Raphael Thys
Earth’s spin axis has shifted in recent years because of groundwater extraction, which redistributes the weight of the planet’s water.AlexSava/iStock
While spinning on its axis, Earth wobbles like an off-kilter top. Sloshing molten iron in Earth’s core, melting ice, ocean currents, and even hurricanes can all cause the poles to wander. Now, scientists have found that a significant amount of the polar drift results from human activity: pumping groundwater for drinking and irrigation.
“The very way the planet wobbles is impacted by our activities,” says Surendra Adhikari, a geophysicist at NASA’s Jet Propulsion Laboratory and an expert on Earth’s rotation who was not involved in the study. “It is, in a way, mind boggling.”
Imagine spinning a basketball on your finger. If you can keep the ball balanced, it will spin evenly along its axis. But add a little bit of weight to one part of the ball or remove it from another, and the ball will rapidly become unbalanced, wobbling and shifting its spin axis. Earth’s spin axis also wobbles, its North Pole tracing out a roughly 10-meter-wide circle every year or so. The center of this wobble also drifts over the long term; lately, it has been tilting in the direction of Iceland by about 9 centimeters per year.
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Clark R. Wilson, a geophysicist at the University of Texas at Austin, and his colleagues thought the removal of tens of gigatons of groundwater each year might affect the drift. But they knew it could not be the only factor. “There’s a lot of pieces that go into the final budget for causing polar drift,” Wilson says.
The scientists built a model of the polar wander, accounting for factors such as reservoirs filling because of new dams and ice sheets melting, to see how well they explained the polar movements observed between 1993 and 2010. During that time, satellite measurements were precise enough to detect a shift in the poles as small as a few millimeters.
Dams and ice changes were not enough to match the observed polar motion. But when the researchers also put in 2150 gigatons of groundwater that hydrologic models estimate were pumped between 1993 and 2010, the predicted polar motion aligned much more closely with observations. Wilson and his colleagues conclude that the redistribution of that water weight to the world’s oceans has caused Earth’s poles to shift nearly 80 centimeters during that time. In fact, groundwater removal appears to have played a bigger role in that period than the release of meltwater from ice in either Greenland or Antarctica, the scientists reported Thursday in Geophysical Research Letters.
The effect was amplified because much of the water was removed in the northern midlatitudes, in groundwater depletion hot spots such as northwestern India and the western United States. If the biggest loss of water had occurred closer to the equator or the poles, the effect would have been smaller, Wilson says.
The extra wobble is far too small to affect weather or seasons, Wilson says. But the findings could enable scientists to use polar drift as a tool to gauge other phenomena. Ki-Weon Seo, a geophysicist at Seoul National University and the study’s lead author, says it offers a way to check how much sea level rise the groundwater pumping is causing, as the water flows downstream and ultimately reaches the ocean.
The new paper helps confirm that groundwater depletion added approximately 6 millimeters to global sea level rise between 1993 and 2010. “I was very happy” that this new method matched other estimates, Seo says. Because detailed astronomical measurements of the polar axis location go back to the end of the 19th century, polar drift could enable Seo to trace the human impact on the planet’s water over the past century.