By pumping water out of the ground, humans have shifted such a large mass of water that the Earth tilted nearly 80 centimeters (31.5 inches) east in the last thirty years.
Earth’s rotational poles, which are the points around which the planet rotates, move with respect to the surface during a process called polar motion in response to the distribution of Earth’s mass. The same physical principle can be seen on a much smaller scale during a hammer throw competition, as the swirling mass of the “hammer” – a metal ball attached by a steel wire to a grip – forces the athlete to wobble around the center of rotation.
Over geological time, the gravitational pull of Sun and Moon, growing or shrinking ice sheets and the slow drift of the continents will move mass around and cause Earth’s axis to shift, but human activity can cause significant changes on a much smaller timescale.
In a 2016 study, the authors argued that an anomalous eastward shift of the rotational poles 0ver the past 115 years was caused by anthropogenic climate change. As the planet warms in response of increasing atmospheric levels of greenhouse gases, melting ice sheets and glaciers caused sea-levels to raise, redistributing large volumes of water and forcing Earth’s center of rotation to shift respectively.
But melting ice and snow alone couldn’t explain the observed polar shift.
In a new study, researchers modeled the drift of Earth’s axis and the movement of water – first, with only ice sheets and glaciers considered, and then adding in different scenarios of groundwater redistribution.
“Our study shows that among climate-related causes, the redistribution of groundwater actually has the largest impact on the drift of the rotational pole,” said lead author Ki-Weon Seo.
Groundwater has been used extensively in modern times for irrigation, potable water, and industrial activity. Scientists previously estimated humans pumped 2,150 gigatons of groundwater, equivalent to more than 6 millimeters (0.24 inches) of sea level rise, from 1993 to 2010.
The water pumped out of the ground and runoff adds mass to the oceans, making the continents lighter, especially in the northern hemisphere.
The location of the groundwater matters for how much it could change polar drift in the model; redistributing water from the mid-latitudes had a larger impact on the rotational pole. During the study period, the most water was redistributed in western North America and northwestern India, both at mid-latitudes.
The rotational poles normally change by several meters within about a year, so changes due to groundwater pumping don’t run the risk of catastrophic consequences. But this new model provides a tool to study humanity’s past use of groundwater.
“Observing changes in Earth’s rotational pole is useful for understanding continent-scale water storage variations,” Seo said. “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 study “Drift of Earth’s Pole Confirms Groundwater Depletion as a Significant Contributor to Global Sea Level Rise 1993–2010” was published in the journal Geophysical Research Letters (2023). Material provided by Rebecca Dzombak for the American Geophysical Union.