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Axis Upheaval


If the West Antarctic Ice Sheet melted, Earth’s axis would shift. The amount of sea level change caused by this change would vary dramatically from place to place. In two locations—one in a virtually uninhabited region in the Indian Ocean (lower right) and one centered on the U.S. (upper left)—sea level would rise about an additional 1.3 meters (around 4 feet).

Reprinted with permission of AAAS.
JX Mitrovica et al., Science 323:753 (2009)

If you stand anywhere on the equator, you’re spinning around Earth’s pole at about 1,000 miles an hour. The centrifugal force of this motion causes Earth to bulge slightly. The diameter of the planet at the equator is about 25 miles greater than the diameter measured between the poles. The ocean’s surface also bulges slightly with Earth’s rotation around its axis. If Earth’s axis of rotation moved, the location of the ocean bulge would also move, causing sea level to go up in some places and down in others. The equatorial bulge of the rest of planet would also change, though much more slowly. The interior of the planet, infinitely stiffer than seawater, would take many millennia to adjust its shape in response to a change in Earth’s rotation.

Polar ice sheets are not located symmetrically on Earth’s poles. Therefore, in light of their huge mass, Earth’s axis of rotation changes as they grow or shrink. Jerry Mitrovica has calculated that if the West Antarctic Ice Sheet melted in its entirety, the southern pole of Earth’s axis would move about half a mile toward the tip of South America. This change alone would raise sea level about 4 feet in two roughly circular regions, one in the Indian Ocean and the other centered on the US.  This calculation demonstrates that movement, or wandering, of the poles is a substantial effect. It also demonstrates another way that changes in sea level are highly variable around the globe. Mitrovica takes polar wandering into account when inferring the relationship between changes in local sea level since the Pliocene observed at Pliomax sites and global average sea level change.

Learn about other adjustments the Pliomax team makes to its field measurements: