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By Peter Rejcek
Terry Wilson’s office at Ohio State University seems
pretty typical of a college professor, especially one
who splits her time teaching the fundamentals of geology
to undergraduates while managing one of the largest and
most ambitious projects of the International Polar Year
(IPY).
Stacks of papers sit in relatively neat piles on the
floor, leaving just enough of an aisle to maneuver in
and out of the office. Various filing cabinet drawers
are pulled out to their maximum extent, revealing cramped
and stuffed folders. The sheer weight of the paperwork
seems enough to cause a dimple in the earth that supports
her office.
Should that material suddenly blow away, vanish in a
freak windstorm, the ground underneath would eventually
rebound to its original state, slowly sighing in relief
as the burden of weight goes away.
In Antarctica, where the weight of its mighty ice sheets
have squashed the earth’s crust below, Wilson and
an international team of scientists are studying a real
phenomenon called post-glacial rebound. The work is part
of an ambitious project, called POLENET, for Polar Earth
Observing Network.
POLENET has many goals, from helping to predict sea level
rise to learning more about the earth below the crust
and down to its core. It’s one of the flagship projects
of IPY External U.S. government site . Tom Wagner, program
director of Earth Sciences at the National Science Foundation’s
Office of Polar Programs External U.S. government site
, doesn’t mince his words when describing the value
of POLENET. “Five years from now, this will be the
most important science to come out of the International
Polar Year,” he said.
The $4.5 million project, led by scientists with the
Byrd Polar Research Center External Non-U.S. government
site at OSU, will install several dozen GPS receivers,
seismic sensors and other instruments around the West
Antarctic ice sheet, wherever a piece of rock pokes through
the ice.
The effort began last austral summer in Antarctica, and
will continue through the 2011-12 field season. Collaborators
include scientists at NASA’s Jet Propulsion Laboratory,
New Mexico Tech, Penn State, University of Memphis, University
of Texas Institute for Geophysics and Washington University.
On the rebound
The POLENET GPS receivers aren’t like a handheld
unit that you might use to set a waypoint to your favorite
backcountry trail or to navigate city streets. These instruments
can measure with millimeter-level accuracy, as one important
experiment measures the rate at which the bedrock of the
continent moves vertically.
How can the earth move in such a way? Well, there’s
a lot of ice in Antarctica, but there was even more during
what’s known as the last glacial maximum, when ice
sheets draped across parts of Europe and North America
in the northern hemisphere and Chile and Argentina in
the southern hemisphere. The ice was at its maximum extent
about 20,000 years ago, then began to shrink.
The ice sheets never disappeared from Antarctica, of
course, but they lost a lot of mass during the current
interglacial period, well before the Industrial Revolution
kicked temperature rise into high gear. How much ice has
disappeared? Well, that’s what Wilson and her team
hope to find out as they measure the rate of rebound now
that all that weight is gone. As you might imagine, it’s
a slow, lengthy process. “We’re looking for
a long-term signal that shows us this rate of uplift due
to ice mass loss since the last glacial maximum,”
explained Wilson, associate professor of Earth sciences
at Ohio State.
Elastic response
But there are more forces at work than this ancient signature.
The TransAntarctic Mountains DEFormation (TAMDEF) External
Non-U.S. government site project — a related program
that measured bedrock movement with GPS stations along
the mountain range that splits the eastern and western
halves of the continent — discovered using continuous
year-round GPS data that the crust also responds to short-term
ice mass loss and gain.
“People with GPS measurements have been able to
measure the deflection of the surface from adding snow
in the winter and removing snow in the summer,” Wilson
said. In fact, the earth at places such as the Amazon
Basin reacts strongly to this elastic effect, as a sizable
portion of South America sinks when the river floods and
then rises more than 5 to 6 centimeters when the waters
recede each year.
Many scientists, citing satellite data, believe West
Antarctica is losing ice almost as quickly as Greenland
because of global warming. The crust below the ice sheet
should have an elastic response to that massive, modern
evacuation of ice. The idea is if you can capture the
rate of elastic response, a relatively short-term event,
you can get a true measure of the size of the ice sheet
and how much mass it is losing. That would finally put
a realistic number on future sea level rise as Antarctica
continues to shed weight faster than a contestant on TV’s
“The Biggest Loser.”
“It’s kind of serendipity in a way that we’re
getting this array of instruments out in time to start
recording some of those dynamic changes, particularly
in West Antarctica and up toward the Antarctic Peninsula,
that are surprising everyone,” Wilson said.
Making waves
The data from seismometers, which record seismic waves
from earthquakes, are also a key part of the equation.
The material below the ice sheets is not uniform. It may
be thick, dense and stiff. Or perhaps it’s weak and
warm. The seismic waves can tell scientists what sits
below the ice sheets, which indicates how quickly the
crust may move up and down.
“The speed at which [seismic waves] go is a direct
recorder of the physical properties of the earth,”
Wilson explained. “Integration of seismic and GPS
is really important for understanding both short- and
long-term ice mass balance. … It’s not entirely
straightforward because you have these superimposed signals.”
Seismic waves pass quickly through dense rock, but move
slower through warm and “gooey” material. Under
one area in West Antarctica, the material is weak and
warm where the crust has pulled apart, meaning the rebound,
the long-term response, may have already occurred.
“The signal from the last glacial maximum could
be gone because it’s relaxed that signal completely
in the length of time that’s transpired, versus if
we had really cold, dense lithosphere, then you’d
have quite a lot of signal going on today,” Wilson
said.
Saving GRACE
The data from POLENET will be particularly useful to
a separate project involving a pair of satellites that
measure gravity field changes on Earth, according to Wilson.
The Gravity Recovery and Climate Experiment (GRACE) External
Non-U.S. government site , operated by the Center for
Space Research at the University of Texas at Austin, measures
changes in gravity by making accurate measurements of
the distance between the two satellites, using GPS and
a microwave ranging system.
Gravity is related to mass, so as mass grows or shrinks,
such as with an ice sheet, the satellites can detect the
changes. But one variable missing in its calculation is
the rebound effect that POLENET measures. Wilson said
a recent series of scientific papers point out that the
biggest source of error in models attempting to account
for Antarctica’s ice mass is the rebound calculation.
In the next few years that should change. “They
will have this very accurate means to track changes in
the mass in the ice in a way that they intended to do,”
Wilson said of the satellite-based study.
“This is one of the reasons that POLENET got funded.
It will make GRACE a very, very powerful tool,” noted
Mike Willis, a postdoctoral fellow at Ohio State, who
works on POLENET and its sister network in Greenland,
GNET.
In the future, using these tools, scientists will be
able to say whether Antarctica grew or shrunk on at least
a monthly basis, according to Willis. “We don’t
even know that at this time. We have hints of it, but
that’s based on models — which are probably
wrong,” he noted.
Setting up the network
The United States’ component of the five-year project
will establish 52 stations, though not all contain both
GPS receivers and seismometers. Some 28 countries are
involved in POLENET in some form or fashion, though the
United States, Italy, the United Kingdom, New Zealand
and Germany are the principal partners in the West Antarctica
venture.
Each station runs year-round, using solar power in the
summer and several hundred pounds of batteries to keep
the instruments juiced through the winter. An Iridium
modem sends the GPS data in real-time, but the seismic
information is too large to send via satellite, so those
sites require physical visits.
Stephanie Konfal, a graduate student at OSU, was a member
of this past season’s installation crew, which traveled
across West Antarctica on ski-equipped aircraft to set
up the stations. On a good day, she said, several people
could install one station in about three to four hours.
“It’s a pretty big setup per site,” she
said.
The team spent most of its time working out of Patriot
Hills, the only privately run camp on the continent, used
mainly by adventurous types for mountaineering expeditions
and ski trips to the South Pole.
“It was nice to be out in the field,” Willis
said. “I hadn’t lived in a tent since 1998-1999.
This was nice to be back to the old school. Old school
but cushy — I mean the food was glorious.”
Even more glorious for Wilson is the network that is
taking shape. A map of the continent shows variously colored
points where POLENET and other GPS and seismometer stations
are located. The dots fringe the entire coast of Antarctica,
even East Antarctica, while even more freckle the vast
interior of West Antarctica.
“It’s pretty spectacular,” Wilson said.
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