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March 23, 2007
GRAVITY
MEASUREMENTS HELP MELT ICE MYSTERIES
Greenland is cold and hot. It's a deep freezer
storing 10 percent of
Earth's ice and a subject of fevered debate. If something should melt
all that
ice, global sea level could rise as much as 7 meters (23 feet).
Greenland and Antarctica
- Earth's two biggest icehouses - are
important indicators of climate change and a high priority for
research, as
highlighted by the newly inaugurated International Polar Year.
Just
a few years ago, the world's
climate scientists predicted that Greenland
wouldn't have much impact at all on sea level in the coming decades.
But recent
measurements show that Greenland's
ice cap is
melting much faster than expected.
These
new data come from the
NASA/German Aerospace Center's Gravity Recovery and Climate Experiment
(Grace).
Launched in March 2002, the twin Grace satellites circle the globe
using
gravity to map changes in Earth's mass 500 kilometers (310 miles)
below. They
are providing a unique way to monitor and understand Earth's great ice
sheets
and glaciers.
Grace measurements have revealed that in just four years, from 2002 to
2006, Greenland
lost between 150 and 250 cubic kilometers (36
to 60 cubic miles) of ice per year. One cubic kilometer is equal to
about 264
billion gallons of water. That's enough melting ice to account for an
increase
in global sea level of as much as 0.5 millimeters (0.019 inches) per
year,
according to Isabella Velicogna and John Wahr of the University
of Colorado,
Boulder.
They published their results in the
scientific journal Nature last fall. Since global sea level has risen
an
average of three millimeters (0.1 inch) per year since 1993, Greenland's
rapidly increasing contribution can't be overlooked.
"Before Grace, the change of Greenland's ice sheet was inferred by a
combination of more regional radar and altimeter studies pieced
together over
many years, but Grace can measure changes in the weight of the ice
directly and
cover the entire ice sheet of Greenland every month," says Michael
Watkins, Grace project scientist at NASA's Jet Propulsion Laboratory,
Pasadena,
Calif. However, as anyone who has ever been concerned about his or her
weight
knows, a number on a scale is just the beginning. In the five years
that Grace
has been flying, scientists have found ways to make the most of this
new set of
observations.
"Grace
has a big footprint," says Watkins. "We can locate
regions of greatest loss, but we can't see individual glaciers."
However,
Grace's spatial resolution is continually improving. In the most recent
studies, he says, Grace has observed large ice losses in the southeast
of Greenland,
while other areas, such as the west coast,
have shown losses as well.
While
Greenland
is losing ice, it's also acquiring some new ice through precipitation.
Scientists at NASA's Goddard
Space Flight
Center, Greenbelt,
Md.,
used Grace to determine that ice losses far surpass ice gains. A new
way of
analyzing the data allowed them to get a picture of regional changes.
While
snow added 60 cubic kilometers (14 cubic miles) of ice mass to
Greenland's
interior each year between 2003 and 2005, the low-lying coast areas of Greenland lost nearly three
times as much ice - 172 cubic
kilometers (41 cubic miles) - each year during the same period.
To confirm just how much of the mass Grace detects in Greenland and Antarctica is due to snow and
ice, scientists also have
to determine the contributions from another source, Earth's changing
crust.
"When Grace sees a change in polar gravity," says Watkins, "part
of it is today's ice melt and part is what is called post-glacial
rebound."
"A long time ago during the last ice age, this region was pushed down
by
even more snow and ice, and now this mantle wants to come back, or
rebound," explains Erik Ivins, a JPL Earth scientist and Grace science
team member.
One way to look at the problem, says Ivins, is to imagine a bathtub
filling up
with water from a faucet but losing water from holes in the bottom of
the tub.
At the same time, the bathtub may be changing shape.
Ivins and his colleagues are refining the computer models used to
understand
and predict post-glacial rebound. It turns out that beneath the ice
sheet
covering Greenland,
the mantle isn't changing
the shape of the ?bathtub" very fast. "This tells us that the large
mass changes Grace detects in the southeastern region of Greenland
aren't due to post-glacial rebound," says Ivins.
As Grace celebrates its fifth birthday and begins its extended mission,
"we're getting the picture into better focus," says Watkins,
"and we're going to have a new wave of discoveries. Improving the
post-glacial
rebound model is going to help, especially in Antarctica,
where post-glacial rebound has a big effect on the gravity signal.
We're also
going to be able to pinpoint areas of loss and better understand how
the losses
change from one particular year to the next. This will tell us more
about the
types and mechanisms of ice mass loss so we can make better predictions
in the
future."
While Grace provides a new and independent way to study Earth's ice
sheets, it
will take a combination of different tools, including laser altimeters,
radar,
and field studies, to sort out more clearly what's happening. "All
technologies have different strengths and weaknesses," says Watkins.
"Grace is the new piece. It shows us the big picture, while other
measurements
look at a smaller scale. We need to use them all together."
"We have to pay attention," Velicogna adds. "These ice sheets
are changing much faster than we were expecting. Observations are the
most
powerful tool we have to know what is going on, especially when the
changes -
and what's causing them - are not obvious."
For
more information and images,
visit:
http://www.nasa.gov/vision/earth/lookingatearth/grace-20070320.html
##
Contact:
Alan Buis
NASA Jet Propulsion Laboratory
818-354-0474
This text is
derived from:
http://www.nasa.gov/vision/earth/lookingatearth/grace-20070320.html
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