We’ve pumped so much groundwater that we’ve nudged Earth’s spin

Based on climate models, scientists previously estimated humans pumped 2,150 gigatons of groundwater, equivalent to more than 6 millimeters (0.24 inches) of sea level rise, from 1993 to 2010. But validating that estimate is difficult.

One approach lies with the Earth’s rotational pole, which is the point around which the planet rotates. It moves during a process called polar motion, which is when the position of the Earth’s rotational pole varies relative to the crust. The distribution of water on the planet affects how mass is distributed. Like adding a tiny bit of weight to a spinning top, the Earth spins a little differently as water is moved around.

“Earth’s rotational pole actually changes a lot,” said Ki-Weon Seo, a geophysicist at Seoul National University who led the study. “Our study shows that among climate-related causes, the redistribution of groundwater actually has the largest impact on the drift of the rotational pole.”

Water’s ability to change the Earth’s rotation was discovered in 2016, and until now, the specific contribution of groundwater to these rotational changes was unexplored. In the new study, researchers modeled the observed changes in the drift of Earth’s rotational pole and the movement of water — first, with only ice sheets and glaciers considered, and then adding in different scenarios of groundwater redistribution.