by Mary Caperton Morton Friday, December 21, 2018
A perfectly round desk globe spins evenly on a fixed axis, but that’s not the case with Earth, which wobbles as the position of its spin axis — the imaginary line running between the North and South poles — slowly drifts over time. In a new study in Earth and Planetary Science Letters, scientists suggest that there are three main reasons for the movement of Earth’s spin axis, called polar motion.
The position of Earth’s spin axis shifts by about 10 centimeters per year, or roughly 10 meters over a century. Using a combination of observational and model-based data spanning the 20th century, a team led by Surendra Adhikari of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., found that melting glacial ice, glacial rebound and mantle convection all contribute to this polar motion. The effect of glacial rebound — the land rising in response to glacial melt after the last Ice Age — has been reported previously.
“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,” Adhikari said in a statement. “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.”
Melting glacial ice in Greenland has redistributed more than 7,500 gigatons of water weight into the world’s oceans, shifting the spin axis of the planet. The researchers also implicated the mantle, in which mass is redistributed by plate tectonics and convection, but its contribution to polar motion relative to those of glacial melting and rebound is not yet known.
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