For decades, humans have been pumping so much water out of the ground that it has caused Earth’s axis of rotation to shift, according to a new study published last week in the journal Geophysical Research Letters.
As the Earth spins, its rotational poles—where the imaginary line of the axis passes through the planet’s crust—naturally move a bit. Anything from ocean currents, to shifting molten rock in the mantle, to the melting of glaciers caused by climate change can lead to a shift in the distribution of mass across the globe and coax the axis to drift. Water stored in artificial reservoirs and seasonal changes in atmospheric winds can play a role as well, write the study authors. In the 20th century, Earth’s axis shifted about four inches per year, according to NASA.
But global warming isn’t the only human-caused factor moving the planet’s poles. The new study estimates that between 1993 and 2010, the pumping of groundwater and the resulting sea-level rise caused the poles to drift by about 2.6 feet.
“The very way the planet wobbles is impacted by our activities,” Surendra Adhikari, a geophysicist at NASA’s Jet Propulsion Laboratory who did not contribute to the research, tells Science’s Warren Cornwall. “It is, in a way, mind boggling.”
“Every mass moving around on the surface of the Earth can change the rotation axis,” Ki-Weon Seo, a co-author of the new study and a geophysicist at Seoul National University, tells Nature News’ Davide Castelvecchi.
The amount of shifting caused by removing groundwater doesn’t affect the length of a day or the seasons, as Clark Wilson, a co-author of the study and geoscientist at the University of Texas at Austin, tells New Scientist’s James Dinneen.
In the new study, the researchers used a computer model to look at the effects of different factors on the shift of the poles. When they didn’t include groundwater removal in their model, their predictions did not match the level of shift that scientists have observed. But when they took into account the massive amount of pumped water—which totaled more than two trillion tons between 1993 and 2010—their model fit the real-world observations.
From this analysis, the researchers estimate that the axis is moving 1.7 inches per year due to groundwater removal. Of the factors the study looked at, pumping groundwater was the second-largest contributor to the axis drifting, behind the melting Greenland ice sheet.
The paper calls attention to just how much water humans have pumped, as Manoochehr Shirzaei, a geophysicist at Virginia Tech who did not contribute to the study, tells New Scientist. “The precise number doesn’t matter really,” he tells the publication. “What matters is that the volume is so gigantic that it can impact the polar drift of the Earth.”
Groundwater removed from sites at the Earth’s midlatitudes, such as in the U.S. and India, has an outsize impact on polar drift, compared with extraction at the equator or the poles. However, most of the pumping has occurred in these high-impact zones, causing the water removal to have a bigger effect on the axis, Wilson tells Science.
Previous research has estimated that extracted groundwater, which eventually flows into the oceans, caused 6.24 millimeters of sea-level rise between 1993 and 2010. Between 2006 and 2015, global sea levels rose about 3.6 millimeters per year, and the new research confirmed that groundwater pumping has been a major contributor.