Environment

Planet Earth Wobbles As It Spins

Since 1899, the Earth's axis of spin has shifted about 34 feet (10.5 meters). Now, research quantifies the reasons why and finds that a third is due to melting ice and rising sea levels, particularly in Greenland — placing the blame on the doorstep of anthropogenic climate change.

Another third of the wobble is due to land masses expanding upward as the glaciers retreat and lighten their load. The final portion is the fault of the slow churn of the mantle, the viscous middle layer of the planet.

Also, Earth's spin isn't perfectly even, as scientists know thanks to slight wiggles in the movements of the stars across the night sky that have been recorded for thousands of years. Since the 1990s, space-based measurements have also confirmed that the Earth's axis of rotation drifts by a few centimeters a year, generally toward Hudson Bay in northeastern Canada.

Researchers knew that a proportion of this wobble was caused by glacial isostatic adjustment, an ongoing process since the end of the last ice age 16,000 years ago. As the glaciers retreat, they relieve the land underneath of their mass. Gradually, over thousands of years, the land responds to this relief by rising like bread dough. (In some places on the edges of the ancient ice sheets, the land might also collapse because the ice had forced it to bulge upward.)

The remaining proportion of Earth’s wobble is accounted for by the melting of the Greenland ice cap and melting glaciers redistributing mass as well as the convective movement of the Earth’s mantle as hotter material rises and cooler material sinks.