Sea level rise due to groundwater withdrawal has caused the Earth’s rotational pole to wander nearly a meter in two decades.
By pumping water out of the ground and moving it elsewhere, humans have shifted such a large mass of water that the Earth tilted nearly 80 centimeters (31.5 inches) east between 1993 and 2010 alone, according to a new study published in Geophysical Research Letters.
For years, scientists have been aware of the gentle movement to the south of the planet’s rotational axis, the imaginary rod around which it turns. So for the longest time, the movement was from the geographic North Pole and toward Canada. Suddenly, though, it made a sharp turn and started heading east, according to the New York Times.
In 2019, Digital Journal reported on a study that the Earth’s magnetic pole was moving at about 50 km (30 miles) a year, a rapid increase since the 1980s. At that time, it was believed that the shifting of the magnetic North was due to a high-speed jet of liquid iron ln the Earth’s outer core beneath Canada.
But almost forgotten, and yet very relevant was a study published in 2013 in the Journal Geophysical Research Letters. Researchers found that accelerated melting of the polar ice sheets and mountain glaciers had changed the way mass was distributed around the planet enough to influence its spin on the rotational axis.
Further study has led to some of those same scientists having identified another factor that’s had the same kind of effect: colossal quantities of water pumped out of the ground for crops and households.
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.
The researchers discovered that 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, reports Science Daily.
“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.”
“I’m not surprised that it would have an effect” on Earth’s spin, said Matthew Rodell, an earth scientist at NASA’s Goddard Space Flight Center. But “it’s impressive they were able to tease that out of the data, and that the observations they have of the polar motion are precise enough to see that effect.”
Human activity and the global climate are factors
You can’t feel it, but our planet’s rotation is nowhere near as smooth as that of the globe on your desk. But if you really think about it, as the Earth moves through space, it is like a poorly thrown Frisbee.
This is partly because it bulges at the Equator and partly because air masses are constantly whirling through the atmosphere and water is sloshing around in the oceans, pulling the planet ever-so-slightly this way and that.
And we also need to remember that Earth’s crust and mantle are springing back after being covered for millenniums by gigantic ice sheets, rebounding like an air mattress after a person gets off.
But the biggest changes have been brought about by human activity. These include the melting of mountain glaciers and the Greenland and Antarctic ice sheets, changes in soil moisture, groundwater depletion, and our impounding of water behind dams.
The location of the groundwater matters for how much it could change polar drift; redistributing water from the midlatitudes has a larger impact on the rotational pole. During the study period, most water was redistributed in western North America and northwestern India, both at mid-latitudes.
Countries’ attempts to slow groundwater depletion rates, especially in those sensitive regions, could theoretically alter the change in drift, but only if such conservation approaches are sustained for decades, Seo said.
“I’m very glad to find the unexplained cause of the rotation pole drift,” Seo said. “On the other hand, as a resident of Earth and a father, I’m concerned and surprised to see that pumping groundwater is another source of sea-level rise.”
“Observing changes in Earth’s rotational pole is useful for understanding continent-scale water storage variations,” Seo said. “Polar motion data are available from as early as the late 19th century. So, we can potentially use those data to understand continental water storage variations during the last 100 years. Were there any hydrological regime changes resulting from the warming climate? Polar motion could hold the answer.”
