Many people might imagine Earth’s shape as a perfect sphere, but it’s not; it’s an oblate spheroid, with high mountains and deep ocean trenches that distribute mass unevenly and make the planet resemble a lumpy potato. The whole thing is also spinning like a top, and if you move enough mass from one place to another, the planet will wobble as it spins.

Earth’s inner core seems to be slowing its spin

The spin axis, or the imaginary line around which the planet rotates, is known as Earth’s rotational pole. The pole is tilted about 23.5 degrees relative to our plane of orbit around the sun, which is why we have seasons. The exact position of this pole changes a lot as the planet wobbles around, a phenomenon called polar motion, said Ki-Weon Seo, a geophysicist at Seoul National University and the lead author of the study.

“It changes every moment — around 10 meters a year,” Seo said. “Winds, [ocean] currents, barometric pressure or glaciers — any kind of mass change can cause polar motion.”

A 2016 study demonstrated that climate-driven changes in water mass distribution can cause Earth’s poles to drift. But the activities examined in that study didn’t perfectly explain the observed polar motion.

The shift in Earth’s tilt is far too small to impact weather or seasons, Seo said. However, the team of scientists discovered that polar drift can be used to estimate the effect of groundwater pumping on sea level rise.

When water is sucked from the ground to irrigate crops and meet global freshwater demands, it eventually travels through rivers and other pathways into the world’s oceans. From 1993 to 2010, the researchers found, groundwater pumping shifted enough mass to contribute to 0.24 inches in global sea level rise.

“Groundwater pumping is one of the few management decisions that can be made about how to slow the rate of sea level rise,” said Famiglietti. “We are really having an impact on this planet, and we really need to be better stewards of Earth’s resources.”