Water storage in dams causing minute shifts in Earth’s poles: Study

For centuries, water storage structures including dams have been built to ensure water supply in dry months. These dams have come to acquire special significance in the wake of climate change.

But, a recent research study has revealed that over the past two centuries, humans have locked up enough water in dams to shift Earth’s poles slightly away from the planet’s axis of rotation. The study compiled a global database of artificial water impoundment (dams) from 1835 to 2011 and found that construction of nearly 7,000 dams during this period shifted the poles about a meter (3 feet) in total and caused a 21-millimeter (0.83-inch) drop in global sea levels. Together, these dams hold enough water to fill the Grand Canyon twice.

Any movement of mass within the Earth or on its surface changes the orientation of the rotation axis relative to the crust, a process termed true polar wander (TPW). In this study, the authors computed TPW driven by the impoundment of water in globally distributed dams from 1835 to 2011. The calculation is based on a recent, comprehensive database of water impoundment, and the resulting polar wander path and magnitude is significantly different from an earlier estimate of the signal.

The results demonstrate another way human activities have affected the planet, according to the study authors. The polar shift is small, but it could help scientists understand how the poles will move if major glaciers and ice sheets melt due to climate change.

“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,” said Natasha Valencic, a graduate student in Earth and planetary sciences at Harvard University and lead author of the new study. “We’re not going to drop into a new ice age, because the pole moved by about a meter in total, but it does have implications for sea level.”

In the study, Valencic and her colleagues used a global database of dams to map the locations of each dam and the amount of water each impounds. They analyzed how the water impoundment from 6,862 dams shifted Earth’s poles from 1835 to 2011.

Their results showed global dam building caused Earth’s poles to shift in two distinct phases. From 1835 to 1954, many dams were built in North America and Europe, shifting these areas toward the equator. The North Pole moved 20.5 centimeters (8 inches) toward the 103rd meridian east, which passes through Russia, Mongolia, China, and the Indochina Peninsula.

Then, from 1954 to 2011, dams were built in East Africa and Asia, and the pole shifted 57 centimeters (22 inches) toward the 117th meridian west, which passes through western North America and the South Pacific.

Over the entire period from 1835 to 2011, the poles moved about 113 centimeters (3.7 feet), with about 104 centimeters (3.4 feet) of movement happening in the 20th century.

The results show that researchers need to take water impoundment into consideration when calculating future sea level rise. In the 20th century, global sea levels rose by 1.2 millimeters per year on average, but humans trapped a quarter of that amount behind dams—a significant fraction, according to Valencic. And sea level rise does not happen uniformly around the globe.

“Depending on where you place dams and reservoirs, the geometry of sea level rise will change,” she said. “That’s another thing we need to consider, because these changes can be pretty large, pretty significant.”