Groundwater pumping alters Earth’s spin

According to a recent study ¹, humans have displaced a significant volume of water from the ground and relocated it, resulting in an Earth shift of approximately 80 centimetres (31.5 inches) eastward solely between 1993 and 2010.

According to prior climate models, researchers had initially estimated that humans extracted around 2,150 gigatons of groundwater between 1993 and 2010, which is equivalent to a sea level rise of over 6 millimetres (0.24 inches). However, verifying this estimation has proven to be challenging.

An avenue for investigation involves the Earth’s rotational pole, which serves as the axis around which the planet rotates. The Earth’s rotational pole undergoes a phenomenon known as polar motion, where its position varies in relation to the planet’s crust. The distribution of water across the globe influences the distribution of mass. Similar to how a spinning top is affected by even a minute increase in weight, the Earth’s rotation is subtly altered as water is displaced and relocated.

“The Earth’s rotational pole experiences significant changes,” stated Ki-Weon Seo, a geophysicist from Seoul National University and the lead researcher of the study. “Our findings indicate that, among various factors associated with climate, the redistribution of groundwater exerts the most substantial influence on the drift of the rotational pole.”

The discovery of water’s capacity to influence the Earth’s rotation was made in 2016, but the precise impact of groundwater on these rotational variations remained unexplored until now. In this recent study, scientists focused on modelling the observed alterations in the drift of the Earth’s rotational pole and the movement of water. Initially, they considered only ice sheets and glaciers, and subsequently introduced various scenarios of groundwater redistribution to assess its specific contribution.

The researchers found that the model accurately aligned with the observed polar drift when they incorporated 2150 gigatons of groundwater redistribution. Without accounting for this factor, the model deviated by 78.5 centimetres (31 inches), equivalent to an annual drift discrepancy of 4.3 centimetres (1.7 inches).

“I am extremely pleased to have identified the previously unexplained factor contributing to the drift of the Earth’s rotation pole,” expressed Seo. “However, as an inhabitant of Earth and a parent, I am both concerned and astonished to discover that pumping groundwater is an additional contributor to sea-level rise.”

“This study is a valuable and significant contribution,” commented Surendra Adhikari, a research scientist at the Jet Propulsion Laboratory who was not involved in the research. Adhikari had previously published a paper in 2016 on the impact of water redistribution on rotational drift. Adhikari acknowledged the researchers’ achievement in quantifying the influence of groundwater pumping on polar motion, emphasizing its notable significance.

The study highlights that the geographical distribution of groundwater plays a crucial role in determining the extent of its impact on polar drift. Redistributing water from the midlatitudes has been found to have a more substantial effect on the Earth’s rotational pole. Notably, during the study period, the largest amount of water was relocated in western North America and northwestern India, both of which are situated at midlatitudes.

According to Seo, if countries make efforts to reduce the rate of groundwater depletion, particularly in those sensitive regions, there is a theoretical possibility of influencing the change in drift. However, Seo emphasizes that for these conservation approaches to have a meaningful impact on polar drift, they would need to be consistently implemented and sustained over several decades.

Typically, the rotational pole undergoes natural fluctuations of several meters within a year, indicating that the changes caused by groundwater pumping do not pose a risk of altering the seasons. However, over geologic time scales, the drift of the rotational pole can indeed have an impact on climate, as highlighted by Adhikari. These long-term shifts in the pole’s position can potentially influence climatic patterns and conditions.

Seo noted the significance of monitoring changes in the Earth’s rotational pole as a means of comprehending variations in continent-scale water storage. Polar motion data has been collected since the late 19th century, presenting an opportunity to potentially analyze those historical records to gain insights into continental water storage changes over the past century. The question arises as to whether there were any alterations in hydrological regimes as a result of climate warming. Polar motion data could potentially hold the answers to these inquiries, aiding in our understanding of the impact of climate change on hydrological systems.