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Past Features

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past feature

Earth's Mass Redistributes

The Earth has "suddenly" started to undergo a major redistribution of its mass from high latitudes to the equatorial regions. Planet Earth is a bit wider around the equator than at the meridian. This slight oblateness (of about 0.3%) results from axial rotation and large-scale mantle convection. If the oblateness decreases with time, then mass is redistributed from equatorial regions to the high latitudes.

For the past 20 years the Earth's oblateness has been observed to be decreasing slightly. Beginning in 1998, scientists have found that a sudden increase has taken place indicating a mass redistribution back to the equatorial regions.

Several mechanisms have been considered that might explain these observations including the melting of Arctic sea ice or Alpine glaciers. Large-scale mass redistribution in the oceans, however, remain a serious candidate.

The recent change occurred in late 1997 to early 1998, at the time of the strongest El Niño event this century. The El Niño-Southern Oscillation is primarily associated with mass transport in the tropical Pacific parallel to the equator, but transport perpendicular to the equator can also occur. The dynamics of this effect are not well understood, but model studies indicate water moves from the subtropics to the tropics.

Future insights into the causes of the unexpected change should come from at least two sources. State-of-the-art ocean circulation models and the recently launched GRACE (Gravity Recovery and Climate Experiment) satellite mission will measure mass redistribution in surface waters with unprecedented precision (1 cm water equivalent) on time scales ranging from a month to several years.

The full report appears in Science magazine in the August 2 issue.



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