Human activity has profound impacts on the Earth’s upper mantle

Unsustainable irrigation and drought have caused changes that have empty almost all of the waters of the Aral Sea since the 1960s, extending all the way to the Earth’s upper mantle, the layer below the Earth’s crust. This is perhaps the deepest recorded example of human activity that will change the solid inner earth.

“To do something that will affect us [upper mantle] It’s like whoa.” Sylvain Barbott At the University of Southern California. “It shows how powerful it is to change the environment.”

The Aral Sea in Central Asia was once one of the largest waters in the world, covering almost 70,000 square kilometers. However, Soviet irrigation programs that began in the 1960s and later droughts empty the oceans. By 2018, it had shrunk by almost 90% and lost about 1,000 cubic kilometres of water.

Wang Ten At Peking University in China, I was interested in the Aral Sea after reading a book about the consequences of this environmental disaster on the surface of the earth. “We’ve noticed that these huge mass changes stimulate the deep Earth’s response,” he says.

He and his colleagues, including Barbot, used satellite measurements to track subtle changes in the elevation of the oceans that were empty between 2016 and 2020. Much of the ocean water disappeared decades ago, but it was found that the uplifts were underway, with on average rising surfaces about 7 millimeters a year.

Next, we used a model of the crust and mantle beneath the Aral Sea to test the mantle beneath the Aral Sea when it came to leading to the uplift of this observed pattern. “We found that the observations were perfectly compatible with a deep response to this change,” says Barbot.

When the weight of the water was removed, the shallow crust first responded, according to the model. This prompted a response at a depth of 190 km from the surface as the viscous rocks in the upper mantle creeped up to fill the blanks. “The uncurved things create space and the rocks want to flow into it,” Barbot says. This delayed reaction in hot, weak areas of the mantle, called the athenosphere, is why the uplift is ongoing, even decades after the water is removed, he says.

The upper mantle rebound is known to occur after other major changes in surface mass, such as glacier advancement and retreat, says Roland Bürgmann At the University of California, Berkeley. But the response to drainage in the Aral Sea may be the deepest example of human-caused changes on solid earth.

Other human-induced changes, such as filling large reservoirs and pumping groundwater, are said to have also caused rebounds. Manoochehr Shirzaei At Virginia Tech. But the wider range of the Aral Sea means the impact of emptying it is likely to run deeper, he says.

In addition to explaining the enormous scale of human activity, the uplift below the Aral Sea offers an extraordinary opportunity to estimate small differences in viscosity of the mantle, particularly under the interior of the continent, Bürgmann says. “It’s really important for people trying to understand plate tectonics to know how that layer behaves under the continent.”