Researchers Measure Antarctic Ice Shelf Tides from Space for the First Time

Radar project sets the stage for upcoming high-tech satellite laser experiments

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 radar altimeter data 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."

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.

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