Groundwater Pumping Is Beginning To Affect the Earth’s Rotation
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Groundwater pumping nudged the Earth nearly 80 centimeters east of its normal rotation axis between 1993 and 2010, a new study has found. Led by scientists from Seoul National University and published in the journal Geophysical Research Letters, the research also found that the human pumping of groundwater is the most significant climate-related contributor to polar drift.
The importance of groundwater
The term “freshwater” might conjure up mental images of vast lakes and glacial peaks, but in reality, most of the world’s freshwater exists as groundwater. Flowing in the cracks and crevices between soil, sand and rock, this groundwater replenishes our freshwater lakes and other areas of surface water when it naturally bubbles to the surface.
But groundwater can also be extracted manually, and it has been used extensively as a source for drinking water, irrigation and other industrial activities in modern times. When this water is pumped out of the ground and used elsewhere, it redistributes how weight is spread around the globe, making the continents comparatively lighter and adding weight to the world’s oceans.
The Earth’s rotational pole – the point around which the globe rotates, which is distinct from its magnetic pole – does shift naturally through a process called “polar motion”. There are many different causes for polar motion, for example, earthquakes can cause small but abrupt changes to this motion as they alter the volume distribution of Earth’s solid mass. Like adding weight to one side of a spinning top, such events affect how the world spins on its axis.
Similarly, changes to water weight distribution can also impact polar motion. This was first discovered in a 2016 study, however, the specific contribution of groundwater pumping to these observed changes was unknown.
“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.”
Polar motion model reveals true impact of groundwater pumping
In this latest study, the research team modeled the drift of the Earth’s rotational pole and the movement of water between 1993 and 2010. At first, they considered only the movement of ice sheets and glaciers, before adding in different groundwater redistribution scenarios.
They found that the model only matched the real polar drift observed after the researchers factored in 2150 gigatons of groundwater redistribution. Without it, the model was off by 78.5 centimeters (31 inches), or 4.3 centimeters (1.7 inches) of drift per year.
“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 location of groundwater pumping could also affect how much it changes polar drift. As the researchers explain, redistributing water from the midlatitudes will have a larger effect on the rotational pole. Notably, during this study period, water was redistributed the most in western North America and northwestern India – both at midlatitudes.
“This is a nice contribution and an important documentation for sure,” commented Surendra Adhikari, a research scientist at the Jet Propulsion Laboratory. Adhikari was not involved in this latest study, though he was a coauthor of the 2016 paper which originally identified the impact of water redistribution on rotational drift. “They’ve quantified the role of groundwater pumping on polar motion, and it’s pretty significant.”
Re-examining historical data
The Earth’s rotational pole commonly fluctuates by several meters within a year, the researchers point out, so these groundwater-induced changes are not about to bring on any disastrous consequences. But on geologic time scales, changes to polar drift can begin to have a more significant effect on the climate.
Similarly, a country’s attempts to slow its groundwater depletion rate could counteract some of this change in drift, but only if such conservation approaches are sustained for decades.
Moving on from this research, the researchers say that their next steps may involve looking to the past.
“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.”
Reference: Seo KW, Ryu D, Eom J, et al. Drift of Earth’s pole confirms groundwater depletion as a significant contributor to global sea level rise 1993–2010. Geophys Res Lett. 2023;50(12):e2023GL103509. doi: 10.1029/2023GL103509
This article is a rework of a press release issued by the American Geophysical Union. Material has been edited for length and content.