as fast
as Greenland is in the Arctic.
Scientists generally consider East Antarctica, a much
larger and thicker ice sheet, still a minor character
in the worldwide warming scenario. But there are signs
that even this behemoth may be responding to changing
climate — and parts of the ice sheet may be more
sensitive than first believed.
Researchers from the United States, United Kingdom and
Australia have teamed up to explore one of the last relatively
uncharted areas of East Antarctica, an expanse that may
prove to be the soft underbelly of the ice sheet, the
chink in the armor. In this region, roughly the size of
Mexico, two huge subglacial basins sit well below sea
level, like most of the West Antarctic Ice Sheet.
“That’s probably been the area that’s
controlled the evolution of East Antarctica, the fast
changes over East Antarctica, over the last 10 million
years,” said Don Blankenship , a research scientist
at the Jackson School’s Institute for Geophysics
at the University of Texas at Austin (UTIG) . Blankenship
is also the principal investigator on the project, an
International Polar Year initiative dubbed ICECAP, for
Investigating the Cryospheric Evolution of the Central
Antarctic Plate .
Beginning in January 2009, the ICECAP team will use an
upgraded World War II-era DC-3 aircraft with a suite of
geophysical instruments to measure the thickness of the
ice sheet and the texture, composition, density and topography
of the bedrock below the ice.
“The large basins of East Antarctica — Wilkes
Basin, Aurora Basin, the Adventure Trench and the Astrolabe
Basin — are all well below sea level and are all
connected to the coastline and each other by channels
that are continuously below sea level,” explained
Andrew Wright , post-doctoral researcher on the ICECAP
project from the University of Edinburgh in the UK .
“Some of the biggest glaciers in Antarctica …
each of which is associated with one of the deep East
Antarctic basins, have recently been showing signs of
surface lowering similar to those observed on the big
glaciers in West Antarctica,” he added. ICECAP will
focus its work on the Wilkes and Aurora basins.
Why is it a problem if that ice sits below sea level?
Increases in atmospheric temperatures alone can’t
melt these blocks of ice. The ocean plays a key role as
warm water flows up under ice shelves and melts them from
below. This speeds the flow of the ice sheet, which is
grounded to the bedrock, into the water.
Scientists estimate the West Antarctic Ice Sheet could
raise sea levels about six meters if wholesale melting
and breakup occurred. All the ice in East Antarctica could
push sea levels by more than tenfold of that estimate
were it all to disintegrate, though much of it is above
sea level. Still, the ice in the low-lying area targeted
by ICECAP is at least a couple of kilometers thick.
“It has several times as much sea level [potential]
over these sub-marine basins as West Antarctica does,”
Blankenship noted. “The same sorts of instabilities
that apply to West Antarctica apply to East Antarctica.”
Moving west to east
Blankenship and colleagues in Texas have been winging
their way above Antarctica’s frozen wastes for nearly
20 years doing aerogeophysical surveys of the ice sheets
and the rock below. In collaboration with colleagues like
Martin Siegert , a glaciologist and ICECAP principal investigator
who heads the School of GeoSciences at the University
of Edinburgh , Blankenship has mapped out the West Antarctic
Ice Sheet using a suite of geophysical instruments.
But he, Siegert and others are also interested in the
subglacial lakes that run below East Antarctica like a
municipal waterworks. Lake Vostok is the most famous of
the subglacial reservoirs, but there are dozens of these
bodies of water,which don’t freeze because of the
enormous pressures of the ice above them.
A recent study published online in the journal Nature
Geoscience used satellite data that revealed a complex
network of subglacial plumbing in which water periodically
cascades from one lake to another. Water acts as a lubricant,
reducing friction at the base of the ice and making the
ice flow faster.
Blankenship said it is important for the science community
to learn more about East Antarctica’s hydrological
system and to map out the boundaries between ice and rock.
That will help the researchers understand the ice sheet’s
evolution and what role the lakes play in its formation
and dynamics.
“The big problem is that very little was known about
the boundary conditions in East Antarctica,” Blankenship
said. “There are a few places that have been covered
pretty well, but the majority of the place is unexplored.”
Some work took place in the 1970s, but little has transpired
since then.
Blankenship and Jack Holt , also with UTIG, worked with
the British Antarctic Survey (BAS) in 2004-5 to explore
what Blankenship called the last piece of unknown West
Antarctica, the Amundsen Sea Embayment . Using airplanes
with radar antennas strapped under the wings, the scientist
created detailed topographic maps of the rocks and sediment
that form the bed on which the ice sits.
“It’s miserable working out there,” Blankenship
said. “The weather is horrible, but we got really
lucky and we were able to do [the whole airborne] program.”
UTIG’s aerogeophysical work, he explained, provides
a sort of roadmap that other researchers use for more
detailed studies of process like the ice-ocean interaction.
“That will be going on for years and years, trying
to understand the Thwaites and Pine Island glaciers, in
particular, in West Antarctica,” Blankenship said.
The time is right to move on to East Antarctica, he added.
“We’re just the leading edge. We always go in
first.”
The U.S. National Science Foundation (NSF) , the U.K.
Natural Environment Research Council, the Australian Antarctic
Division (AAD) and the University of Texas are providing
funding for ICECAP.
The Australians are offering the use of their coastal
research base, Casey Station , as the hub of operations
for the first season of the three-year project. That allows
the researchers to fly the converted DC-3 without relying
on fuel caches in the field. The British and the University
of Texas covered the costs of the plane upgrades, while
the NSF and AAD pay for the personnel and deployments,
according to Blankenship.
“This is a real IPY project, where everybody is chipping
a lot in,” he noted. ICECAP will use the U.S. Antarctic
Program’s McMurdo Station for sorties in future seasons.
Added Wright, “I think the important thing is that
the level of funding necessary, and the need to use the
Antarctic bases [and] logistics of more than one country
in order to access the regions of interest, means that
projects of this scale can only be attempted as collaborations
between nations.”
A flying laboratory
The ICECAP team will fly eight-hour missions out of Casey
Station every good-weather day over a three-week period,
according to Tas van Ommen , a glaciologist with the Australian
Antarctic Division and ICECAP principal investigator.
“The hope is to cover a radial pattern of flight
lines covering most of the Aurora subglacial basin,”
he explained.
The instruments aboard the plane include ice-penetrating
radar to record echoes from the ice surface and base as
well as from within the ice sheet, the University of Edinburgh’s
Wright explained. Radar antennas installed beneath the
wings of the aircraft transmit and receive the signals.
“Data [are] recorded in flight, but require processing
to make it useful,” he said. “We will also measure
and record changes in the Earth’s gravity and magnetic
fields, which, when combined with the radar measurements
of ice thickness, can be used to determine information
about the rock types and large-scale geology beneath the
ice sheet.”
The aircraft also sports a magnetometer within a tail-boom
that sticks about 3 meters from the rear of the aircraft.
“Accurate position information is important for this
kind of survey, so GPS and aircraft altitude [and] orientation
sensors are an important part of our equipment,”
Wright said.
Going way back
The survey will also help pinpoint the location where
scientists might expect to find ice that is more than
one million years old. The Australians are particularly
interested in drilling an ice core that could provide
a paleoclimate record that would complement data from
marine sediment cores, according to van Ommen.
Scientists can use ice cores to recreate past climate
by analyzing the trapped gases and dust in the ice. The
oldest ice core record, also from East Antarctica, stretches
back about 800,000 years.
The million-year mark is not just a psychological barrier
to break, van Ommen explained. “We see in the marine
sediment record that before about 1.3 million years …
ice age cycles beat with a rhythm of around 41,000 years.
For the last 800,000 years, though, ice ages have waxed
and waned on a 100,000-year cycle.”
Said Blankenship, “People really want to understand
what throws the switch between 40,000 and 100,000 years.
In other words, what does it take to change the climate
like that, and it’s down there right around 1 million
years.”
Theories exist, van Ommen said, “and one general
line of argument implicates a background change in overall
planetary [carbon dioxide]. Understanding this has clear
implications for predicting where we head in a high CO2
future, and in our general understanding of mechanisms.”
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