Holiday special
The seasonal movement of water between north and south polar regions regularly stretches and compresses the Earth, say US researchers.
Geophysicists analysing data from the Global Positioning System were initially puzzled when a small annual Earth movement remained after they accounted for known phenomena such as tides and plate tectonics.
A careful investigation showed the effect was “hidden in the theory” describing the behavior of an elastic body like the Earth, says Geoffrey Blewett of the University of Nevada in Reno.
Each winter, snow and soil moisture accumulates in cold polar regions and the atmosphere becomes denser. The elastic body theory predicts that this extra weight should slightly compress the hemisphere experiencing winter. In the north, the maximum compression should occur in February or March, when the most water has accumulated.
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The polar region should then dry out during spring and summer, while moisture settles out at the opposite pole. The result is an annual oscillation of Earth’s shape. And this is exactly the pattern shown in the GPS data.
Hemispheric shift
Sites near the north pole sank up to three millimeters in February and March, while regions near the equator moved about 1.5 millimeters north. Then the cycle reversed, with moisture accumulating near the south pole causing it to sink about three millimeters during the southern hemisphere’s winter in August and September.
“The northern hemisphere and the southern hemisphere compress,” Blewett told New Scientist. The water that “pushes down” the poles oscillates between the hemispheres.
Once they understood the oscillation pattern, the geophysicists were able to estimate that a load of about 1016 kilograms shifts between the hemispheres each year.
Journal reference: Science (vol 294, p 2342)
