AsianScientist (Jul. 28, 2023)– In a recent study, a multinational team of scientists have analyzed changes in the Earth’s rotational pole, caused by displacement of large masses of ground water. Their findings published in Geophysical Research Letters reveal that excessive groundwater pumping is not only adding to sea level rise but also causing the planet to tilt on its axis.

The way mass is distributed across the globe can cause its rotational pole—an imaginary line passing through the North and South Poles—to shift and wobble in a phenomenon called polar motion. Just how putting a small weight on a spinning top would make the toy spin differently. Earth’s axial wobble has been linked with long-term processes like the melting of Greenland’s ice into the oceans as global temperatures climbed during the 20th century.

“Earth’s rotational pole actually changes a lot,” said Ki-Weon Seo, a geophysicist at Seoul National University who led the study. “Our study shows that among climate-related causes, the redistribution of groundwater actually has the largest impact on the drift of the rotational pole.”

Groundwater constitutes an immense amount of water held in porous rocks beneath the Earth’s surface called aquifers. Humans have primarily extracted groundwater for irrigation and drinking water. However, about 80% of the extracted groundwater eventually finds its way into the oceans, resulting in water being transferred from the Earth’s continents to its vast oceans.

Even though water’s capacity to affect the Earth’s spin axis was known since 2016, the specific influence of groundwater depletion was unexplored. To find that out, Seo’s team modelled the polar motion observed between 1993 and 2010 that accounted for water movement associated with thinning ice sheets, melting glaciers and water stored in reservoirs.

Their results showed that without including the estimated 2,150 gigatons of groundwater pumped during this period, the model was off by 78.5 centimeters (31 inches) eastward, or an annual drift of 4.3 centimeters (1.7 inches). This extracted groundwater that could have ended up in the oceans would amount to a global sea level rise of 6.24 millimeters.

“I’m very glad to find the unexplained cause of the rotation pole drift,” Seo said. “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 Earth’s polar drift usually wanders by several meters every year, making the impact from groundwater depletion negligible on our immediate weather and seasons. Over geologic time scales, however, this axial change could hold greater climate consequences. Seo suggested that countries located at mid-latitudes, which exert the most influence on the rotational pole, have the opportunity to adopt and maintain improved conservation approaches to change this drift.

The greater concern from these findings was groundwater’s contribution to sea levels. As sea levels rise, communities may find themselves forced to migrate to higher grounds due to retreating shorelines and the possible surge in the occurrence of more dangerous hurricanes and typhoons. Unsustainable pumping of groundwater could also lead to a reduction in water flow from natural streams and disrupt delicate aquatic ecosystems.

Looking ahead, Seo emphasized that polar drift could prove valuable in exploring our past and gaining a deeper understanding of the impacts of climate change.

“Observing changes in Earth’s rotational pole is useful for understanding continent-scale water storage variations,” Seo said. “Polar motion data are available from as early as the late 19th century. So, we can potentially use those data to understand continental water storage variations during the last 100 years. Were there any hydrological regime changes resulting from the warming climate? Polar motion could hold the answer.”

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Source: Seoul National University ; Image: Shelly Liew/Asian Scientist Magazine