Human-Constructed Dams Have Shifted the Earth's Poles, Scientists Say
Over the last 200 years, humans have constructed nearly 7,000 massive dams, impounding enough water to nudge the Earth's poles by about three feet (one meter) and cause a 0.83-inch (21-millimeter) drop in global sea levels, according to a new study in Geophysical Research Letters.
This drift is possible because Earth's solid crust forms a hard shell around a molten layer of gooey magma. This means that whenever a significant amount of mass is redistributed across the planet's surface, the outermost rock layer wobbles, shifting relative to Earth's molten interior. When this happens, different areas on the Earth's surface end up directly over the planet's rotational axis. As a result, the planet's poles pass through different surface locations than before, a phenomenon known as true polar wander.
'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 redistributes mass around the world,' Natasha Valencic, a graduate student at Harvard University and lead author of the new study, said in a statement.
In the study, Valencic and her team analyzed a previously published global database of dams to figure out their locations, the volume of water they store, and how that stored water has impacted Earth's mass distribution. Previously, the database revealed that 6,862 large dams built between 1835 and 2011 contributed to a decrease in sea levels. Collectively, these dams hold enough water to fill the Grand Canyon twice.
The results showed that global dam-building has caused Earth's poles to shift in two phases. The first phase, from 1835 to 1954, coincided with a boom in dam construction in North America and Europe. These areas shifted toward the equator, and as a result, the North Pole moved about 8 inches (25 centimeters) toward the 103rd meridian east, a line that passes through Russia, Mongolia, and China.
During the second wave of dam construction, between 1954 and 2011, most dams were built in Asia and East Africa. As a result, the North Pole shifted 22 inches (57 centimeters) toward the 117th meridian west, which passes through western South America and the South Pacific. Polar wander is not linear; instead, it follows a wobbly path, which is why the total shift does not add up precisely to 3.7 feet.
While the results are relatively subtle, they highlight the need for researchers to account for water stored in dams when predicting future sea level rise. In the 20th century, global sea levels rose 4.7 to 6.7 inches (12 to 17 centimeters), but humans trapped about a quarter of that volume behind dams, according to Valencic.
'Depending on where you place dams and reservoirs, the geometry of sea level rise will change,' she said. 'That's another factor we need to consider, because these changes can be quite large and significant.'
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