Earth’s axis changed by extraction of underground water – 06/30/2023 – Environment

A look through the window of an airplane in flight leaves no doubt of humanity’s ability to radically transform the surface of the planet it lives on. Less obvious is the fact that our agricultural activities are shifting the Earth’s axis of rotation. But that’s exactly what is happening.

The finding was made by the group led by Ki-Weon Seo, a geophysicist at the National University of Seoul, in South Korea, and was published in the latest edition of the journal Geophysical Research Letters.

The notion that the Earth’s geographic poles, that is, the points through which the planet’s axis of rotation passes, are slightly mobile, has been consolidated for some time. This is because the Earth’s gravitational field is not perfectly equal at all points on the surface.

It is not difficult to understand this. Where there is more mass, gravity is slightly greater; where there is less, it is less. And our planet is dynamic, with masses of water and ice each year shifting with the seasons, creating a wobble that in turn produces a seasonal wobble of the axis.

All this is very small compared to the scale of the planet, so that nobody needs to worry about an instability on the axis that would be capable of radically disturbing the climate on the globe (recalling that it is precisely the degree of inclination of it in relation to the orbit of the Earth). Earth around the Sun that produces the seasons).

Still, researchers have noticed an additional shift from the seasonal movement that defies explanation. Even taking into account the establishment of dams on rivers (which naturally increase the amount of water, therefore mass, on land) and the melting of glaciers and continental polar ice caps (which increase the amount of water in the oceans), the displacement predicted from the geographic pole did not quite line up with the observations.

The key insight of Seo and his colleagues was to remember the potential effects of extracting groundwater. There are recent data, obtained mainly with NASA’s Grace satellite, of small variations in the Earth’s gravitational field, mainly due to the displacement of water. It operated between 2002 and 2017 and had a replacement launched in 2018, the Grace Follow-On.

These measurements clearly showed the increase in the mass of the oceans, accumulating more water. It has always been difficult, however, to specify the contribution of groundwater extraction by human activity. A global hydrological model released in 2010 suggested that, from 1993 to that year, around 2,150 gigatons of water pumped from the ground would have ended up in the oceans, leading to a contribution of 6.24 mm in mean sea level in the period.

These emptyings of underground water reservoirs occurred in particular in northwest India and western North America.

The problem, of course, is well known: with the aim of mainly promoting irrigation of plantations, water pumping systems are depleting these resources faster than nature can replace them. (The term “renewable resource” can hide all sorts of sins; water is indeed a renewable resource, but a problem arises if you consume it locally faster than the Earth system can return it to where it was taken from. .)

The challenge was confirming that this was actually happening at the scale suggested by the model. Seo’s group saw this as an opportunity to kill two birds with one swipe: explain the still-ununderstood motion of the geographic pole and find a beacon to corroborate the loss of groundwater to the oceans.

They took data from the 2010 model and plugged it into another, which calculated the impact of various water movements on the axis of rotation. And then, surprise, the alignment between prediction and observation got much better, almost exact.

“In combination with other well-understood sources of water redistribution, such as melting polar ice sheets and mountain glaciers, the good agreement with observations of polar motion serves as independent confirmation of the model’s estimate of soil water extraction.” , the researchers wrote.

More: the work demonstrates that the extraction of water from the subsoil, alone, produces a displacement of 4.36 centimeters per year from the north geographic pole, towards the Novaya Zenlya islands, Russia.

With this, it was demonstrated that the extraction of water from the subsoil is the second largest effect on the displacement of the axis, second only to the post-glacial adjustment (a natural effect that involves the accommodation of the Earth’s crust after all the mass of ice that has gathered at the poles in the last glacial period, which ended about 11,000 years ago).

Interestingly, all the other more appreciable effects have something to do with human activities: dams, loss of glaciers and ice in Antarctica and Greenland. Smaller impacts, such as soil moisture, atmospheric pressure, ocean floor pressure, winds and sea currents, do not even make the list, so modest is their influence.

“It is a solid article in terms of data”, comments Álvaro Crósta, a geologist from Unicamp (State University of Campinas) not involved with the study. “It’s interesting to see this, that you have an overexploitation of aquifers and this is contributing to sea level rise — and, by extension, pole shift. But the effect is small, it’s something even difficult to measure.”

In any case, the Anthropocene (the geological epoch that is still much debated among specialists, marked by the moment when humanity starts to have global impacts comparable to – and sometimes superior to – the forces of nature), seems to have finally arrived. And it affects, amazingly, even how our planet revolves around its own axis.

_{The Planeta em Transe project is supported by the Open Society Foundations.}