Extracting groundwater for irrigation and drinking water has shifted Earth’s rotational pole by 31.5 inches (80 cm) between 1993 and 2010, according to a study published in Geophysical Research Letters. The redistribution is equivalent to moving 2,150 gigatons of water from land to ocean and has raised global sea level by about 0.24 inches (6 mm).

Ki-Weon Seo, geophysicist at Seoul National University and lead author, said the redistribution of groundwater now outweighs any other climate-related cause of polar drift. “Earth’s rotational pole actually changes a lot,” Seo explained. “Our study shows that, among climate-related causes, the redistribution of groundwater has the largest impact on the drift of the rotational pole.”

The principle is simple: moving mass on a spinning body changes how it spins. Like adding a tiny bit of weight to a spinning top, the Earth rotates slightly differently as water is redistributed.

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NASA research in 2016 first demonstrated that changes in water storage could perturb the planet’s rotation, but the new work supplies hard numbers. Seo’s team ran several simulations of observed polar motion. Only the model that included the 2,150-gigaton groundwater transfer matched the real-world drift recorded by satellites and astronomical measurements.

Surendra Adhikari, a research scientist at NASA’s Jet Propulsion Laboratory who worked on the 2016 study, said the updated research is important. “They’ve quantified the role of groundwater pumping on polar motion, and it’s pretty significant,” he said in a statement.

Location matters. Water redistributed from mid-latitudes exerts the strongest leverage on the spin axis, so intensive pumping under western North America and north-western India has played an outsized role in the observed tilt.

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Now that scientists can see the effect so clearly over a relatively short, recent interval, they plan to mine historical records of Earth’s orientation for clues to past groundwater depletion.

“Observing changes in Earth’s rotational pole is useful for understanding continent-scale water-storage variations,” Seo said. The data could also help conservationists and water managers design policies that curb further sea-level rise and other climate impacts—provided those policies are implemented in time.

Story Source: The study was published in Geophysical Research Letters