text: Unlike a plastic globe of Earth, the real planet Earth isn’t perfectly round and doesn’t spin smoothly. The imaginary line around which Earth spins – passing through the North and South Poles – is called its spin axis. Scientists have known for a long time that Earth’s spin axis drifts and wobbles. Measurements over the 20th century show that Earth’s spin axis drifts by about 4 inches (10 cm) per year. Over the course of a century, that’s more than 11 yards (10 meters). This week (September 19, 2018), NASA scientists announced they’ve used observational and model-based data spanning the 20th century in order to identify – for the first time – three processes primarily responsible for this drift.
  The three processes are mass loss due to melting ice (mostly in Greenland), the lifting of land masses as glaciers have melted since the last ice age (aka glacial rebound), and the slow creeping motion of rocky material in Earth’s interior mantle, caused by convection currents carrying heat from our planet’s interior to its surface (this third process is called mantle convection).
  Scientists call the drift of Earth’s spin-axis its polar motion. Surendra Adhikari of NASA’s Jet Propulsion Laboratory in Pasadena, California is first author on the new paper describing the causes for the drift. He said:
  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.
  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.
  His team’s statement said:
  In general, the redistribution of mass on and within Earth – like changes to land, ice sheets, oceans and mantle flow – affects the planet’s rotation. As temperatures increased throughout the 20th century, Greenland’s ice mass decreased. In fact, a total of about 7,500 gigatons – the weight of more than 20 million Empire State Buildings – of Greenland’s ice melted into the ocean during this time period. This makes Greenland one of the top contributors of mass being transferred to the oceans, causing sea level to rise and, consequently, a drift in Earth’s spin axis.
  While ice melt is occurring in other places (like Antarctica), Greenland’s location makes it a more significant contributor to polar motion.
  Coauthor Erik Ivins, also of JPL, explained:
  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.
  Previous studies identified glacial rebound as the key contributor to long-term polar motion. In the new study, which relied heavily on a statistical analysis of such rebound, scientists figured out that glacial rebound is likely to be responsible for only about a third of the polar drift in the 20th century.
  Bottom line: Scientists thought the drift in Earth’s spin axis was primarily due to the lifting of land masses as ice has melted since the last ice age. A new study shows this mechanism can account for only a third of Earth’s spin axis drift, and names two other causes as well.