A study published in the November issue of the journal Earth and Planetary Science Letters said the wobble — scientifically labeled as "polar motion" — is caused by three factors: melting ice in Greenland, land area rising as ice sheets melt, and changes in Earth's mantle, a mostly rocky layer inside Earth between its outer crust and the core.
Rising temperatures during the 20th century have caused ice to melt in Greenland. Researchers say 7,500 gigatons of ice — equal to the weight of more than 20 million Empire State Buildings — melted into the ocean over that time.
The ice melt, combined with Greenland's location on Earth, plays a role in how the Earth wobbles. "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, it will have a bigger impact on shifting Earth's spin axis than a mass that is right near the Pole," said Eric Ivins, a researcher at the Jet Propulsion Laboratory and co-author on the study, in a statement.
