Between 1993 and 2010, humans extracted and moved so much of our planet's groundwater that it contributed to the migration of Earth's poles.
Just the contribution of groundwater redistribution resulted in a polar shift of 80 centimeters (31.5 inches) towards the east, according to a new analysis led by geophysicist Ki-Weon Seo of Seoul National University in South Korea.
These findings allowed scientists to confirm that previous estimates of groundwater depletion caused by human activity are equivalent to a total sea level rise of 6 millimeters during that time.
Researchers conducted this work to better understand the phenomenon of polar motion and the contribution made by changes in Earth's water distribution. In 2016, scientists made a breakthrough in figuring out why Earth's rotational poles wander: the distribution of terrestrial water storage.
Now Seo and his colleagues have determined how much humans moving groundwater around has contributed to that wander.
"Earth's rotational pole actually changes a lot," Seo says. "Our study shows that among climate-related causes, the redistribution of groundwater actually has the largest impact on the drift of the rotational pole."
It makes sense when you think about it. Earth spins about its rotational axis a lot like a spinning top. When the distribution mass around that axis changes and becomes uneven, the axis shifts to compensate.
Climate change has a big effect on this. As the frozen parts of the world melt, such as glaciers and ice sheets, the distribution of water over Earth's surface changes, and the poles – the ends of the rotational axis – move around.
This effect became prominent in the early 1990s, and much work has been done to determine the role water redistribution plays in it. But the impact of groundwater extraction alone had not been isolated.
Based on climate models, scientists estimated in 2010 that humans had pumped around 2,150 gigatons of groundwater between 1993 and 2010, around 6 millimeters of sea level rise, but this was difficult to confirm observationally.
Seo and his colleagues tackled the problem using observational data of polar motion and modeling. First, they modeled polar motion only considering the contribution of ice melt from glaciers, ice sheets, and sea ice. Then, they added different levels of groundwater extraction to their models.
This took them closer to the observed motion, but the model only became a match when they used the 2,150 gigaton estimate.
The estimate provided the exact contribution of groundwater extraction. With no inclusion of groundwater contribution, the model was off by 78.48 centimeters.
Between 1993 and 2010, groundwater extraction pushed Earth's poles at a rate of 4.36 centimeters per year. (It's probably still exerting an influence, but the team's work is only based on data up to 2010.)
"I'm very glad to find the unexplained cause of the rotation pole drift," Seo says. "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."
However, the findings could help mitigate further polar motion. The biggest effect is felt when groundwater is extracted from mid-latitudes. The researchers found that most of the groundwater extraction between 1993 and 2010 occurred at mid-latitudes, primarily from North America and the North of India.
If these regions made a concerted effort to limit groundwater extraction rates, that could help slow polar motion and sea level rise. However, such an effort would have to be ongoing over a long period, at least decades, the researchers say.
But working towards mitigating human-influenced effects on climate is a long game. The sooner started, the better.
"Observing changes in Earth's rotational pole is useful for understanding continent-scale water storage variations," Seo says.
"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."
The research has been published in Geophysical Research Letters.