A couple of decades ago, scientists at NASA’s Jet Propulsion Lab they estimated that the filling of the Three Gorges Dam, China’s massive hydroelectric project, involved the movement of a mass of water so vast that it was capable of lengthening 0.06 microseconds Earth day. The Chinese dam is a unique case due to its magnitude, but it is just one of the tens of thousands of reservoirs that populate our planet.
The overall impact of these reservoirs has now come under study.
Shifting the poles. A new study has found evidence of the geological impacts that the construction of dams can have on a global scale. They observed that the construction of dams had caused a slight shift of the geographic poles of the planet,
Geographic poles? As children we were probably taught that the geographic poles and magnetic poles of our planet were located at different points in their respective polar circles. Surely they would teach us that the magnetic poles tend to move slowly over time while the geographic poles remain static, forming the Earth’s axis of rotation.
This is more or less true, but not entirely precise: geographic poles can also move. The reason is in the very structure of the Earth. Our planet is not a uniform block of rock but is made up of several concentric layers.
While the outer layer is essentially solid, the inner ones are not. As if it were a mat on a swimming pool, the Earth’s crust “floats” on the mantle, which implies that it can move more or less independently, displaced by the physical forces applied in each case.
Redistributing the mass. Forces that in turn depend in part on the distribution of the masses that we find in this superficial part of the planet, a distribution that is altered every time we introduce changes in the geography of our planet. These changes include the displacement of large masses of water into the swamps, water that under “normal” conditions would have returned to the ocean carried by the rivers on which we build dams.
“As we trap water behind dams, we not only remove water from the oceans, lowering sea levels, we also distribute mass differently around the world,” explained in a press release Natasha Valencic, co-author of the study.
6,862 reservoirs. The team responsible for the study analyzed the impact of the accumulation of water in 6,862 reservoirs, an accumulation of water that represents twice the volume of the Grand Canyon of the Colorado. The team observed that between 1835 and 2011 this accumulation of water could be responsible for almost a meter of displacement in the Earth’s axis of rotation with respect to the planet’s surface.
The displacement would be divided into two stages. The first between 1835 and 1954, in which the pole would have moved 20.5 cm towards the 103rd eastern meridian as a result of the construction of dams in Europe and North America. The second, between 1954 and 2011, would have been dominated by construction in Asia and East Africa, moving the pole 57 centimeters towards the 117th meridian west.
Details of the study were published in an article in the magazine Geophysical Research Letters.
More than reservoirs. The implications of these geographical changes are not extreme: after all, the tectonic plates do not stop moving at our feet despite the fact that their rhythms are usually slower. However, knowing these small movements can help us understand some of the geological dynamics and the impact of humans on them.
“We are not going to fall into a new ice age because the pole has changed by around one meter in total, but it does have implications for sea level,” Valencic clarified.
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Image | Ali Madad Sakhirani
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