Around the turn of the millennium, the Earth’s rotation began to become unbalanced, and no one could say exactly why.
For decades, scientists had watched the average position of our planet’s axis of rotation, the imaginary rod it revolves around, gently drift south away from the geographic North Pole and toward Canada. Suddenly, however, it made a sharp turn and began to head east.
Over time, investigators came to a startling realization of what had happened. The accelerated melting of the polar ice caps and mountain glaciers had changed the way mass was distributed around the planet enough to affect its rotation.
Now some of the same scientists have identified another factor that has had the same effect: colossal amounts of water pumped from the ground for crops and households.
“Wow,” Ki-Weon Seo, who led the research behind the latest discovery, recalled thinking when his calculations showed a strong link between groundwater withdrawal and drifting off the Earth’s axis. It was a “great 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-stressed areas like the American West. When water is pumped out of the ground but not replenished, the land can sink, damaging homes and infrastructure and also reducing the amount of underground space that can hold water afterwards.
Between 1960 and 2000, global groundwater depletion more than doubled, to about 75 trillion gallons a year, scientists estimate. Since then, satellites measuring variations in Earth’s gravity have revealed the staggering rate at which groundwater resources have dwindled in certain regions, including India and California’s Central Valley.
“I’m not surprised 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 that they were able to extract that from the data,” said Dr. Rodell, citing the authors of the new study, which was published this month in the journal Geophysical Research Letters. “And that the observations they have of the polar motion are accurate enough to see that effect.”
Earth’s axis hasn’t drifted enough to affect the seasons, which are determined by the planet’s tilt. But fine patterns and variations in the planet’s rotation are hugely important to the satellite-based navigation systems that direct planes, missiles and mapping apps. This has helped motivate researchers to try to understand why the axis moves and where it might go next.
You don’t feel it, but the rotation of our planet is not nearly as smooth as that of the globe on your desk.
As it moves through space, the Earth wobbles like a poorly thrown Frisbee. This is partly because it bulges out at the equator and partly because air masses are constantly swirling through the atmosphere and water sloshes in the oceans, pulling the planet back and forth just a tiny bit.
And then there’s that wandering ashes.
One of the main causes is that the earth’s crust and mantle bounce back after being covered for millennia by giant ice caps, which rebound like a mattress without the burden of a sleeper. This has steadily changed the mass balance around the planet.
More recently, the equilibrium has also changed due to factors more closely related to human activities and the global climate. These include the melting of mountain glaciers and the ice sheets of Greenland and Antarctica, changes in soil moisture and our impoundment of water behind dams.
Another big factor, according to Dr. Seo and his colleagues, is the depletion of groundwater. In terms of its effect on Earth’s axis, pumping water from the ground was second in magnitude between 1993 and 2010, second only to postglacial adjustment of the Earth’s crust, the study found.
Other forces may also pull the Earth’s axis in its new direction, but are not yet fully understood, said Clark R. Wilson, a geophysicist at the University of Texas at Austin and another author of the study. “It’s possible, for example, that there’s something going on in the Earth’s liquid core that’s also contributing,” he said.
Still, the latest discovery points to new possibilities for using information about Earth’s rotation to study climate, said Dr. Wilson.
Because scientists collected highly accurate data about the position of the Earth’s axis throughout much 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 became available.
It’s a possibility that Dr. Seo says he has already started exploring.