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http://www.jpl.nasa.gov/news/news.php?feature=6332
NASA Study Solves Two Mysteries About Wobbling Earth
Jet Propulsion Laboratory
April 8, 2016
Using satellite data on how water moves around Earth, NASA scientists
have solved two mysteries about wobbles in the planet's rotation -- one
new and one more than a century old. The research may help improve our knowledge of past and future climate.
Although a desktop globe always spins smoothly around the axis running
through its north and south poles, a real planet wobbles. Earth's spin
axis drifts slowly around the poles; the farthest away it has wobbled
since observations began is 37 feet (12 meters). These wobbles don't affect our daily life, but they must be taken into account to get accurate results from GPS, Earth-observing satellites and observatories on the ground.
In a paper published today in Science Advances, Surendra Adhikari and
Erik Ivins of NASA's Jet Propulsion Laboratory, Pasadena, California, researched how the movement of water around the world contributes to Earth's rotational wobbles. Earlier studies have pinpointed many connections between processes on Earth's surface or interior and our planet's wandering ways.
For example, Earth's mantle is still readjusting to the loss of ice on
North America after the last ice age, and the reduced mass beneath that continent pulls the spin axis toward Canada at the rate of a few inches
each year. But some motions are still puzzling.
A Sharp Turn to the East
Around the year 2000, Earth's spin axis took an abrupt turn toward the
east and is now drifting almost twice as fast as before, at a rate of
almost 7 inches (17 centimeters) a year. "It's no longer moving toward
Hudson Bay, but instead toward the British Isles," said Adhikari. "That's
a massive swing." Adhikari and Ivins set out to explain this unexpected change.
Scientists have suggested that the loss of mass from Greenland and Antarctica's
rapidly melting ice sheet could be causing the eastward shift of the spin axis. The JPL scientists assessed this idea using observations from the NASA/German Aerospace Center Gravity Recovery and Climate Experiment (GRACE) satellites, which provide a monthly record of changes in mass around Earth. Those changes are largely caused by movements of water through everyday processes such as accumulating snowpack and groundwater depletion. They calculated how much mass was involved in water cycling between Earth's
land areas and its oceans from 2003 to 2015, and the extent to which the
mass losses and gains pulled and pushed on the spin axis.
Adhikari and Ivins' calculations showed that the changes in Greenland
alone do not generate the gigantic amount of energy needed to pull the
spin axis as far as it has shifted. In the Southern Hemisphere, ice mass
loss from West Antarctica is pulling, and ice mass gain in East Antarctica
is pushing, Earth's spin axis in the same direction that Greenland is
pulling it from the north, but the combined effect is still not enough
to explain the speedup and new direction. Something east of Greenland
has to be exerting an additional pull.
The researchers found the answer in Eurasia. "The bulk of the answer is
a deficit of water in Eurasia: the Indian subcontinent and the Caspian
Sea area," Adhikari said.
The finding was a surprise. This region has lost water mass due to depletion of aquifers and drought, but the loss is nowhere near as great as the
change in the ice sheets.
So why did the smaller loss have such a strong effect? The researchers
say it's because the spin axis is very sensitive to changes occurring
around 45 degrees latitude, both north and south. "This is well explained
in the theory of rotating objects," Adhikari explained. "That's why changes
in the Indian subcontinent, for example, are so important."
New Insight on an Old Wobble
In the process of solving this recent mystery, the researchers unexpectedly came up with a promising new solution to a very old problem, as well.
One particular wobble in Earth's rotation has perplexed scientists since observations began in 1899. Every six to 14 years, the spin axis wobbles
about 20 to 60 inches (0.5 to 1.5 meters) either east or west of its general direction of drift. "Despite tremendous theoretical and modeling efforts,
no plausible mechanism has been put forward that could explain this enigmatic oscillation," Adhikari said.
Lining up a graph of the east-west wobble during the period when GRACE
data were available against a graph of changes in continental water storage for the same period, the JPL scientists spotted a startling similarity
between the two. Changes in polar ice appeared to have no relationship
to the wobble -- only changes in water on land. Dry years in Eurasia,
for example, corresponded to eastward swings, while wet years corresponded
to westward swings.
When the researchers input the GRACE observations on changes in land water mass from April 2002 to March 2015 into classic physics equations that
predict pole positions, they found that the results matched the observed east-west wobble very closely. "This is much more than a simple correlation," coauthor Ivins said. "We have isolated the cause."
The discovery raises the possibility that the 115-year record of east-west wobbles in Earth's spin axis may, in fact, be a remarkably good record
of changes in land water storage. "That could tell us something about
past climate -- whether the intensity of drought or wetness has amplified
over time, and in which locations," said Adhikari.
"Historical records of polar motion are both globally comprehensive in
their sensitivity and extraordinarily accurate," said Ivins. "Our study
shows that this legacy data set can be used to leverage vital information about changes in continental water storage and ice sheets over time."
GRACE is a joint NASA mission with the German Aerospace Center (DLR) and
the German Research Center for Geosciences (GFZ), in partnership with
the University of Texas at Austin. For more information on the mission,
visit:
http://grace.jpl.nasa.gov
http://www.csr.utexas.edu/grace
NASA uses the vantage point of space to increase our understanding of
our home planet, improve lives and safeguard our future. NASA develops
new ways to observe and study Earth's interconnected natural systems with long-term data records. The agency freely shares this unique knowledge
and works with institutions around the world to gain new insights into
how our planet is changing.
For more information about NASA's Earth science activities, visit:
http://www.nasa.gov/earth
News Media Contact
Alan Buis
Jet Propulsion Laboratory, Pasadena, California
818-354-0474
Alan.buis@jpl.nasa.gov
Written by Carol Rasmussen
NASA Earth Science News Team
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