Groundwater pumped up from the earth and moved elsewhere to quench the thirst of humans and their activities resulted in the earth’s axis tilting nearly 80 cm to the east, a new study has found.
The same study also found that nearly 2,150 billion tonnes of groundwater has been pumped and drained into the oceans in 1993-2010, making it one of the important contributors to global sea-level rise.
The earth’s rotational pole is the point along which the planet rotates. This point, which lies on the axis of rotation of the planet, moves in a process called polar motion. In other words, the location of the earth’s pole varies relative to the earth’s crust.
Unlike a globe, which has a fixed axis and rotates stably, the earth’s axis wobbles. It’s more like a spinning top gone off kilter, where the earth’s rotational pole tends to wander in a circular pattern several meters wide every year due to the weather, seasonal changes, the molten core, and even powerful hurricanes.
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Scientists have been able to track this motion relative to astronomical phenomena such as the centres of bright galaxies or quasars.
They have also known for a long time that the movement of water can affect the earth’s rotation. A study published in 2016, for example, showed how the movement of water around the world contributed to the wobble in the earth’s axis.
But the role of groundwater hadn’t been considered before.
To bridge this gap, a group of scientists at the Seoul National University led by Professor Ki-Weon Seo used a climate model that linked the shift in the earth’s axis with the movement of water through melting ice caps and glaciers. The numbers did not line up.
The scientists added the effects of water stored in reservoirs and dams but to no avail.
The model only matched the observed drift of the axis once they added groundwater to the equation.
“I’m very glad to find the unexplained cause of the rotation pole drift,” Dr Seo said in a press release. “On the other hand, as a resident of earth and a father, I’m concerned and surprised to see that pumping groundwater is another source of sea-level rise.”
The scientists also said that the location of groundwater depletion is important because that affects how much the axis wanders. Using their model, they found that pumping groundwater from midlatitude areas would impact the drift the most. They also found that the most amount of groundwater redistribution took place in northwest India and western North America, both situated at mid-latitudes.
The surveys conducted during the study period, and as part of the study, also found that enough groundwater was pumped from underground reservoirs or aquifers to cause the global sea level rise of 6.24 mm between 1993 and 2010.
Groundwater depletion has been a particular concern across India since the last decade. About 95% of India’s groundwater depletion was traced to north India where groundwater is primarily used for irrigation.
Punjab, Haryana, Delhi, and western Uttar Pradesh have critical groundwater levels due to the indiscriminate use of groundwater, while Rajasthan and Gujarat have low groundwater levels due to arid climate. Groundwater availability is also low in parts of Karnataka, Tamil Nadu, Telangana, and Andhra Pradesh due to the crystalline nature of the aquifers found here.
