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July 31,
2002
Scripps Contacts:
Mario Aguilera or Cindy Clark: (858) 534-3624
E-mail: scrippsnews@ucsd.edu
Researchers
Measure Antarctic Ice Shelf
Tides from Space for the First Time
Radar project sets the stage for upcoming high-tech satellite
laser experiments
Images available upon request
 In efforts to determine how Antarctica is changing-whether due
to natural or human-produced climate change-scientists use satellite and
radar technologies to monitor the height and thickness of the continent's
ice shelves. How are global warming and sea temperature changes affecting
the thickness of these massive floating ice blocks?
The height changes due to climate can be very small, perhaps only an inch
or so per year. In contrast, the ocean tides that flow underneath ice
shelves can push them up and down by several feet over the course of a
day, and this large effect can make it difficult to measure the small
climate-related changes with satellites. Now, researchers at Scripps Institution
of Oceanography at the University of California, San Diego, and Earth
and Space Research of Seattle have measured Antarctic ice shelf tides
from space for the first time. Through their research, the effect of tides
can be removed more accurately and thus climate-related changes can be
tracked more closely.
Helen Amanda Fricker of Scripps tapped information from the European Space
Agency's European Remote Sensing (ERS) Satellite, which beamed radar signals
to the Antarctic surface. Every 35 days, as the satellite orbited over
Antarctica, the radar signals would hit the ice shelves and bounce back
to the satellite, allowing scientists to calculate how the height of the
ice shelves was changing. On floating ice, surface height can be used
to estimate the ice thickness.
Fricker's information was combined with calculations for Antarctic tides
developed by Laurie Padman of Earth and Space Research, together setting
the groundwork for a clear measurement of how the ice shelves change.
"Ice shelves are floating ice blocks, so if the ocean underneath
them is warming, it will increase the melting under the ice shelves and
the ice is going to get thinner," said Fricker, of the Cecil H. and
Ida M. Green Institute of Geophysics and Planetary Physics at Scripps.
"Antarctic ice shelves can be sensitive areas in terms of climate
change. We want to monitor their thickness and see if they're in steady-state
or whether they are changing with time because of changes in climate."
Fricker said the ice shelves can play a critical role in buttressing,
or holding back, ice from detaching from the Antarctic continent. Removing
them, she said, may increase the flow of ice off the continent.
"As that ice melts, it will increase sea level around the world.
It's important to monitor not only the grounded ice on the continent and
how that's changing, but the floating ice as well," said Fricker.
"To do this, we need accurate repeat measurements of ice shelf height
and we have to remove the tidal signal because that will mask the true
ice shelf elevation."
Fricker and Padman's analysis served as a successful "proof of concept"
for upcoming studies investigating Antarctic ice shelves and climate change.
The collaborative study, published in a recent issue of Geophysical Research
Letters (GRL), details their analysis of eight years' worth of ERS information
using synthetic aperture radar (SAR) signals concentrated on the 500-mile-wide
Filchner-Ronne Ice Shelf in Antarctica's Weddell Sea.
"This was a first attempt," said Padman. "Now that we have
these results we are encouraged to improve our model of tides by using
more sophisticated analysis techniques and combining the new data with
numerical models based on the physics of ocean tides."
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The next step will take the form of a new satellite called ICESat being
prepared by NASA for launch later this year. A new instrument on ICESat,
the Geoscience Laser Altimeter System (GLAS), will be the first to measure
ice shelves using a sophisticated space-based laser instrument. GLAS will
beam laser pulses 40 times per second, from approximately 400 miles above
the Earth's surface, and time each pulse to determine the surface height
with an accuracy of better than six inches. Over time this will result
in a determination of the surface height change with
an accuracy of better than half an inch per year.
"GLAS will be the first spaceborne laser altimeter to cover Antarctica.
It will have a much smaller footprint on the ground than the radar altimeter
and be able to give us much more accurate measurements than ERS,"
said Fricker.
Fricker and Padman's research for the GRL study was supported by NASA
and the National Science Foundation Office of Polar Programs.
# # #
Journalists may request a copy of the paper from Harvey Leifert at hleifert@agu.org.
Please indicate whether you prefer PDF or fax and provide your contact
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