Groundwater extraction shifted the Earth’s axis: What a new study says

It was carried out by Ki-Weon Seo, Taewhan Jeon, Jae-Seung Kim, Kookhyoun Youm of the Seoul National University (South Korea), Dongryeol Ryu of the University of Melbourne (Australia), Jooyoung Eom of the Kyungpook National University (South Korea), Jianli Chen of the Hong Kong Polytechnic University (Hong Kong), Clark R Wilson of the University of Texas at Austin (USA).

Although the shift isn’t significant enough to have real-life consequences, the study shows that humans have extracted so much water from the ground that it has impacted the planet’s axis and contributed to global sea level rise.

Earth’s axis keeps shifting

Earth spins around an imaginary axis which passes through the north pole, its centre of mass and the south pole — just like a top spins around its spindle. Scientists for years have known that the poles and the axis keep shifting naturally as the mass distribution in and on the planet changes. This phenomenon is known as “polar motion”.

For instance, rocks slowly circulating inside Earth’s mantle causes the planet’s mass to shift, leading to a change in the position of the rotational axis. Lead author of the latest research, Ki-Weon Seo, told The Indian Express that the shift of the axis, in fact, “varies about several metres in a year.”

There are several other reasons responsible for polar motion like ocean currents and even hurricanes. But this phenomenon is also impacted by human activities. In 2016, a team of researchers demonstrated that climate-driven changes in water mass distribution, led by the melting of glaciers and ice in Greenland, can cause Earth’s axis to drift. Five years later, another study said climate change was causing the rotational axis to shift more than usual since the 1990s.

Findings of the new study

To carry out the study, Seo and his team took observational data spanning 17 years and a computer model to find out which factors affected the Earth’s rotation of axis the most. Initially, the team wasn’t able to match their prediction with the level of shift that scientists have observed in previous years.

“I calculated variations of the spin axis using many kinds of data including atmospheric pressure, ocean bottom pressure, artificial reservoirs behind dams, polar ice, mountain glacier, wind, current and finally groundwater. The estimated spin axis variations didn’t agree with the observation when excluding the groundwater effect. After including it, estimation agreed really well with observation,” said Seo, adding that the redistribution of groundwater was found to be the largest contributor to the drift of the rotational pole.

The study also noted that the groundwater extraction from North America and northwestern India, both located at the Earth’s midlatitudes, had an outsized impact on the polar motion in comparison to the extraction taking place in poles or equators.

Seo explained: “Mass change on the equator or pole cannot affect change in the rotational pole. Rotational pole change is actually associated with the moment of inertia of the Earth, which is sensitive to midlatitude mass change.”

The water sucked out from the ground for irrigation and meeting the world’s freshwater demands, eventually, goes into the oceans. Seo and his team confirmed that groundwater extraction is one of the major contributors to the global sea level rise. Their calculations matched with previous research, which estimated that groundwater extraction raised global sea levels by 6.24mm between 1993 and 2010.