Around the turn of the millennium, the Earth’s spin began to get out of hand, and no one could say why.
For decades, scientists have been watching the average position of our planet’s axis of rotation, the imaginary bar around which it revolves, drifting gently south away from the geographic North Pole and toward Canada. Suddenly, however, it made a sharp turn and began heading east.
Over time, researchers came to an amazing realization about 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 influence its spin.
Now some of the same scientists have identified another factor that has had the same kind of effect: colossal amounts of water drawn from the soil for crops and homes.
“Wow,” recalled Ki-Weon Seo, who led the research behind the latest discovery, when his calculations showed a strong link between groundwater extraction and the drift of the Earth’s axis. It was a “big surprise,” said Dr. Seo, a geophysicist at Seoul National University.
Water experts have long warned about the consequences of overusing groundwater, particularly as water from groundwater aquifers becomes an increasingly vital resource in drought-stricken areas like the western United States. When water is removed from the ground but not replenished, land can subside, damaging homes and infrastructure and also reducing the amount of space below ground that can hold water thereafter.
Between 1960 and 2000, depletion of groundwater worldwide more than double, to about 75 trillion gallons a year, scientists estimate. Since then, satellites measuring variations in Earth’s gravity have revealed the astonishing degree to which groundwater supplies have dwindled in particular regions, including India and the Central Valley of California.
“I’m not surprised that it has 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 deduce that from the data,” said Dr. Rodell, referring to the authors of the new research, which was published this month in the journal Geophysical Research Letters. “And that the observations they have of polar motion are precise enough to see that effect.”
Earth’s axis has not shifted far enough to affect the seasons, which are determined by the planet’s tilt. But fine patterns and variations in the planet’s spin matter a lot to satellite-based navigation systems that guide planes, missiles, and mapping apps. This has helped motivate researchers to try to understand why the axis is moving and where it might go next.
You can’t feel it, but our planet’s rotation isn’t as smooth as the globe on your desk.
As you move through space, the earth is shaking Like a badly 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 churning in the oceans, pushing the planet ever so slightly from side to side.
And then, there’s that wandering axis.
One of the main causes is that the Earth’s crust and mantle are recovering after being covered for millennia by gigantic sheets of ice, rebounding like a mattress without the load of a sleeper. This has been constantly changing the mass balance across the planet.
More recently, the balance has also been disturbed by factors more closely linked to human activity and the global climate. These include the melting of mountain glaciers and the Greenland and Antarctic ice sheets, changes in soil moisture, and our accumulation of water behind dams.
Another important factor, according to the study by Dr. Seo and his colleagues, is the depletion of groundwater. In terms of the effect on Earth’s axis, the pumping of water from the subsurface was second in magnitude, between 1993 and 2010, second only to the post-glacial adjustment of the planet’s crust, the study found.
Other forces could also be pulling Earth’s axis in its new direction, but they’re not yet fully understood, said Clark R. Wilson, a geophysicist at the University of Texas at Austin and another study author. “It’s possible, for example, that there is something in Earth’s fluid core going on that is also contributing,” he said.
Still, the latest discovery points to new possibilities for using information about the Earth’s spin to study climate, Dr. Wilson said.
Because scientists have collected very precise data on the position of the Earth’s axis for much of the 20th century, they could use it to understand changes in groundwater use that took place before the most modern data were available and reliable.
It’s a possibility that Dr. Seo says he has already begun to explore.
