Groundwater depletion equals Climate Change? California Partially to Blame!

Climate model estimates show significant groundwater depletion during the 20th century, consistent with global mean sea level (GMSL) budget analysis. It has been proposed that an important anthropogenic contribution is sea level rise due to groundwater depletion resulting from irrigation.

Scientists previously calculated that between 1993 and 2010, humanity pumped 2,150 gigatons of groundwater, or more than 6 millimeters (0.24 inches), of sea level increase, based on climate models. It’s challenging to verify that estimate, though.

A new study suggests that humans have pumped so much water out of the ground and moved it elsewhere that the Earth has tilted approximately 80 centimeters (31.5 inches) east between 1993 and 2010 alone.

California pumps 17.4 billion gallons per day of groundwater for all purposes, 2.4 times as much as the second-ranked state — Texas (7.2 bgd). According to NASA and European Space Agency data released on February 8, parts of the California aqueduct on the west side of the San Joaquin Valley, near Avenal, sank more than two feet between 2013 and 2016 as farmers pumped records amounts of groundwater during the state’s historic drought.

Earth’s rotational pole changes a lot. This study has shown that among climate-related causes, the redistribution of groundwater has the largest impact on the drift of the rotational pole.

In 2016, it was found that water might alter how the Earth rotated, but up until now, the precise role of groundwater in these rotational shifts has not been studied. In the current study, scientists used computer simulations to simulate the reported changes in the drift of the Earth’s rotational pole and the movement of water, initially simply taking into account ice sheets and glaciers and then including several scenarios of groundwater redistribution.

Only after the researchers included 2150 gigatons of groundwater redistribution did, the model match the observed pole drift. Without it, the model was off by 78.5 cm (31 inches), or 4.3 cm (1.7 inches) of drift annually.

Ki-Weon Seo, a geophysicist at Seoul National University who led the study, said, “I’m very glad to find the unexplained cause of the rotation pole drift. 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.”

Surendra Adhikari, a research scientist at the Jet Propulsion Laboratory, said, “This is a nice contribution and important documentation for sure. They’ve quantified the role of groundwater pumping on polar motion, and it’s pretty significant.”

How much groundwater could alter polar drift depends on where it is; water redistributing from the midlatitudes has a greater effect on the rotational pole. The two midlatitude regions of western North America and northwest India had the greatest water redistribution throughout the research period.

Theoretically, efforts by countries to limit groundwater depletion rates, particularly in those vulnerable areas, could affect the change in drift, but only if such conservation measures are maintained for decades.

Adhikari said, “The rotational pole normally changes by several meters within about a year, so changes due to groundwater pumping don’t run the risk of shifting seasons. But on geologic time scales, polar drift can impact climate.”

Seo said, “Observing changes in Earth’s rotational pole is useful for understanding continent-scale water storage variations. 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.”

Journal Reference:

1. Ki-Weon Seo, Dongryeal Ryu, et al. Drift of Earth’s Pole Confirms Groundwater Depletion as a Significant Contributor to Global Sea Level Rise 1993–2010. Geophysical Research Letters. DOI: 10.1029/2023GL103509