Between 1993 and 2010, humans extracted and moved so much of Earth’s groundwater that it contributed to the shift of the Earth’s poles.
A new analysis led by geophysicist Seo Ki-won of Seoul National University in South Korea found that groundwater redistribution alone caused a pole shift of 80 cm (31.5 inches) to the east.
With these findings, scientists were able to confirm that previous estimates of groundwater depletion due to human activity equated to a total sea level rise of 6 millimeters over the period.
The researchers did this to better understand the polar motion phenomenon and its contributions from changes in Earth’s water distribution. In 2016, scientists made a breakthrough in figuring out why Earth’s rotational poles drift—the distribution of terrestrial water reservoirs.
Now Seo and his colleagues have determined how much human movement of groundwater contributed to that wandering.
“Earth’s rotational poles actually change a lot,” says Seo. “Our study shows that, among climate-related causes, redistribution of groundwater actually has the greatest effect on shifts in the magnetic poles.”
It makes sense when you think about it. The Earth rotates around its axis like a top. If the distributed mass around that axis changes and becomes uneven, the axis moves to compensate.
Climate change has a huge impact on this. As the frozen parts of the world, such as glaciers and ice sheets, melt, the distribution of water on Earth’s surface changes and the poles, the ends of the axis of rotation, shift.
This effect became prominent in the early 1990s and much work has been done to determine the role of water redistribution. However, groundwater extraction alone did not have an impact.
Based on climate models, scientists estimated in 2010 that humans pumped about 2,150 gigatons of groundwater between 1993 and 2010, raising sea levels by about 6 millimeters, but this has been difficult to confirm observationally.
Seo and his colleagues solved the problem using observational data from polar motion and modeling. First, they modeled polar motion by considering only the contribution of ice melting from glaciers, ice sheets and sea ice. We then added different levels of groundwater extraction to the model.
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This brought it closer to the observed movement, but the model only matched when using the 2,150 gigatonne estimate.
Estimates provided an accurate contribution of groundwater extraction. The model without groundwater contribution differed by 78.48 cm.
Between 1993 and 2010, groundwater extraction pushed the Earth’s poles at a rate of 4.36 cm per year. (It may still have an impact, but the team’s work is based only on data through 2010.)
“We are very happy to find the unexplained cause of the rotating pole drift,” said Seo. “On the one hand, as a resident and father of the Earth, I am concerned and alarmed that pumping groundwater is another cause of sea level rise.”
However, this discovery may help mitigate further polar motion. The greatest effect is felt when groundwater is extracted in mid-latitudes. Researchers found that between 1993 and 2010, most groundwater extraction occurred in mid-latitudes, primarily in North America and northern India.
A concerted effort by these regions to limit the rate of groundwater extraction could help slow polar motion and sea level rise. But such an effort would have to be long-term, at least decades, the researchers say.
But working to mitigate human impact on the climate is a long-term challenge. The sooner you start, the better.
“Observing changes in Earth’s rotational poles is useful for understanding continent-scale changes in water storage,” says Seo.
“Polar motion data are available as early as the late 19th century. Therefore, we can potentially use these data to understand changes in continental water storage over the past 100 years. Have there been changes in the hydrological regime due to a warming climate? Polar motion keep the answer.”
This study geophysical research letter.
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