Is the Earth getting crooked? This may be because humans are over-extracting groundwater

Produced by: Science Popularization China

Author: Earth's Gravity (Popular Science Creator)

Producer: China Science Expo

Editor's note: In order to expand the boundaries of cognition, the China Science Popularization Frontier Science Project has launched a series of articles on the "Unknown Realm", which provides an overview of the exploration results that break through the limits in deep space, deep earth, deep sea and other fields. Let us embark on a journey of scientific discovery and get to know the amazing world.

The rapid development of human society has had an increasingly greater impact on the earth. The impact of environmental pollution and global climate change on the earth is obvious to all. However, recently, scientists have also discovered that even human behavior of extracting groundwater has had a significant impact on the earth - the earth's axis of rotation shifted by about 80 centimeters between 1993 and 2010!

What is the axis of rotation?

To explain how human extraction of groundwater causes the Earth's axis of rotation to shift, we may need to review some geography knowledge we learned in junior high school - the Earth's axis of rotation.

As we all know, the alternation of day and night on Earth is caused by the continuous rotation of the Earth. In order to quantitatively describe and analyze the rotation of the Earth in scientific observation, scientists have imagined that there is such an axis that runs through the North and South Poles of the Earth. Generally speaking, the Earth rotates around this axis. Take the globe as an example. The Earth is lying at an angle, and its North and South Poles are penetrated by an axis. If you fiddle with it a little, the globe will rotate around it.

Globe

(Image source: pickpik)

But the actual situation is much more complicated than the simplified model of a globe. When the real earth rotates, its axis of rotation will naturally swing.

This may be a bit abstract, so we can use a spinning top as an analogy. When you play with a spinning top, it is similar to the earth and will roughly rotate around the axis in its center (this is also an imaginary axis), but during the rotation, its axis will constantly shake left and right.

This is the case with the Earth's axis of rotation, except that the Earth's rotation speed is much slower than a gyroscope, and the Earth's volume is also extremely large, which results in the actual deviation speed of the Earth's axis being relatively slow.

The gyroscope's axis of rotation is very obvious

(Image source: Wikipedia)

It is precisely because of the constant movement of the Earth's rotation axis (also known as polar shift) that it has caused great inconvenience to scientists' geodetic work: the coordinate system itself is moving, and the points in the coordinate system naturally cannot be represented by fixed (X, Y) values.

To solve this problem, they artificially defined the reference axis CIO (Conventional International Origin) of the Earth's rotation axis based on the average position of the poles in 1900. Using this reference rotation axis as a standard, scientists mapped the approximate situation of polar shift.

Function of polar motion in arc seconds and time in days

(Image source: Wikipedia)

How does groundwater extraction cause the axis of rotation to shift?

Since discovering that the Earth's axis of rotation has periodic shifts, scientists have analyzed the causes of polar drift. The most general explanation is that the uneven distribution of mass on the Earth leads to polar drift. We can do this experiment ourselves using a gyroscope - just find a place on the gyroscope and put a small weight, such as a small piece of wood, a small steel ball, or even chewed gum, on the gyroscope with tape. When you pull out the gyroscope, you will find that it is crooked when it spins.

Back to the Earth, on the one hand, the Earth itself is not an object of regular shape, which leads to the fact that the mass distribution of the Earth is uneven in structure.

On the other hand, the earth is still an active planet, and various activities on the planet will lead to uneven mass distribution. Inside the earth, there are three layers of structure consisting of crust, mantle and core. Outside the earth, the atmosphere and hydrosphere are constantly moving, and these movements often lead to temporary changes in the mass distribution of the earth. For example, the upward movement of large magma bodies in the core and mantle, the relative movement of air currents in the atmosphere in winter and summer (the familiar Siberian cold air mass is one of them), and the melting of glaciers and ice caps caused by climate warming, and the flow of large masses of water into the ocean, etc.

The inner and outer layers of the Earth are in motion, and the activity of any layer may lead to uneven distribution of the Earth's mass.

(Image source: Wikipedia)

Then, it is easy to understand that extracting groundwater will cause the Earth's polar shift - human activities extract a large amount of groundwater from the strata, and after using the groundwater, at least 80% of this groundwater becomes surface water again, and returns to the sea through various channels (such as evaporation after agricultural irrigation, sewage discharge after industrial and daily use, etc.). This naturally redistributes the mass of the Earth's surface, and the intensity of this redistribution is very large.

According to research, between 1993 and 2010, the total amount of groundwater extracted by humans was about 2.15 trillion tons, and the discharge of this groundwater into the ocean has caused the global sea level to rise by 6mm. More importantly, most of this extracted groundwater comes from the northern hemisphere - North America, India, the Middle East, etc. This water can eventually be regarded as evenly distributed in the world's oceans, which naturally leads to a more obvious polar shift, with an average speed of 4.36 cm per year. During these years, the pole has moved 78.48 cm to 64.16° east longitude.

Global distribution of groundwater loss (left) and sea level rise (right)

(Image source: Reference 1)

Although humans have known since 2016 that human activities have changed the polar shift and made it larger, the logic at that time was that it had little to do with groundwater. Instead, it was that human activities caused global warming, which caused the melting of the Arctic and Antarctic ice sheets. On the one hand, the ice turned into water, and on the other hand, the ice layer that was originally pressing on the Arctic and Antarctic strata melted, causing part of the earth's crust to rebound (just like a spring being pressed down by a heavy object, the spring will rebound after the heavy object disappears). These have changed the global mass distribution.

However, this study believes that human behavior of extracting groundwater is actually the most important controlling factor after the melting of ice sheets.

Comparison of theoretical calculations without considering groundwater polar shift (dashed blue line), considering groundwater polar shift (solid blue line), and the observed global polar shift (red)

(Image source: Reference 1)

What are the serious consequences of pole shift?

According to current research, the current degree of polar shift will not have much impact on the Earth. However, this is only the impact of polar shift on the Earth. The truly serious consequences come directly from the two factors that cause polar shift. The consequences of melting ice sheets are self-evident. If they continue, sea levels will rise and many densely populated and economically developed cities will be submerged by sea water.

As for over-extraction of groundwater, its harm is even more far-reaching - it may take hundreds of thousands or even millions of years to recover. This is because groundwater does not exist in the form of a whole body of water underground, but in the form of pore water in the underground rock formation. That is to say, when we extract groundwater, we do not extract it from an underground river or underground lake, but from the pores of the rock formation. The water in these pores will be enriched in the well and then be continuously extracted.

The underground rock layers and the pores in the rock layers are the source of groundwater.

(Image source: BRGM-CO2GeoNet, used under CC license)

When not extracted, pore water is actually a supporting structure in the rock. Once the pore water disappears, the rock is equivalent to losing a supporting point, and then it will be compacted under the weight of the upper rock layer, and its pores will naturally become smaller or even disappear. At this time, even if we inject groundwater back, we cannot restore these pores.

On the one hand, the rock layer becomes denser due to the loss of porosity, just like when we press a sponge, the sponge becomes flat. From a macroscopic perspective, this will show as surface subsidence.

Looking at the world, Jakarta, the capital of Indonesia, is the city with the most serious surface subsidence in the world (17 cm per year). It is expected that due to the combined impact of rising sea levels, most of its areas may be completely below sea level by 2050. Indonesia has already begun planning to relocate its capital (of course, surface subsidence is only one of the reasons for the relocation).

Jakarta Settlement Rate

(Image source: Reference 2)

On the other hand, once the groundwater in coastal cities is extracted, the pores become empty, and seawater will penetrate into these pores to fill the original fresh water position. As a result, these places will experience seawater intrusion, which will seriously affect the local water supply for many years to come.

In addition, since some groundwater is located deep in the earth, its source comes from the slow infiltration of surface water. Once it is extracted, it takes hundreds of thousands or even millions of years to replenish it. If it is contaminated by seawater or due to inappropriate extraction methods, it is equivalent to almost permanently losing this part of groundwater.

Humans currently have a significant impact on all aspects of the Earth's operation. While we should be proud of this, we may also need to express some concern about the future of humanity and the Earth.

References:

[1] Seo KW, Ryu D, Eom J, et al. Drift of Earth's pole confirms groundwater depletion as a significant contributor to global sea level rise 1993–2010[J]. Geophysical Research Letters, 2023, 50(12): e2023GL103509.

[2] Bott LM, Schöne T, Illigner J, et al. Land subsidence in Jakarta and Semarang Bay–The relationship between physical processes, risk perception, and household adaptation[J]. Ocean & Coastal Management, 2021, 211: 105775

.