The Earth spins steadily, but not without a wobble. While we imagine the North and South Poles as fixed points, they’re not exactly anchored. Over time, they shift slightly, influenced by how mass is distributed across the globe.
Ice sheets melt, tectonic plates shift, and oceans swell. But now, researchers have revealed that humans, through dam building, have also nudged the planet’s spin ever so slightly.
In a quiet but measurable way, our thirst for water control and hydroelectric energy has shifted the Earth’s poles. From the earliest major dams in the 19th century to the massive reservoirs constructed in Asia and Africa during the 20th century, the effect has grown stronger.
According to a study published in the journal Geophysical Research Letters, this phenomenon has moved Earth’s rotational axis by more than a meter (3.3 feet) since 1835.
Dam building shifts Earth’s poles
True polar wander refers to the shifting of Earth’s solid outer shell relative to its spin axis. This motion occurs when mass on the planet’s surface moves around.
Think of Earth like a spinning basketball. Place a lump of clay on one side, and the ball shifts slightly to balance the extra weight. When this happens on Earth, the solid crust adjusts its position, moving relative to the axis of rotation. The poles then pass through new locations on the surface.
Traditionally, this phenomenon has been linked to natural processes like glacial melting. But the recent study by Natasha Valencic and her team at Harvard University shows that human activity, specifically water impoundment from dam construction, has become another significant contributor.
The findings suggest that this artificial redistribution of mass has created a measurable shift in the Earth’s pole positions.
“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 Valencic. That mass, once fluid and ocean-bound, is now stored in still reservoirs scattered across continents.
Earth’s poles moved in two phases
Using data from the Global Reservoir and Dam (GRanD) database, the researchers mapped the water storage of 6,862 artificial reservoirs from 1835 to 2011. The dam analysis revealed a striking, non-linear shift in the Earth’s poles.
From 1835 to 1954, dam building focused heavily in North America and Europe. This caused the North Pole to move about eight inches toward 103.4°E, a direction passing through Russia and parts of Asia.
After 1954, the pattern of dam construction changed. Major projects emerged across East Africa and Asia. As a result, the pole reversed direction, moving 22.5 inches toward −117.5°E, roughly through the western United States and into the South Pacific.
These changes did not follow a smooth path. The polar motion was highly variable, influenced by the size and location of dams being built over time.
Across the entire study period, the poles shifted about 44.6 inches. Notably, 40.9 inches of that motion occurred during the 20th century. This finding suggests that modern infrastructure, rather than long-term natural processes alone, has become a key factor in Earth’s orientation.
Dams and Earth’s pole positions
To track these subtle shifts, the team used a detailed physical model. They combined gravitationally self-consistent sea level theory with equations for rotational stability.
The researchers accounted for how Earth’s crust responds elastically to mass changes, and how these changes, in turn, influence Earth’s spin.
The Earth model used, known as PREM (Preliminary Reference Earth Model), includes variations in Earth’s density and elasticity. The team also incorporated seepage, the loss of stored water into surrounding soils, using a parameterization method from earlier work by Chao et al. (2008).
Despite excluding about 28 percent of the global impoundment volume, mainly from smaller dams, the results remained consistent.
The missing volume came mostly from minor reservoirs, each contributing less than 0.024 cubic miles, and these had minimal influence on the final polar calculations.
To test the significance of these smaller dams, the team plotted how pole position changed with each added reservoir. After about 6,000 of the largest were included, the results stabilized. This confirmed that the many small, excluded dams had negligible overall effect.
Dam water storage and sea levels
Global sea levels are rising today, but during the 20th century, the creation of reservoirs temporarily offset this trend. The researchers found that the 6,862 dams in their study led to a sea level drop of 0.86 inches between 1900 and 2011.
This artificial dip matters. While it’s small compared to the rise caused by melting glaciers and warming oceans, it complicates calculations.
“Depending on where you place dams and reservoirs, the geometry of sea level rise will change. That’s another thing we need to consider, because these changes can be pretty large, pretty significant,” said Valencic.
In the first half of the 20th century, the mean polar motion rate from impoundment was about 0.12 inches per year.
In the second half, it rose to 0.37 inches per year, a threefold increase. The pole’s changing pace mirrors the rapid expansion of dam building after 1950.
Revisiting old assumptions
Previous studies underestimated the effect of dam-induced polar wander. One earlier estimate in 2008 suggested a mean movement of −0.06 inches per year in the east direction and −0.31 inches per year northward across the 20th century.
But Valencic’s team found it was actually −0.12 inches per year and −0.09 inches per year, respectively. This difference matters because it rebalances the budget of polar motion.
Scientists trying to understand how much glaciers, groundwater, or ocean changes contribute to Earth’s spin now have to subtract out the dam effect. Without this correction, estimates about ice loss and sea level rise remain incomplete.
Interestingly, the influence of dam building didn’t act alone in shifting Earth’s poles. The researchers also modeled how the oceans responded to displaced water.
While water impoundment dominated the first half of the century, the second half saw both dam storage and ocean mass redistribution playing nearly equal roles in shaping Earth’s polar path.
Subtle shift in Earth’s poles
This study began as a classroom project at Harvard but quickly grew into a deeper investigation of Earth’s shifting balance. It reveals that something as mundane as a dam, one of civilization’s most common tools, has reshaped the geography of our spinning planet.
The pole has not leapt across continents. It has crept, step by engineered step, with every new concrete wall holding back a river. While not disastrous on its own, this shift reveals a truth: our actions reach farther than we often imagine.
Every dam, every floodgate, becomes part of the Earth’s momentum. What we store on land doesn’t just change rivers. It nudges Earth’s rotation, redistributes the weight of water, and contributes silently to a planet that, while spinning, is never truly still.
The study is published in the journal Geophysical Research Letters.
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