Sea level rise is caused by changes in the earth’s axis, according to a new study that places polar tilt alongside known contributing factors like the melting Greenland ice sheet or lost mountain glaciers. And humans are again the cause: The tilt and related sea level rise are linked to how much water humans have pumped out of the ground.

“Earth’s rotational pole actually changes a lot,” explains Ki-Weon Seo, a geophysicist at Seoul National University who led the research team. “Our study shows that among climate-related causes, the redistribution of groundwater actually has the largest impact on the drift of the rotational pole.”

That’s because the precise location of Earth’s rotational pole, around which the planet rotates, moves in relation to the planet’s crust. How water is distributed has impacts on mass. Seo and his colleagues explain that it’s like adding a tiny bit of weight to a spinning top, with the Earth spinning a little differently because water moved.

It’s been known since 2016 that water has the ability to change the earth’s rotation, but scientists haven’t been clear about the impacts. The new study, published in Geophysical Research Letters by the South Korean scientists and their international colleagues, finds a tilt of nearly 80 centimeters (31.5 inches) to the east between 1993 and 2010 alone.

The modeling work was based on observations of polar drift matched with the movement of water. When the scientists were able to account for an estimated 2,150 gigatons of groundwater redistribution, they could identify how much polar motion was associated with it.

Specifically, the groundwater removal was linked to polar drift toward 64.16°E at a speed of 4.36 centimeters per year. “Neglecting groundwater depletion … leads to a trend that is more westward than observed,” the paper authors said, noting that the shift may not be factored into existing projections.

The findings confirm that groundwater changes are the second largest cause for sea level rise in the last few decades. Where they occur matters, too. Groundwater redistributed in the global midlatitudes has a larger impact, with northwestern India and western North America showing the greatest impacts during the study years.

Conservation measures, especially in these sensitive regions, could change the polar drift rates but only if groundwater depletion changes were sustained for decades, Seo said.

“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 next step could be a look toward the past, Seo said.

“Polar motion data are available from as early as the late 19^th century,” he said. “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.”

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