2023-06-16 16:43:10
These are proportions to keep in mind, and which we nevertheless neglect: fresh water represents only 2.5% of the water on Earth, the vast majority corresponding to salt water.
This precious fresh water is distributed between glaciers (68.7%), underground water tables (30.1%), permafrost (0.8%) and that which is permanently exchanged between the surface – course of water, lakes – and the atmosphere (0.4%).
According to a report carried out by the Food and Agriculture Organization of the United Nations (FAO), agriculture is the sector that takes the most fresh water with irrigation (70%), far ahead industry (19%) and domestic use (11%). Especially in groundwater.
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Only part of the fresh water withdrawn by man is returned, at the outlet of factories or power stations, for example. But not necessarily in the same physical (temperature) and chemical (pollution) state… or in the same place. However, these water displacements result in a modification of the Earth’s rotation, according to a study published in the journal Geophysical Research Letters (Ki-Weon Seo et al. 2023).
Earth’s rotation: what are we talking about?
A 2022 study had suggested that the rotation of the Earth was influenced by extreme climatic and geodynamic events. However, it was a question of a phenomenon called “precession of the equinoxes”, a change in the axis of rotation of the Earth under the effect of the forces of attraction exerted by the Moon and the Sun on our planet, and whose period (cycle) is approximately 26,000 years.
In the new study, it is by no means precession that we are talking about, but a completely different type of variation in the rotation of our planet. This is the “movement of the pole”, a movement whose lines form a kind of spiral.
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The motion of the pole results from three components: the Chandler oscillation [due au fait que la Terre n’est pas une sphère parfaite] over a 14-month cycle; an oscillation caused by the seasonal movement of air and water masses on Earth over a 12-month period; and finally, the irregular drift of the pole caused in the first place by the slow deformations of the earth’s crust.
Impact of Man on the movement of the pole: yes, but to put it into perspective
In 2016, a previous study in Science Advances argued that water displacements could influence the motion of the pole over a 10-year period (S. Adhikari & ER Ivins, 2016).
To go further, a team led by the Seoul National University (South Korea) modeled the observed changes in the movement of the pole according to the movements of water on Earth. First by considering only ice sheets and glaciers, then by adding different scenarios of groundwater redistribution by humans.
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However, their model made it possible to obtain a result corresponding to the polar drift actually observed between 1993 and 2010, on the sole condition of including a redistribution of groundwater to the tune of 2,150 gigatonnes. A figure that was not chosen at random, since it corresponds to the mass of water pumped by man from the groundwater between these dates according to the calculations of another study (Y.wada et al. 2010).
Without this groundwater parameter, the model deviates from reality by about 78.5 centimeters, or a drift of 4.3 cm per year, the authors estimate. An impact that must however be put into perspective, since the two main oscillations of the movement of the pole, the Chandler oscillation and the oscillation governed by the (natural) seasonal flows of air and water, correspond to a order of magnitude of several meters, respectively over 14 months and over a year.
Displacement of fresh water by humans… and sea level rise
“I am very happy to have found the unexplained cause of the pole drift”said Ki-Weon Seo, geophysicist at Seoul National University and first author of the study, quoted by the American Geophysical Union in a press release relayed by Phys (15/06/2023). “On the other hand, as an Earth dweller and a father, I am concerned and surprised to see that groundwater pumping is another source of sea level rise”he nuanced.
Indeed, the previous study by Wada et al (2010) matched the mass of 2,150 gigatonnes of groundwater to a rise in water levels of about 6 millimeters between 1993 and 2010. This contribution, however, must also be put into perspective.
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According to the UN Panel of Experts on Climate (IPCC), the sea level has already risen by about twenty centimeters between 1900 and 2018, and is expected to rise by another forty centimeters. by 2100 if the rise in global temperature is limited to less than 2°C, or even several meters in the worst scenarios. It is man-made global warming – both melting continental ice and swelling the volume of water – that should alert us more.
On the same topic :
⋙ How to save water?
⋙ Why is the rotation of the Earth accelerating?
⋙ Could the rotation of the Earth’s core be adjusted to changes in the length of the day? A study suggests so, cautiously
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