Excessive groundwater depletion has knocked Earth’s axis off-kilter

Groundwater pumped from the earth and moved elsewhere to quench the thirst of human activity has resulted in the Earth’s axis tilting nearly 80 cm to the east.
| Photo Credit: Yousuf Mohammed/The Hindu

Groundwater pumped from the earth and moved elsewhere to quench the thirst of human activity has resulted in the Earth’s axis tilting nearly 80 cm to the east, a study conducted between 1993 and 2010 has shown. 

Scientists estimated that nearly 2,150 gigatons of groundwater have been pumped and drained into the oceans, making it one of the important contributors to global sea-level rise. However, there was no direct observational proof that backed this estimate. 

Now, a study published in the Geophysical Research Letters has used the Earth’s rotational pole to determine the veracity of the estimate.

The Earth’s rotational pole is the point along which the planet rotates. This point, also called the axis of the Earth, moves in a process called polar motion which is when the Earth’s pole varies relative to the crust. 

Unlike a globe which has a fixed axis and rotates stably the Earth’s axis wobbles. It is 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, season changes, the molten core and even hurricanes. 

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Scientists have been able to track this motion by looking at astronomical phenomena such as centres of bright galaxies or quasars by comparing their changing positions in the sky as the Earth’s axis shifted.

Scientists have also known for a long time that the movement of water can affect Earth’s rotation. A study published in 2016, showed how the movement of water around the world contributed to the wobble in the Earth’s axis. 

However, the role of groundwater had not 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 which aimed to link the shift in the Earth’s axis and the movement of water through melting ice caps and glaciers. However, the numbers did not line up.

The scientists added the effects of water stored in reservoirs and dams but to no avail. 

The model finally reflected the observed drift of the axis once groundwater was added 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.”

Scientists also said that the location of groundwater depletion is important as it would affect how much the axis wandered. Using the climate model, they found that pumping groundwater from midlatitude areas would impact the drift the most. 

They found that the most amount of groundwater redistribution took place in northwestern India and western North America, both situated at mid-latitudes. 

The surveys conducted during the study period suggested 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. 

States such as Punjab, Haryana, Delhi and western Uttar Pradesh have clocked critical groundwater levels due to indiscriminate use of groundwater, while Rajasthan and Gujarat have low groundwater levels due to arid climate. Groundwater availability is low in parts of Karnataka, Tamil Nadu, Telangana and Andhra Pradesh due to the crystalline nature of the aquifers found here.