It began with a simple idea: hold back rivers to store water. But over nearly two centuries, this effort — multiplied across nearly 7,000 dams worldwide — has done something no one expected. It has moved the planet.
In a new study published in Geophysical Research Letters, a team of geophysicists from Harvard University shows that the construction of large dams has shifted Earth’s magnetic poles by more than one meter. This movement, known as true polar wander, doesn’t mean the North Pole is racing off toward Siberia. Rather, Earth’s crust — the outer shell we live on — has slowly crept across the inner, rotating layers, nudging our geographic orientation.
“Any movement of mass within the Earth or on its surface changes the orientation of the rotation axis relative to the crust,” the researchers wrote. One of the most powerful such movements in recent history, it turns out, came from humans rather than natural forces.
The Planet We Tilted
The story begins in 1835, when the first of what would become thousands of dams was built. These human-made reservoirs store enormous volumes of water — enough, the study notes, to fill the Grand Canyon twice. While this has helped societies irrigate crops, generate power, and control floods, it has also created a new kind of global force.
“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 in a different way around the world,” said Natasha Valencic, a doctoral student in geophysics at Harvard and the lead author of the study.
To understand this effect, Valencic and colleagues analyzed 6,862 artificial reservoirs built between 1835 and 2011. They relied on the most comprehensive global dam database to date, the GRanD database, which accounts for roughly 72% of the world’s known water impoundment. Using advanced modeling, they calculated how the shifting mass of water affected Earth’s rotation.
From 1835 to 1954, most dams were built in Europe and North America. This pulled the crust slightly away from these regions, shifting the North Pole about 20.5 centimeters toward 103.4°E, which roughly passes through Russia and Mongolia.
After 1954, a second wave of dam construction swept through Asia and East Africa. This reversed the trend, pushing the pole another 57.1 centimeters in the opposite direction, toward −117.5°E, which slices through North America and the South Pacific. In total, Earth’s poles have shifted about 1.13 meters (3.7 feet) due to dams along a wobbly, non-linear path.
Shifting Poles, Shrinking Seas
This motion is subtle but meaningful.
“A meter is pretty big in regards to polar motion,” Jim Davis, a geodesist at Columbia University’s Lamont-Doherty Earth Observatory, who was not involved in the research, told Scientific American. “We have to know the polar motion accurately or else the errors will propagate into other systems on which we depend on a daily basis,” he said, citing GPS, astronomy, and climate modeling as key examples.
The pole shift also highlights how human engineering affects planetary systems in complex and often invisible ways.
The researchers found that water stored in these dams has caused a global sea level drop of around 21.8 millimeters, or nearly an inch. That’s about a quarter of the sea level rise we might otherwise have seen in the 20th century. The effect is especially significant in today’s warming world, where melting ice sheets and thermal expansion are pushing oceans ever higher.
“We’re not going to drop into a new ice age, because the pole moved by about a meter in total,” Valencic said. “But it does have implications for sea level.”
Moreover, the effects are not uniform across the globe. The redistribution of mass changes the geometry of sea level rise, meaning some regions might experience higher or lower seas depending on how and where water is impounded.
“Depending on where you place dams and reservoirs, the geometry of sea level rise will change,” Valencic explained. “That’s another thing we need to consider, because these changes can be pretty large, pretty significant.”
From Mesopotamia to Mitrovica
The idea of using dams to tame water dates back to ancient Mesopotamia. But only in the past century has the scale become truly planetary. The pace accelerated after World War II, driven by rapid industrialization and development.
The Harvard team’s work builds on foundational research by NASA scientist Benjamin Fong Chao, who first modeled how large reservoirs could affect Earth’s spin. In 2008, Chao showed that when China’s massive Three Gorges Dam is full, it slows Earth’s rotation by 0.06 microseconds — or 60 billionths of a second.
Still, until now, no one had mapped out the complete polar wander path caused by dam building over time. Valencic’s team did just that, using models of Earth’s rotation and gravitational response developed by geophysicist Jerry X. Mitrovica and colleagues. They showed that 88% of the total polar shift occurred after 1954, in tandem with a global boom in dam construction.
Interestingly, the team also found that missing data — the roughly 28% of small reservoirs not included in the GRanD database — likely made little difference to their final results. The largest 6,000 dams appear to account for nearly all the polar movement, due to their outsized share of the impounded water.
A New Geological Epoch?
The idea that human activity is powerful enough to shift Earth’s poles sounds like science fiction. But it’s yet another data point in the case for the Anthropocene, the proposed geological epoch marked by human dominance of natural systems.
True polar wander is typically driven by forces like shifting tectonic plates or the melting of giant ice sheets. But increasingly, humans are among the drivers — through groundwater extraction, glacier loss, and now, dams.
A 2023 study estimated that groundwater depletion alone caused a polar shift of 4.36 centimeters per year between 1993 and 2010. Another found that climate-driven ice melt may shift the poles by 27 meters by the end of the century.
In this context, the dam-induced shift of just over a meter might seem modest. But it’s also a stark reminder of how even well-intentioned engineering projects could potentially cause an imbalance.
