When looking at the Earth from afar it appears to be a perfect sphere, but that actually isn’t the case. Because Earth isn’t uniform on all sides due to land masses that shift and change over time, our planet actually wobbles a bit when it spins. Now, a new study by researchers with NASA’s Jet Propulsion Laboratory and several universities and science centers has pinpointed the causes of Earth’s imperfect spin, called “polar motion,” and they found that humans are contributing to it.

The researchers used a wealth of data gathered over 100 years to build mathematical models to trace the causes of the wobble and found that three factors are at play, and mankind is responsible for one of them.

Two of the three factors identified by the scientists are glacial rebound and mantle convection. Glacial rebound happens when thick ice sheets physically push down on land masses, compressing them, but then release that pressure upon melting. The land then balloons back up over time, causing Earth’s spin to wobble as if slightly off-axis. The effects of the last ice age, which would have compressed a huge amount of land across many continents, is still being felt today in the form of glacial rebound.

Mantle convection, the other uncontrollable factor in Earth’s wobble, relates to our planet’s inner workings. The plates on Earth’s surface are in constant flux due to the movement of liquid rock far beneath our feet. The researchers believe these currents also contribute to the planet’s imperfect spin.

The third and final factor identified by the scientists is the massive loss of ice on Greenland and other areas, which is the direct result of global warming thanks to human activities. The researchers estimate that Greenland has lost roughly 7,500 gigatons, or 7,500,000,000,000 metric tons of ice due to global warming.

All that ice loss has happened in the 20th century, and greenhouse gas production has been cited as the primary culprit. Losing all that mass has caused a significant shift on the planet and has contributed to the wobble as well.

“With these three broad contributors identified, scientists can distinguish mass changes and polar motion caused by long-term Earth processes over which we have little control from those caused by climate change,” NASA writes. “They now know that if Greenland’s ice loss accelerates, polar motion likely will, too.”

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