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, according to new research.
Earth’s outermost solid layer sits atop goopy molten rock, so it can move relative to the magma below it. Anytime mass is redistributed around the planet’s surface, like when ice sheets grow or shrink, this outermost rock layer wobbles and moves around. Imagine slapping a lump of clay onto one side of a spinning basketball: to maintain momentum, the part of the ball with the clay on it will shift slightly toward its equator and away from its axis of rotation. When this happens on Earth and the outermost rock layer wobbles around, different areas of the surface end up sitting directly over the axis of rotation. The geographic poles then pass through different spots on the surface than before, a process called true polar wander.
A new study in Geophysical Research Letters finds the construction of nearly 7,000 dams from 1835 to 2011 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.
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 new 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.”
