Accelerated melting of Earth's glaciers in recent years has forced the planet to let a notch out of its belt as its midsection gains girth, according to a study released today.
The increased water flowing into the oceans each year since 1997 is equal to a square block of ice that would cover much of Utah, accounting for about half of a mysterious equatorial bulge first reported in August.
The other contributing half appears to involve changing ocean currents that have redirected water from polar regions to tropical areas.
The puzzling redistribution of mass actually involved a measurement of Earth's gravitational field.
Earth has never been exactly spherical; it has always been somewhat pumpkin-shaped. Since the last Ice Age, though, the planet has gotten rounder and rounder as ground beneath the polar regions, relieved of the weight from ice, rebounds.
Between 1997 and 1998, however, that rounding tendency suddenly reversed, satellite data showed, and the equator seems to have been getting fatter ever since. Scientists who announced the reversal could not explain it, but they were nearly certain the answer was in the oceans.
The new study seems to confirm that suspicion."Our solution to the puzzle is rather straightforward, and involves redistribution of water mass from high to low latitudes, in quantities sufficient to counteract the ongoing effects of post-glacial rebound," said Jean Dickey, a researcher at NASA's Jet Propulsion Laboratory in Pasadena, Calif., who led the latest work.
The study found less water in the so-called Antarctic Circumpolar Current, and more water in the tropical Pacific and Indian Oceans. The results are laid out in the Dec. 6 issue of the journal Science.
Long-term changes in ocean currents may well be normal, like extended versions of the El Nino cycle, but researches don't have data over long enough time periods to know what exactly goes on.
The melting of mountain glaciers, on the other hand, is of greater concern to scientists pondering the extent and effects of global warming. Other research teams have catalogued the phenomenon in many regions of the planet. Evidence continues to mount that a warming atmosphere is fueling accelerated melting worldwide.
"The enhanced glacial melting amounts to several hundred cubic kilometers of water per year during the time of the most rapid changes [1997-98]," Dickey told SPACE.com. "The transport [of water] toward the equator in the ocean had a similar magnitude."
Dickey's team worked with data supplied by the scientists who made the original discovery of the planet's swelling midsection. They also analyzed recent data on the oceans, land ice, the atmosphere, and groundwater. The study did not consider possible effects of polar ice sheets in Greenland and Antarctica, for which not enough information was available.
Dickey cautioned that the study is not entirely conclusive, as the changes in sea level are measured in millimeters and represent a "daunting task" that requires numerous corrections to account for various known factors, such as natural short-term fluctuations.
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