Over the past two centuries, the construction of thousands of dams has done more than just tame rivers – it has shifted the Earth’s North Pole about a meter from its original position. By storing billions of liters of water, these reservoirs have fundamentally altered the globe's mass distribution, causing both a drop in global sea level and a drift in the planet’s axis.
But how does this kind of mass redistribution of water cause the shift in the axis of rotation? Earth’s lithosphere, the outermost rocky layer, rests upon a semi-fluid layer of the upper mantle. This setup allows the surface to shift when mass is redistributed, whether from melting or growing ice sheets, or in this case, human‐made reservoirs.
Imagine a spinning ball that has a gooey blob of clay (on the inside, not on the surface). If this blob changes from being perfectly centered on one side, the spinning ball will subtly adjust itself. The entire ball will wobble and reorient itself so that the heavier side moves closer to its equator of the spin. This will cause the points on the outer surface, which were once at the poles, to drift away from the axis of rotation. The movement of the poles to new locations is called the true polar wander.
Scientists already knew that the reallocation of enormous volumes of water could impact the true polar wander path, but it had never been accurately measured. To examine the effect, researchers at Harvard University inspected and mapped the location of about 7,000 of Earth’s biggest dams, constructed from 1835 to 2011, to calculate how impounded water redistributed Earth’s mass.
“As we trap water behind dams, not only does it remove water from the oceans, thus leading to a global sea level fall, it also distributes mass in a different way around the world,” explained the lead author of the study, Natasha Valencic.
The team found that the dam building has lowered the sea levels by 21 millimeters (0.83 inches), which is enough to fill the Grand Canyon twice. Researchers then mapped the polar shift into two phases.
From 1835 to 1954, rampant dam construction in North America and Europe tilted the North Pole by 20.5 cm (8 in) toward the 103rd meridian east, which crosses Russia, Mongolia, China, and Indochina.
After 1954, dams popped up in East Africa and Asia. The pole shifted by 57 cm (22 in) toward the 117th meridian west, towards the west of North America and the South Pacific.
In total, the poles traveled about 113 cm (3.7 ft), with over 100 cm of movement happening in the 20th century alone.
“Depending on where you place dams and reservoirs, the geometry of sea level rise will change,” says Valencic. “That’s another thing we need to consider, because these changes can be pretty large, pretty significant.”
The study has been published in Geophysical Research Letters.
Source: AGU
