Here, the researchers compare the observed polar motion (red arrow, “OBS”) to the modeling results without (dashed blue arrow) and with (solid blue arrow) groundwater mass redistribution. The model with groundwater mass redistribution is a much better match for the observed polar motion, telling the researchers the magnitude and direction of groundwater’s influence on the Earth’s spin. Credit: Seo et al. (2023), Geophysical Research Letters
Key points:
- The shifting of mass and consequent sea level rise due to groundwater withdrawal has caused the Earth’s rotational pole to wander.
- In the last 20 years, the Earth tilted nearly 80 centimeters (31.5 inches) east.
- Redistributing water from the midlatitudes has a larger impact on the rotational pole.
Over the last 20 years, humans have pumped so much groundwater that Earth has tilted nearly 80 centimeters (31.5 inches) east between 1993 and 2010 alone.
Water’s ability to change the Earth’s rotation was discovered in 2016, but until now, the contribution of groundwater to these rotational changes was not studied.
In this new study, published in Geophysical Research Letters, researchers first modeled the observed changes in the drift taking into account only ice sheets and glaciers. They subsequently modeled different scenarios of groundwater redistribution.
According to the findings, the model only matched the observed polar drift once the researchers included 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.
Researchers say the location of the groundwater matters for how much the Earth could change polar drift. For example, redistributing water from the midlatitudes has a larger impact on the rotational pole. During the study period, most water was redistributed in western North America and northwestern India, both at midlatitudes.
Countries’ attempts to slow groundwater depletion rates, especially in sensitive regions, could theoretically alter the change in drift, but only if such conservation approaches are sustained for decades, said Ki-Weon Seo, a geophysicist at Seoul National University who led the study.
The next step for this research could be looking into the past.
“Observing changes in Earth’s rotational pole is useful for understanding continent-scale water storage variations,” said Seo. “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.”