NASA'S Grace Gravity Mission Weighs in on Earth's Changing Climate

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JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
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http://www.jpl.nasa.gov

Alan Buis (818) 354-0474
Jet Propulsion Laboratory, Pasadena, Calif.

Gretchen Cook-Anderson (202) 358-0836
NASA Headquarters, Washington,
D.C.

NEWS RELEASE: 2004-224
September 9, 2004

NASA'S Grace Gravity Mission Weighs in on Earth's Changing Climate

For the first time, scientists have demonstrated that precise
measurements of Earth's changing gravity field can effectively monitor
changes in the planet's climate and weather.

This finding comes from more than a year's worth of data from the
Gravity Recovery and Climate Experiment, or Grace. Grace is a
two-spacecraft, joint partnership of NASA and the German Aerospace
Center.

Results published in the journal Science show that monthly changes in
the distribution of water and ice masses could be estimated by
measuring changes in Earth's gravity field. The Grace data measured
the weight of up to 10 centimeters (four inches) of groundwater
accumulations from heavy tropical rains, particularly in the Amazon
basin and Southeast Asia. Smaller signals caused by changes in ocean
circulation were also visible.

Launched in March 2002, Grace tracks changes in Earth's gravity field.
Grace senses minute variations in gravitational pull from local
changes in Earth's mass. To do this, Grace measures, to one-hundredth
the width of a human hair, changes in the separation of two identical
spacecraft in the same orbit approximately 220 kilometers (137 miles)
apart.

Grace maps these variations from month to month, following changes
imposed by the seasons, weather patterns and short-term climate
change. Understanding how Earth's mass varies over time is an
important component necessary to study changes in global sea level,
polar ice mass, deep ocean currents, and depletion and recharge of
continental aquifers.

Grace monthly maps are up to 100 times more accurate than existing
ones, substantially improving the accuracy of many techniques used by
oceanographers, hydrologists, glaciologists, geologists and other
scientists to study phenomena that influence climate.

"Measurements of surface water in large, inaccessible river basins
have been difficult to acquire, while underground aquifers and deep
ocean currents have been nearly impossible to measure," said Dr. Byron
Tapley, Grace principal investigator at the University of Texas Center
for Space Research in Austin, Texas. "Grace gives us a powerful new
tool to track how water moves from one place to another, influencing
climate and weather. These initial results give us great confidence
Grace will make critical contributions to climate research in the
coming years," he added.

"The unparalleled accuracy of the Grace measurements opens a number of
new scientific perspectives," said Dr. Christoph Reigber of
GeoForschungsZentrum Potsdam in Germany. "Observations of mass
variations over the oceans will assist in interpreting annual signals
in long-term sea-level change that have become an important climate
change indicator," Reigber said.

Dr. Michael Watkins, Grace project scientist at NASA's Jet Propulsion
Laboratory, Pasadena, Calif., said the results mark the birth of a new
field of remote sensing. "Over the past 20 years, we've made primitive
measurements of changes in Earth's gravity field over scales of
thousands of kilometers, but this is the first time we've been able to
demonstrate gravity measurements can be truly useful for climate
monitoring," he said.

"The Grace gravity measurements will be combined with water models to
sketch an exceptionally accurate picture of water distribution around
the globe. Together with other NASA spacecraft, Grace will help
scientists better understand the global water cycle and its changes,"
Watkins added.

The University of Texas Center for Space Research has overall mission
responsibility. German mission elements are the responsibility of
GeoForschungsZentrum Potsdam. Science data processing, distribution,
archiving and product verification are managed under a cooperative
arrangement between JPL, the University of Texas and
GeoForschungsZentrum Potsdam.

For more information about Grace on the Internet, visit
http://www.csr.utexas.edu/grace
or http://www.gfz-potsdam.de/grace

.. For information about NASA programs on the Internet, visit
http://www.nasa.gov/