Here, the researchers compare the observed polar motion (red arrow, “OBS”) to the modeling results without (dashed blue arrow) and with (solid blue arrow) groundwater mass redistribution. The model with groundwater mass redistribution is a much better match for the observed polar motion, telling the researchers the magnitude and direction of groundwater’s influence on the Earth’s spin. Credit: Geophysical Research Letters (2023). DOI: 10.1029/2023GL103509
By pumping out huge quantity of ground water and moving it elsewhere, human civilization has shifted such a large mass of water that the Earth tilted nearly 80 centimeters (31.5 inches) east between 1993 and 2010 alone, according to a new study published in Geophysical Research Letters.
The study (Ki-Weon Seo, Dongryeol Ryu, Jooyoung Eom, Taewhan Jeon, Jae-Seung Kim, Kookhyoun Youm, Jianli Chen, Clark R. Wilson, Drift of Earth’s Pole Confirms Groundwater Depletion as a Significant Contributor to Global Sea Level Rise 1993–2010, 15 June 2023, https://doi.org/10.1029/2023GL103509) said:
Based on climate models, 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. But validating that estimate is difficult.
One approach lies with the Earth’s rotational pole, which is the point around which the planet rotates. It moves during a process called polar motion (PM), which is when the position of the Earth’s rotational pole varies relative to the crust. The distribution of water on the planet affects how mass is distributed. Like adding a tiny bit of weight to a spinning top, the Earth spins a little differently as water is moved around.
Climate model estimates show significant groundwater depletion during the 20th century, consistent with global mean sea level (GMSL) budget analysis. However, prior to the Argo float era, in the early 2000’s, there is little information about steric sea level contributions to GMSL, making the role of groundwater depletion in this period less certain.
The scientists said:
A useful constraint is found in observed PM. In the period 1993–2010, they found that predicted PM excitation trends estimated from various sources of surface mass loads and the estimated glacial isostatic adjustment agree very well with the observed. Among many contributors to the PM excitation trend, groundwater storage changes are estimated to be the second largest (4.36 cm/yr) toward 64.16°E. Neglecting groundwater effects, the predicted trend differs significantly from the observed. PM observations may also provide a tool for studying historical continental scale water storage variations.
Key Points Of The Study Findings
- Earth’s pole has drifted toward 64.16°E at a speed of 4.36 cm/yr during 1993–2010 due to groundwater depletion and resulting sea level rise
- Including groundwater depletion effects, the estimated drift of Earth’s rotational pole agrees remarkably well with observations
The study report said:
Melting of polar ice sheets and mountain glaciers has been understood as a main cause of sea level rise associated with contemporary climate warming. It has been proposed that an important anthropogenic contribution is sea level rise due to groundwater depletion resulting from irrigation. A climate model estimate for the period 1993–2010 gives total groundwater depletion of 2,150 GTon, equivalent to global sea level rise of 6.24 mm. However, direct observational evidence supporting this estimate has been lacking.
In the study, the scientists show that the model estimate of water redistribution from aquifers to the oceans would result in a drift of Earth’s rotational pole, about 78.48 cm toward 64.16°E. In combination with other well-understood sources of water redistribution, such as melting of polar ice sheets and mountain glaciers, good agreement with PM observations serves as an independent confirmation of the groundwater depletion model estimate.
The study report’s conclusion said:
Global climate model estimates indicate that groundwater depletion is a significant contributor to GMSL rise. Since the launch of GRACE, observations of time-variable gravity show large amounts of groundwater depletion and resulting sea level rise. Prior to the GRACE mission, GMSL budgets indicated declining groundwater, but confirmation from direct observations was lacking on a global scale. Independent confirmation of groundwater’s contribution to GMSL changes might come from variations in Earth’s dynamic oblateness (J2) and PM. It has been found that J2 is not especially useful for this purpose because of the geography (low latitudes) of aquifers that have been depleted in this period, and uncertainty in GIA predictions regarding J2. On the other hand, PM excitation, for 1993–2010 is sensitive to global groundwater changes, and observed and predicted PM excitations agree well with each other. It has also been found that groundwater depletion was the second largest (4.36 cm/yr) component of PM excitation trend toward 64.16°E during 1993–2010. Among known sources of PM excitation, groundwater changes are particularly important to explain the component, trending toward 90° east longitude. Neglecting groundwater depletion in the PM excitation budget leads to a trend that is more westward than observed. Various choices for estimates of other surface mass load contributions lead to the same conclusion. This confirms that groundwater depletion is a major source of GMSL rise during the last a few decades as previously indicated by these models.
Water’s ability to change the Earth’s rotation was discovered in 2016, and until now, the specific contribution of groundwater to these rotational changes was unexplored. In the new study, researchers modeled the observed changes in the drift of Earth’s rotational pole and the movement of water — first, with only ice sheets and glaciers considered, and then adding in different scenarios of groundwater redistribution.
The model only matched the observed polar drift once the researchers included 2150 gigatons of groundwater redistribution. Without it, the model was off by 78.5 centimeters (31 inches), or 4.3 centimeters (1.7 inches) of drift per year.
“This is a nice contribution and an important documentation for sure,” said Surendra Adhikari, a research scientist at the Jet Propulsion Laboratory who was not involved in this study. Adhikari published the 2016 paper on water redistribution impacting rotational drift. “They’ve quantified the role of groundwater pumping on polar motion, and it’s pretty significant.”
The location of the groundwater matters for how much it could change polar drift; redistributing water from the midlatitudes has 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 midlatitudes.
Countries’ attempts to slow groundwater depletion rates, especially in those sensitive regions, could theoretically alter the change in drift, but only if such conservation approaches are sustained for decades, said one of the scientists involved with the study.
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 have an impact on climate, Adhikari said.
