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Star trails above the Three Gorges Dam in Yichang, Hubei province, China. Large dams like this one are shifting the Earth’s poles away from its axis of rotation.<\/p>\n
Cynthia Lee\/Alamy Stock Photo<\/p>\n
Over the past 200 years, humans have stored hundreds of billions of liters of water in thousands of dams<\/a> around the world. That shift of water is pushing Earth\u2019s geographic poles away from its axis of rotation<\/a>, a new study published in Geophysical Research Letters has found, contributing to a phenomenon called true polar wander<\/a>.<\/p>\n In an ideal model of our planet, the axis around which Earth rotates<\/a> goes through the fixed geographic poles, or what we typically call the North and South Poles. But in reality, the axis pierces through the surface of the planet at points some distance away from the geographic poles, says Jim Davis, a geodesist at Lamont Doherty Earth Observatory at Columbia University, who was not involved in the study. The distance between the two is called the polar motion, or true polar wander, and it\u2019s crucial to know for a number of purposes.<\/p>\n \u201cAstronomers need it. GPS depends on it. There\u2019s a lot of things that depend on knowing how the Earth is rotating at the moment,\u201d Davis says. \u201cWe have to know the polar motion accurately or else the errors will propagate into other systems on which we depend on a daily basis.\u201d<\/p>\n On supporting science journalism<\/p>\n If you’re enjoying this article, consider supporting our award-winning journalism by subscribing<\/a>. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.<\/p>\n True polar wander happens because the surface of Earth rotates to redistribute mass and stabilize its spin<\/a>. Imagine our planet as a beach ball spinning on someone\u2019s finger. If someone stuck a wad of gum on the ball, the ball might wobble on its axis of rotation and move to adjust to the increase in mass, Davis explains. On Earth, instead of a wad of gum, there are shifting ice sheets and movements of mantle rock<\/a> that cause the planet to adjust itself. Though dams move a smaller amount of mass than these other phenomena, they still have an appreciable effect.<\/p>\n