A new study is suggesting that human influence is affecting the planet in an unexpected way: human-driven global warming appears to be accelerating the shift the planet’s axis. Although scientists have known about the slow wobble in the Earth’s axis for a long time, NASA researchers have found that the planet’s shrinking ice sheets are altering the Earth’s balance, causing the pole to drift one-third faster than it would if the planet’s temperature had remained stable.
Although the sheer mass of the Earth keeps it spinning (more-or-less) regularly, it doesn’t do so perfectly, and just like the wobble that eventually creeps into the spinning of a toy top, the Earth also exhibits a slow wobble, called "polar motion", on its axis. Although gravitational influences are the primary driver behind the 25,772-year precession of the equinox, short-term irregularities in this motion are generated by the fact that the Earth is not a perfect sphere, but rather continually experiences changes in balance at different points in the globe, as elements both on and under the surface of the planet evolve. Traditionally, it has been assumed that there three main influences affecting polar motion: glacial rebound, mantle convection, and the melting of surface ice. Combined, these three factors account for a third of the shift in Earth’s axis.
"The traditional explanation is that one process, glacial rebound, is responsible for this motion of Earth’s spin axis. But recently, many researchers have speculated that other processes could have potentially large effects on it as well," explains lead study author Surendra Adhikari of NASA’s Jet Propulsion Laboratory. "We assembled models for a suite of processes that are thought to be important for driving the motion of the spin axis. We identified not one but three sets of processes that are crucial — and melting of the global cryosphere (especially Greenland) over the course of the 20th century is one of them."
Glacial rebound, sometimes referred to as isostatic rebound, was originally considered to be the largest influence of the three: The weight of the massive glaciers that formed during the last ice age pressed down on the Earth’s crust, depressing the area beneath the ice; conversely, the effect raises the land surrounding the ice sheet, much like a weight placed on one end of a seesaw will raise the unencumbered end. After the ice melts, the crust that was under the ice slowly rebounds back upward (and the surrounding land back downward), redistributing mass on a continental scale.
Mantle convection is the process of molten rock, heated near the Earth’s core, rising through the mantle toward the crust, where it affects earthquakes, volcanoes, and the slow but steady movement of continents — once again, shifting mass to different areas around the planet.
However, the third factor, the melting of the planet’s ice sheets, particularly the accelerated melt of Greenland’s glaciers, has been found to be a much bigger contributor to the wobble in Earth’s axis than originally assumed. The study conducted by NASA found that ice melt alone has contributed to one-third of polar motion over the course of the twentieth century, with the pole having migrated 10 meters (32.8 feet), at 10.5 centimeters (4.1 inches) per year, over that time. Over the century, an estimated 7.5 trillion tons of ice — the equivalent of more than 20 million Empire State Buildings — melted into the ocean, redistributing a large amount of weight from a strategic point on the Earth’s surface southward.
"There is a geometrical effect that if you have a mass that is 45 degrees from the North Pole — which Greenland is — or from the South Pole (like Patagonian glaciers), it will have a bigger impact on shifting Earth’s spin axis than a mass that is right near the Pole," coauthor Eric Ivins, also with JPL, points out.