As it moves through space, every first year college physics student knows the Earth wobbles like a slightly displaced gyroscope. This is partly because it bulges at the Equator and partly because air masses are constantly whirling through the atmosphere and water is sloshing around in the oceans, pulling the planet ever-so-slightly this way and that. And then, there is the matter of its wandering axis.
For decades, geo-scientists had been watching the average position of our planet’s rotational axis, the imaginary rod around which it turns, gently wander south, away from the geographic North Pole and toward Canada. One such strong frequency component is known as "the Chandler wobble" - centered about a period of 420 days or roughly 1 1/6 yrs. Theoretically, a damping should be present on the order of 10-20 years but not such effect has been observed. This has led geophysicists to posit some kind of random excitation to keep the wobble going. But maybe this has been staring us in the face the whole time.
In time, researchers came to a startling realization about what had happened. Accelerated melting of the polar ice sheets and mountain glaciers had changed the way mass was distributed around the planet enough to influence its spin. The spin axis, to fix ideas, has made a sharp turn and has started heading east. One main cause is that Earth’s crust and mantle are springing back after being covered for millenniums by gigantic ice sheets, rebounding like a mattress unburdened of a sleeper. This has been steadily changing the balance of mass around the planet.
More
recently, the balance has also been
altered 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 impounding of water behind dams.
Another big factor, according to the study by Dr. Ki-Weon Seo and his colleagues at Seoul National University, is groundwater depletion. In terms of the effect on Earth’s axis, pumping up water from underground was second in magnitude, between 1993 and 2010, only to the post-glacier adjustment of the planet’s crust, the study found.
Water experts have long warned of the consequences of groundwater
overuse, particularly 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 shrinking the amount of
underground space that can hold water thereafter.
Between 1960 and 2000, worldwide groundwater depletion more than doubled, to about 75 trillion gallons a year, scientists
estimate. Since then, satellites that measure variations in Earth’s gravity have revealed the
staggering extent to which groundwater supplies have declined in particular
regions, including India and
the Central Valley of California.
At issue is how an initially symmetrically rotating body - like a top or gyroscope - with moment of inertia I, would alter so that its axis 'wanders' and the motion is no longer predictable. For a completely symmetric Earth one would have at any point of its rotation:
I1 = I2
So that, for the condition shown below - where z is the Earth's axis of symmetry:
Now add in climate change factors, such as melting glaciers, and w1 is affected. Then add in the groundwater depletion and w2 is affected. Take the resultant,
w1
i + w 2 j
and the position of Earth's axis is affected, e.g. with w 3 relative to w. To be sure other forces might also be pulling Earth’s axis in its new direction but aren’t yet fully understood, according to Clark R. Wilson, a geophysicist at the University of Texas at Austin and another author of the study. In his words:
"It’s possible, for example, there’s something in Earth’s fluid core that’s going on, that’s contributing as well,”
Even
so, the latest discovery points to new possibilities for using information
about Earth’s spin to study the climate, Dr. Wilson said.
Because
scientists have collected highly precise data on the position of Earth’s axis
during much of the 20th century, they might be able to use it to understand
shifts in groundwater use that took place before the most modern and reliable
data became available. This would mean comparing the changes in the moments of inertia - say arising from the human agency factors (like massive groundwater depletion) with those associated with the established variations such as the Chandler wobble.
We also cannot ignore the effects of deep earthquakes and the mantle changes underlying them, possibly producing discontinuous changes in the inertia tensor large enough to incept major spin axis displacements.
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