Around the turn of the millennium, the Earth's rotation began to change and no one really knew why.
For decades, scientists had watched as the average position of our planet's axis of rotation, the imaginary rod around which it rotates, gradually moved away from the geographic North Pole and toward Canada. But suddenly, it took a sharp turn and began to head east.
Eventually, investigators came to a surprising conclusion about what had happened. The accelerated melting of polar ice sheets and mountain glaciers had changed the way mass was distributed across the planet enough to influence its rotation.
Now, some of the same scientists have identified another factor that has the same kind of effect: colossal amounts of water drawn from the soil for crops and homes.
"Wow," Ki-Weon Seo, who led the research that led to the latest discovery, remembers thinking when his calculations showed a strong relationship between groundwater extraction and Earth's axis drift. It was a "big surprise," said Seo, a geophysicist at Seoul National University.
Melting glaciers at the poles from global warming are affecting Earth's rotation. Photo: AFP
Water experts have long warned of the consequences of groundwater overexploitation, especially as water from underground aquifers becomes an increasingly vital resource in drought-stricken areas such as the western United States.
When water is drawn from underground but not replenished, the earth can sink, damaging homes and infrastructure and also reducing the amount of underground space that can subsequently hold water.
Between 1960 and 2000, the reduction of groundwater worldwide doubled, to almost 300 billion cubic meters per year, scientists estimate.
Since then, satellites measuring variations in Earth's gravity have revealed the staggering magnitude of the reduction in groundwater reserves in certain regions, such as India and California's Central Valley.
"I'm not surprised it has an effect" on Earth's rotation, said Matthew Rodell, an Earth scientist at NASA's Goddard Space Flight Center.
But "it's impressive that they were able to deduce it from the data," he added, referring to the authors of the new research, published this month in the journal Geophysical Research Letters. "And that the observations they have of polar motion are accurate enough to see that effect."
The consequences of the turn
Earth's axis hasn't moved enough to affect the seasons, which are determined by the planet's tilt. But the delicate patterns and variations in the planet's rotation are hugely important for satellite navigation systems that guide planes, missiles and mapping applications. This has led researchers to try to understand why the axis is moving and where it might be headed.
It is not felt, but the rotation of our planet is not at all as smooth as that of the globe we have on the table.
Icebergs in the Arctic Ocean, in a file image. Photo: AFP
As it moves through space, Earth wobbles like a badly launched frisbee. That's partly because it bulges at the equator and partly because air masses are constantly swirling in the atmosphere and water churns in the oceans, pulling the planet slightly back and forth.
And then there is the question of shaft deviation.
Possible causes
One of the main causes is that the crust and mantle of the Earth are returning to their original shape after being covered for millennia by gigantic ice sheets and recover volume like a mattress from which the sleeping person has risen. This has been constantly changing the mass balance across the planet.
More recently, the balance has also been altered by factors more linked to human activity and global climate. These include melting mountain glaciers and ice sheets in Greenland and Antarctica, changes in soil moisture and water accumulation in dams.
Another important factor, according to the study by Seo and colleagues, is the reduction of groundwater. As for the effect on the Earth's axis, the extraction of water from the subsurface was the second factor in magnitude between 1993 and 2010, only surpassed by the adaptation of the planet's crust after the age of glaciers, according to the study.
According to Clark R. Wilson, a geophysicist at the University of Texas at Austin and another of the study's authors, there are other forces that could be pushing Earth's axis in its new direction, but they are not yet fully understood. "It's possible, for example, that there's something in Earth's fluid core that's also contributing," he said.
However, the latest discovery allows us to think about new possibilities for using information about Earth's rotation to study climate, according to Wilson.
Since scientists have collected very precise data on the position of the Earth's axis for much of the twentieth century, they could use it to understand changes in groundwater use that took place before the most modern and reliable data became available.
Seo said he has already begun studying that possibility.
Source: The New York Times
Translation: Elisa Carnelli
CB
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