Groundwater Pumping Affects Mass Distribution, Causing Changes in the Earth’s Rotation

The rising global population and economic developments continue to increase the demand for water supply. Since about 71% of the Earth's surface is covered with water, these demands can be met by surface water availability. However, water stress is experienced in many parts of the world due to the scarcity of safe, usable water in certain areas.

Regions that frequently experience water stress often sort to groundwater as an additional water source. A study in 2010 revealed that a total of 2,150 gigatons of groundwater had been pumped by humans from 1993 to 2010. When the amount of extracted groundwater exceeds the rate of water restoration, persistent groundwater depletion is likely to occur. This reduction of groundwater does not only damage the surface of the Earth but the planet's rotation as well.

Changes in the Earth's Polar Motion

The Earth rotates around a point known as its rotational pole. Since our planet is constantly in motion, then it is no surprise that the rotational axis also shifts. Polar motion is normal and can be categorized into several time scales, such as the yearly wobbles and the Chandler wobbles. During the process of polar motion, the position of the rotational pole changes relative to the Earth's crust.

On a larger time scale, the secular movement of the axis is directed to the West. Recently, scientists observed that the polar drift had shifted eastward toward Greenland. Geophysicists conclude that these unexplainable polar shifts are related to the mass distribution between the land and the ocean.

How is Groundwater Connected to the Polar Drift?

In 2016, researchers from the Jet Propulsion Laboratory at California Institute of Technology discovered that water could change the Earth's rotation. However, no further exploration explains the particular contribution of groundwater to these changes.

A new study conducted by geophysicist Ki-Weon Seo from Seoul National University confirms that groundwater redistribution has the largest impact on the shift of rotational pole among the other climate-related causes. Since how water is distributed on the planet affects mass distribution, then the spin of the Earth changes a little as water is displaced.

Seo and his team developed a model showing the changes in the rotational pole of the Earth and the movement of water. They first used ice sheets and glaciers before adding the conditions in redistributing groundwater.

The observed polar drift matched the model when the extraction of 2150 gigatons of water was included. Without it, the model only shows a 1.7-inch drift every year. It gives new insight into the unexplained rotation pole drift that used to baffle scientists.

The extent to which groundwater could change polar drift is affected by location. Larger impact can be observed in water redistribution along the mid-latitudes. In the study made by Seo and his colleagues, most waters was redistributed in the western region of North America and northwestern India, which are both located in mid-latitudes.

Seo clarifies that although countries attempt to reverse this phenomenon by slowing down the rates of depletion, these conservation approaches can only be realized if they are sustained for decades. The changes in our planet's rotational pole can also be understood by looking at the past activities of the Earth. Since data on polar motion are available from the late 19^th century, these data can be used to understand the variations in continental water storage for the past 100 years.