Baihetan Dam is a huge hydroelectric operation in China. (Image credit: VCG/VCG via Getty Images)

Dams Have Shifted Earth’s Poles

New research has revealed that large dams around the world have shifted the position of Earth’s geographic poles. Published in Geophysical Research Letters on May 23, the study found that the construction of nearly 7,000 dams between 1835 and 2011 has redistributed so much water on the planet’s surface that it’s altered Earth’s mass balance and caused the poles to drift.

Earth’s crust floats atop a semi-fluid mantle, and when mass shifts on the surface—such as through water being stored behind dams—the crust responds. This movement, known as “true polar wander,” happens when Earth’s outer shell adjusts due to changes in how mass is distributed. Scientists have long known that natural processes like melting glaciers and groundwater depletion affect this drift, but this study is the first to quantify the role of human-made reservoirs.

The researchers found that the water stored in dams has effectively lowered global sea levels by about 0.9 inches (23 millimeters), because this freshwater is no longer part of the oceans. This shift in water mass also caused the North and South Poles to migrate by approximately 3.7 feet (1.1 meters) over the study period.

Two main periods of dam-building were particularly influential. Between 1835 and 1954, most major dams were constructed in North America and Europe. This caused the North Pole to shift roughly 8 inches (20 centimeters) eastward, toward the 103° east meridian—passing through parts of Russia, Mongolia, and China. From 1954 to 2011, dam-building ramped up in Asia and East Africa, shifting the pole about 22 inches (57 centimeters) westward, toward the 117° west meridian—across western North America and the South Pacific.

The researchers note that the polar shift hasn’t followed a perfectly straight line; instead, it traces a wobbly, meandering path due to the complex nature of mass redistribution. While these shifts don’t significantly affect most systems on Earth, they do have implications for sea-level studies. In the 20th century, global sea levels rose about 12 to 17 centimeters, but the water impounded in dams accounts for about a quarter of that volume being kept out of the oceans. This means that future projections of sea-level rise must account for such artificial reservoirs, which can slightly reduce or delay the impact depending on the region.

Should future dam projects consider not just ecological and social costs, but also global geophysical effects?