Humans have used enough groundwater to shift Earth’s tilt

Rampant removal of groundwater for drinking and irrigation has altered the distribution of water on Earth enough to shift the planet’s tilt, according to a sweeping new study. The finding underscores the dramatic impact that human activity can have on the planet.

Humans pump most of our drinking water from natural underground reservoirs called aquifers. Researchers calculate that between 1993 and 2010, we removed a total of 2,150 gigatons of groundwater — enough to fill 860 million Olympic swimming pools.

According to the new study, published on June 15 in the journal Geophysical Research Letters, moving all that water has shifted Earth’s tilt 31.5 inches eastward.

Many people might imagine Earth’s shape as a perfect sphere, but it’s not; it’s an oblate spheroid, with high mountains and deep ocean trenches that distribute mass unevenly and make the planet resemble a lumpy potato. The whole thing is also spinning like a top, and if you move enough mass from one place to another, the planet will wobble as it spins.

“I kind of liken it to a waterlogged softball,” said James Famiglietti, a hydrologist at Arizona State University who was not involved in the study. “When a softball or baseball gets soaked, it gets waterlogged, and when you throw it, it wobbles funny. That’s what’s happening here.”

Earth’s inner core seems to be slowing its spin

The spin axis, or the imaginary line around which the planet rotates, is known as Earth’s rotational pole. The pole is tilted about 23.5 degrees relative to our plane of orbit around the sun, which is why we have seasons. The exact position of this pole changes a lot as the planet wobbles around, a phenomenon called polar motion, said Ki-Weon Seo, a geophysicist at Seoul National University and the lead author of the study.

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“It changes every moment — around 10 meters a year,” Seo said. “Winds, [ocean] currents, barometric pressure or glaciers — any kind of mass change can cause polar motion.”

It’s only recently, however, that scientists realized how much human activity can also cause polar motion.

A 2016 study demonstrated that climate-driven changes in water mass distribution can cause Earth’s poles to drift. But the activities examined in that study didn’t perfectly explain the observed polar motion.

With observational data spanning 17 years, Seo and his colleagues used a computer model to simulate which hydrological sources significantly affected Earth’s tilt. To Seo’s surprise, modeled polar drift only matched the observations when groundwater pumping was included.

“We have a lot of dams, and there are a lot of reservoirs on land, so I originally thought that should be very important,” said Seo, “but actually it’s not. After including the groundwater effect, I finally could explain all [of the] observations.”

The shift in Earth’s tilt is far too small to impact weather or seasons, Seo said. However, the team of scientists discovered that polar drift can be used to estimate the effect of groundwater pumping on sea level rise.

When water is sucked from the ground to irrigate crops and meet global freshwater demands, it eventually travels through rivers and other pathways into the world’s oceans. From 1993 to 2010, the researchers found, groundwater pumping shifted enough mass to contribute to 0.24 inches in global sea level rise.

“Groundwater pumping is one of the few management decisions that can be made about how to slow the rate of sea level rise,” said Famiglietti. “We are really having an impact on this planet, and we really need to be better stewards of Earth’s resources.”

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