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past feature
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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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