A ship graveyard in the Aral Sea desert, Uzbekistan
S@OwwL / Alamy
Unsustainable irrigation and drought have emptied nearly all of the Aral Sea’s water since the 1960s, causing changes extending all the way down to Earth’s upper mantle, the layer beneath the planet’s crust. This is probably the deepest recorded example of human activity changing the solid inner Earth.
“To do something that would affect the [upper mantle] is like, whoa,” says Sylvain Barbot at the University of Southern California. “It’s showing you how potent we are at changing the environment.”
The Aral Sea in central Asia was once one of the world’s largest bodies of water, covering almost 70,000 square kilometres. But Soviet irrigation programmes starting in the 1960s, as well as later droughts, emptied the sea. By 2018, it had shrunk by almost 90 per cent and lost around 1000 cubic kilometres of water.
Wang Teng at Peking University in China became curious about the Aral Sea after reading a book about the consequences of this environmental disaster on Earth’s surface. “I realised that such a huge mass change would stimulate the response of the deep Earth,” he says.
He and his colleagues, including Barbot, used satellite measurements to track subtle changes in the emptied sea’s elevation between 2016 and 2020. Although much of the sea’s water disappeared decades ago, they found the uplift is ongoing, with the surface rising by around 7 millimetres per year on average.
They then used a model of the crust and mantle beneath the Aral Sea to test what changes deep below would lead to this observed pattern of uplift. “We find that the observations are completely compatible with a deep response to this change,” says Barbot.
As the weight of water was removed, the shallower crust responded first, according to their model, by unbending. This prompted a response at depths as far as 190 kilometres below the surface, as viscous rocks in the upper mantle crept in to fill the void. “The unbending creates space, and the rocks want to flow into it,” says Barbot. This delayed response in a hot, weak region of the mantle called the asthenosphere is why the uplift is ongoing, even decades after the water was removed, he says.
Rebound in the upper mantle is known to occur after other large changes in mass at the surface, such as the advance and retreat of glaciers, says Roland Bürgmann at the University of California, Berkeley. But the response to the draining of the Aral Sea may well be the deepest example of a human-caused change in the solid Earth, he says.
Other changes caused by humans, such as filling large reservoirs or pumping groundwater, have also caused rebound, says Manoochehr Shirzaei at Virginia Tech. But the wide range of the Aral Sea means the effects of emptying it are likely to run deeper, he says.
In addition to illustrating the sheer scale of human activity, the uplift beneath the Aral Sea offers an unusual opportunity to estimate small differences in the viscosity of the mantle, particularly where it lies beneath the interior of a continent, says Bürgmann. “Knowing how that layer right under continents behaves is really important for people who try to understand plate tectonics.”
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