Earth's Spin Has Shifted—And Humans Are To Blame

Humans have pumped so much groundwater out of the Earth that we have shifted the Earth's rotational pole, according to a study.

In a paper published in the journal Geophysical Research Letters, researchers found that the rotational pole around which our planet spins has drifted around 31 inches east between 1993 and 2010 due to groundwater depletion and resulting sea level rise.

The position of the Earth's rotational pole relative to the crust can be affected by how the mass of water is distributed across the planet.

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

Scientists uncovered the ability of water to alter the Earth's rotation in 2016. But until now, the specific contribution of groundwater to these changes has not been clear.

In the latest study, researchers modeled observed changes in the drift of the rotational pole and the movements of water. Initially, the models only took into account ice sheets and glaciers. Then the authors looked at different scenarios involving the redistribution of groundwater.

The model only matched the drift in the pole observed between 1993 and 2010 if scientists took into account 2,150 gigatons of groundwater redistribution. Without accounting for the groundwater that humans had pumped from below the Earth's surface, the model was off by 31 inches or 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 2,150 gigaton figure—equivalent to 0.23 inches of global sea level rise—comes from previous estimates by scientists regarding groundwater pumping within the 17-year period from 1993 to 2010. But direct observational evidence supporting this estimate has been lacking.

The modeling in the latest study—taking into account the redistribution of water from underground sources into the oceans—has provided independent confirmation of this estimate.

"This is a nice contribution and an important documentation for sure," Surendra Adhikari, a research scientist at the California Institute of Technology's Jet Propulsion Laboratory who was not involved in the study but took part in the research published in 2016, said in a statement. "They've quantified the role of groundwater pumping on polar motion, and it's pretty significant."

Groundwater depletion involves pumping out water from sources such as underground aquifers faster than the liquid can be replenished. This groundwater is used for agricultural purposes as well as being supplied to urban areas.

Once this groundwater has been extracted, it can find its way into the ocean through run-off or via the process of evaporation and precipitation.

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