The observed direction of polar motion, shown as a light blue line, compared with the sum (pink line) of the influence of Greenland ice loss (blue), postglacial rebound (yellow) and deep mantle convection (red). Source: NASA/JPL-Caltech

You may not feel the Earth move beneath your feet, but the planet is wobbling. More precisely, Earth’s polar motion – the spin along the North Pole-South Pole axis – drifted about 4 inches (10 cm) annually during the 20th century.

Glacial rebound and other mass distribution phenomena have been widely cited by NASA scientists as responsible for this change. However, recent research points to other agents for the planet’s wayward wobble.

Analysis of observational and model-based data covering the last century reveals that glacial rebound is the agent of about a third of recent polar drift. The loss of about 7,500 gigatons of ice in Greenland has affected the Earth's spin by redistributing mass into the oceans, causing sea level rise and consequently, a drift in the planetary spin axis.

Its location also makes Greenland a major driver of spin axis shift. A mass at 45 degrees from either pole – and the country is 45 degrees from the North Pole – exerts a greater force on spin axis relative to a mass closer to these points. If Greenland's ice loss accelerates, polar motion likely will, too.

The movement of molten rock in the Earth’s interior is another contributor to alterations in planetary spin. Mantle convection is trending toward a more vertical circulation, spurring mass redistribution which in turn affects polar motion.