Pumping Groundwater Has Changed Earth's Spin, Study Finds

Around the turn of the millennium, Earth's spin started to go awry, and no one really knows why.

For decades, scientists have been observing the average position of our planet's axis of rotation, an imaginary rotating rod, gently wandering south, away from the geographic north pole and towards Canada. However, suddenly, he made a sharp turn and started heading east.

Later, researchers came shocking realization about what had happened. The accelerated melting of polar ice caps and mountain glaciers has changed the way mass is distributed around the planet enough to affect its spin.

Now, those same scientists have identified another factor that has a similar effect: the enormous amounts of water pumped out of the ground for agriculture and households.

“Wow,” Ki-Weon Seo, who led the research behind the latest discovery, recalls the time when his calculations showed a strong connection between groundwater extraction and the deviation of the Earth's axis. It was a “big surprise,” said Dr. Seo, a geophysicist at Seoul National University.

Water experts have long warned of the consequences of overusing groundwater, especially as water from underground aquifers becomes an increasingly vital resource in drought-stricken areas such as the American West. When water is pumped out of the ground but not replenished, the ground can sink, damaging homes and infrastructure, and also shrinking the number of basements that can hold water thereafter.

Between 1960 and 2000, groundwater depletion worldwide more than doubled, to about 75 trillion gallons a year, scientists estimate. Since then, satellites measuring variations in Earth's gravity have revealed surprisingly that groundwater supplies have decreased in certain areas, including India and California's Central Valley.

“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 could tease that out of the data,” says Dr. Rodell, referring to the authors of the new study, who published this month in the journal Geophysical Research Letters. “And that their observations of polar motion are precise enough to see that effect.”

The Earth's axis has not yet been deviated enough to affect the seasons, which are determined by the planet's tilt. But subtle patterns and variations in planetary spin are critical to satellite-based navigation systems that guide aircraft, missiles and map applications. This has helped motivate researchers to try to understand why the shaft is moving and where it is headed next.

You can't feel it, but our planet's rotation isn't as smooth as the globe on your desk.

While moving through space, The earth shakes like a badly thrown Frisbee. This is partly because it is bulging at the Equator and partly because air masses are constantly swirling through the atmosphere and water is flowing in the oceans, tugging the planet slightly to and fro.

And then, there's that wandering shaft.

One of the main causes is that the earth's crust and mantle are reappearing after being covered for thousands of years by giant permafrost, bouncing back like a weightless mattress off a bed. This is constantly changing the mass balance around the planet.

Recently, balance has also been changed by factors more closely related to human activities and the global climate. These include the melting of mountain glaciers and the Greenland and Antarctic ice sheets, changes in soil moisture, and water retention behind dams.

Another big factor, according to research by Dr. Seo and colleagues, is the depletion of groundwater. In terms of effects on the Earth's axis, the pumping of water from underground was second in magnitude, between 1993 and 2010, after post-glacial adjustment in the planet's crust, the study found.

Other forces may also be pulling the Earth's axis in new directions but are not fully understood, said Clark R. Wilson, a geophysicist at the University of Texas at Austin and another author of the study. “It's possible that, for example, there's something in Earth's fluid core that's going on that's also contributing,” he said.

Even so, recent discoveries point to new possibilities for using information about the Earth's spin to study climate, said Dr. Wilson.

Because scientists have collected highly accurate data about the position of the Earth's axis over most of the 20th century, they may be able to use it to understand shifts in groundwater use that occurred before the most modern and reliable data were available.

It is possible that Dr. Seo said he has started exploring.