"[It's as though] you have two automobile-size things, one in Los Angeles and one in San Diego, and you're measuring the distance between them to the size of a red blood cell," Michael Watkins of NASA's Jet Propulsion Laboratory in Pasadena said last fall at a scientific meeting. Watkins was not involved in the new study.
This allows scientists not only to map the Earth's gravity field from space, but also to look for small changes from one year to another.
That ability helped solve the Canadian gravity anomaly.
If a downwelling mantle plume was the culprit, then there wouldn't be any detectable changes, Tamisiea said, because mantle convection occurs on fairly long time scales.
There was a signal, however—the area's gravity is increasing at a rate comparable to that which would occur if you poured two inches (five centimeters) of water across the region each year.
"It's a fairly large signal," Tamisiea said.
It probably means that the region is rebounding around half an inch (about a centimeter) per year.
Global Warming Implications
The new GRACE study is also important for people studying global climate change, Watkins told National Geographic News.
GRACE was designed mostly to study changes in water, such as melting glaciers in Greenland and Antarctica. (Related: "Greenland Ice Sheet Is Melting Faster, Study Says" [August 10, 2006].)
"Water has weight and therefore it has gravitational attraction and GRACE can observe it," Watkins said.
The new measurements, however, helps show post-glacial rebound is also occurring in many places.
"In the case of Greenland it's not big," he said, "but in Antarctica, when GRACE sees a signal of so many centimeters of ice has changed, part of that is rebound."
Thus, Watkins said, "it also allows us to remove an error source for our climate measures."
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