The Earth doesn't just spin while on its axis, it wobbles. And scientists at NASA say they've identified three reasons why it happens.

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.

Several studies have suggested global warming has contributed to an increase in ice melt in Greenland, Antarctica and other parts of the world. A study in February said sea levels could be at least 2 feet higher by the end of the century compared to now because of melting ice.

Last month, scientists observed some of the oldest and thickest ice in the Arctic, north of Greenland, showed signs it was beginning to break up.

Another factor noted by researchers is glacial rebound, a process where land once depressed by heavy glaciers begins to rise. Also, the circulation of material inside the Earth's mantle, called mantle convection, plays a role.

Ivins and lead author Surendra Adhikari said all three factors contribute to a significant redistribution of the Earth's mass, leading to the wobble effect.

"The traditional explanation is that one process, glacial rebound, is responsible for this motion of Earth's spin axis," said Adhikari. "But recently, many researchers have speculated that other processes could have potentially large effects on it as well."

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