Humans pumped so much groundwater out of the Earth that the planet has begun to wobble detectably on its axis, a new study has found.

On its own terms, the magnitude of the new wobble is slight — a matter of millimeters, which puts it in the same approximate speed category as Earth’s slowly drifting continents.

But the findings published earlier this month in Geophysical Research Letters show the extent to which human action — in the form of dam construction, groundwater drilling and the burning of fossil fuels — are impacting the very position of the Earth.

They also provide powerful corroboration for something scientists had long suspected but had remained unproven: the staggering depletion of the world’s groundwater reserves over the past several decades.

The scope of the findings released this month were startling. Between 1993 and 2010, the scientists found, human society — and mostly human agriculture — had depleted 2,510 gigatons of water. 

That’s the equivalent of 600 cubic miles of lost freshwater, more than five times the volume of Lake Erie or half the volume of Lake Huron. 

At that rate, we would run through the equivalent of the 2,900 cubic miles of water that fill Lake Superior by the end of the century. 

That speed of depletion reflects a global threat.

Groundwater is the collective term for waters held below ground, largely among the sand and rock of aquifers — distinct from surface water that courses through lakes and rivers.

Across the world — particularly in India and the American West — pumping underground reserves of fossil groundwater has, for the past century, made it possible to grow water-dependent crops despite near-desert conditions.

For dryland growers of thirsty crops — such as the Saudi alfalfa farms expanding into Arizona’s Sonoran Desert — groundwater also offers a way around the intractable water politics of the arid West.

Despite stringent restrictions on using the region’s diminishing lakes and rivers — most notably the Colorado River — states including Arizona, California, Texas and Nebraska allow landowners broad pumping rights to the water below their land.

The legal reality that groundwater can be used in unlimited quantities — provided it’s being used for an economic purpose — is at odds with the hydrologic reality of the American West, in which groundwater reserves that took thousands of years to form are being exhausted over decades.

In the massive Ogallala Aquifer, which stretches from North Texas to the Dakotas, 90 percent of water pulled from underground goes to crops and cattle — a demand that is draining that aquifer.

“It’s inevitable that we are going to run out of water in this region,” a West Texas agronomist told The Hill last year. 

Parts of the High Plains may run out of groundwater before midcentury, according to KAMR, which is owned by The Hill’s parent company, Nexstar Media Group.

And in the dense, groundwater-fed farm country of California’s Central Valley, months of heavy rains have done little to counter the impacts of decades of over pumping, according to nonprofit news site CalMatters.

More than 5,000 of the state’s wells have gone dry in the past decade, California’s Department of Water Resources found. 

This local problem scales globally: A majority of the global population is dependent on food exports from trade partners (primarily India, Pakistan and the U.S.) who are spending down their groundwater to grow it, according to Nature, a weekly science journal.

“Some countries, such as the USA, Mexico, Iran and China, are particularly exposed to these risks because they both produce and import food irrigated from rapidly depleting aquifers,” the Nature study found.

This month’s study suggests that these local agricultural policies are causing planetary-scale perturbations.

The experiment came as an attempt to solve a scientific mystery more than a century in the making — a question that initially appeared to have little to do with groundwater.  

That story begins with the 1891 discovery of the phenomenon called Chandler wobble, named after the amateur Boston astronomer Seth Carlo Chandler, who discovered it.

An actuary by profession, Chandler determined from the position of the stars that the Earth’s axis — a tilt of approximately 23 degrees — was slightly erratic.

The existence of Chandler wobble was confirmed 70 years later with the dawn of both human spaceflight and the rise of high-tech measuring tools such as atomic clocks and laser ranging.

But it was only with the invention of Global Positioning Systems (GPS) in the late 1980s and their rise to dominance over the next 30 years that the wobble became a matter of life-and-death importance in everything from communication and navigation to war.

“You cannot do positioning with a GPS unless you know where the rotation axis is inside the Earth,” said Clark Wilson of the University of Texas, a co-author on the paper. “So for decades, NASA, the U.S. military and major research universities have gotten very good at tracking slight changes in the Earth’s motion.”

How slight? Thanks to those tools, scientists can now track changes in the Earth’s movement “at the millimeter level,” Wilson said — about a hundred thousand times the accuracy available to the 19th-century astronomers who started the process. That precision is so exact that it allowed researchers to confirm that North America and Europe are moving apart at a rate of 3 millimeters per year.

But as that dataset of axial tilts grew, scientists began to notice something they struggled to explain: The tilt was shifting, inscribing a slight eastward spiral around the North Pole. And none of the exterior forces on the Earth — the way its spin is impacted by the gravitational pull of the sun, moon and other planets — accounted for the change.

Over the past decade, scientists began to test a new theory: that the change had originated from something within the systems of rock, wind and water that make up the planet. In 2013, Wilson was a co-author on a paper that found that the rapid melting of the polar ice sheets and mountainous glaciers was a partial cause of that eastward motion.

The sheer mass of melting ice, the scientists found, had caused the distribution of the entire Earth’s weight to shift enough that the pattern of its motion had begun to shift — both from the movement of the water itself and the slow rebounding of the Earth’s crust as it melted, like a lily pad riding higher in the water after a frog has jumped off.

But the ice melt couldn’t account for all that change in direction — leading Wilson and his colleagues across the globe to wonder if human actions might not be the missing ingredient.

In 2010, an international team released estimates that suggested that in the second half of the 20th century, human society had more than doubled the rate at which it was depleting groundwater. The rates of loss had spiked from 126 cubic km (30 cubic miles) in 1960 to 283 cubic kilometers (67 cubic miles) in 2000.

To put that in other terms, the scientists estimated that by the end of the millennium, human society was exhausting the equivalent of two Lake Tahoes every year — or, said yet another way, more than a third of the water drawn out of wells and aquifers ended up gone for good, with the total supply decreasing each year by 2 percent. 

This month’s paper has confirmed that estimate, Wilson said. “We were trying to confirm whether the groundwater estimate in the literature had been a good one — and this says, basically, it was pretty good.”

(Lead author Ki-Weon Seo was even more succinct: “Wow,” he told The New York Times, recalling his immediate reaction to the findings.) 

Wilson said that two human interventions represented missing, if counterbalanced, puzzle pieces. The mass construction of dams worldwide — with their heavy, crust-displacing reservoirs — had pulled the Earth slightly west, and the loss of groundwater had pulled it east. 

In conjunction with the observed motion from ice sheet melting — itself, in large measure, a result of human-caused climate change — the addition of the groundwater loss and dam construction largely explained the wobble in the axis.

There is a possibility that this coastal wobbling is speeding up, as rising temperatures increase the scale of the meltwater pouring off Greenland — although that will take more study, Wilson said. The historical record of Earth’s water distribution and climate “may be written in the drift of the poles,” he said.

Before precise record-keeping started, “maybe the climate was more stable, or there was less agricultural use of water and fewer dams built,” he said. “So those are the things we’d like to look at.”