Around the turn of the millennium, the Earth’s rotation began to change, and no one could explain why.
For decades, scientists had been watching the average position of our planet’s axis of rotation, the imaginary bar around which it revolves, gently drift south, away from the geographic North Pole and toward Canada. However, it suddenly took a sharp turn and started heading east.
Over time, the researchers realized that 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 rotation.
Now, some of the same scientists have identified another factor: colossal amounts of water drawn from the soil for crops and homes. When water is extracted from the ground, but not replenished, the land can subside, reducing the amount of underground space available thereafter to hold water.
Between 1960 and 2000, groundwater depletion more than doubled worldwide, to about 284 trillion liters a year, scientists estimate. Since then, satellites measuring variations in Earth’s gravity have revealed the impressive degree to which groundwater supplies have dwindled in particular regions, including India and California.
The Earth’s axis has not shifted enough to affect the seasons, which are determined by the planet’s tilt. But the fine patterns and variations in the planet’s rotation are very important to satellite-based navigation systems that guide planes, missiles, and mapping applications.
As it moves through space, the Earth wobbles. This is partly because it is widest 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 returning to their position after having been covered for millennia by gigantic sheets of ice.
This has been constantly changing the mass balance across the planet.
More recently, the balance has also been upset by human activity and weather. 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 is the depletion of groundwater, finds the new study by Ki-Weon Seo, a geophysicist at Seoul National University, and colleagues.
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 is a possibility that Seo says he is already exploring.
By: RAYMOND ZHONG
BBC-NEWS-SRC: http://www.nytsyn.com/subscribed/stories/6797881, IMPORTING DATE: 2023-07-10 21:10:07
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