Ice loss fattening the Earth
The Earth is getting thicker around the middle due to ice loss from the Greenland and Antarctic ice sheets, says a new study by researchers from the Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado Boulder.
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“If you imagine the Earth is like a soccer ball and you push down on the North Pole it would bulge out at its ‘equator’,” said CIRES Fellow Steve Nerem, coauthor of the study with CIRES Fellow John Wahr. “That’s what it looks like —a bit like a slightly squished ball.” We refer to the size of that bulge as the Earth’s ‘oblateness.’”
As the Earth’s shape changes so does its gravity field, a variable that can be measured from satellites, Nerem said. Data from the Gravity Recovery and Climate Experiment (GRACE)—twin satellites launched in 2002 that make detailed measurements of Earth's gravity field to monitor changes in ice mass, the amount of water in the ocean and losses in continental water —enabled Nerem and Wahr to test a theory that the ice loss was changing the oblateness.
Using the GRACE values for ice loss in Greenland and Anarctica, the scientists predicted how that ice loss has changed the Earth’s oblateness since 2002, and their calculations agreed with the changes recorded by laser ranging measurements from a variety of different satellites. “We found that Greenland and Antarctica cause most of this change,” Nerem said. Their results are currently in press in the journal Geophysical Research Letters.
From the time scientists first began measuring the Earth’s shape, they’ve noted it’s not a perfect sphere, Nerem said. The spinning of the planet means, just like any non-rigid spinning object, material tends to move out to the equator. “There is more mass along the equator than there is at the poles.”
Most of the time the scientists have been taking measurements of its shape the Earth has been changing from this elliptical, or oblate shape, to a rounder one as it readjusts to the end of the ice age 20,000 years ago, Nerem said. Since the downward pressure of land-based ice has reduced as the ice melted, the land underneath has “rebounded” causing the Earth to become more spherical, he said.
In the mid-1990s that trend changed, however, as the planet appeared to start flattening out again, Nerem said. Puzzled by this observation, the scientific community came up with theories as to why this might be the case. “But a lot of it was speculation, albeit informed speculation,” he said.
That was until the launching of the GRACE satellite mission. Using the high-resolution GRACE dataset Nerem and Wahr were able to conduct their experiment confirming the relationship between ice mass loss and the shape of the Earth. But this Nerem says is only a starting point. “People have started to suggest that the melting in Greenland and Antarctica have started to affect the Earth’s rotation,” Nerem said. “That is another thing to think about.”
The study was supported by two separate National Aeronautics and Space Administration GRACE Science Team investigations and a Jet Propulsion Laboratory GRACE MEASURES contract. It will be published online in a future edition of Geophysical Research Letters.
Contacts:
Steven Nerem, CIRES, nerem@colorado.edu, 303-492-6721
John Wahr, CIRES, wahr@lemond.colorado.edu, 303-492-8349
Katy Human CIRES, kathleen.human@colorado.edu , 303-735-0196