Humans may be tiny compared to the immensity of Earth, but our activity can alter its dynamics on a planetary scale—for good and ill. Scientists have even discovered that Earth’s rotation has shifted due to our influence.
According to a recent study published in Geophysical Research Letters, humans have pulled Earth’s poles slightly off-kilter by building huge dams. The results shed light on how future forces like ice melt, sea level rise, and continued dam construction might shape our planet’s rotation.
In an ideal model, Earth’s geographic poles in the Arctic and Antarctic align exactly with its axis of rotation. In reality, the poles drift from the axis because they are sensitive to the constant redistribution of mass on the planet’s surface and in its interior. This drift, called true polar wander, is critical to our daily lives, as it influences navigation and timekeeping technologies like GPS, as well as astronomical observations.
“Imagine a spinning balloon and, let's say, a fly lands on it,” said Natasha Valencic, a graduate student in the department of Earth and planetary sciences at Harvard University who led the study. “Basically, the pole will move away from the extra mass. That’s what we see with the Earth.”
Scientists have known for years that the vast amount of water locked up in dams around the world exerts influence on sea levels and other large-scale phenomena. But Valencic and her colleagues are the first to calculate how dams impact true polar wander. The team analyzed a comprehensive sample of nearly 7,000 dams built globally from 1835 to 2011, which collectively contain enough water to fill the Grand Canyon twice.
“We really focus on the big ones,” Valencic said, adding that the dams in the study all contain at least one cubic kilometer of volume. About a quarter of catalogued dams are below that cutoff, but excluding them “likely had a negligible impact” on results, according to the new study.
The team estimated that these colossal water impoundments have caused Earth’s poles to wander a total of about 3.7 feet over the past two centuries—a small amount, but still incredible to think about—and that motion hasn’t all been in the same direction, shifting based on where dams were being built.
From 1835 to 1954, most dam production occurred in North America, which propelled the North Pole in the other direction by about eight inches toward the 103rd meridian east, which runs through Eurasia. From the 1950s to 2011, the dam boom moved to East Africa and Asia, causing the pole to wander 22 inches to the 117th meridian west, which passes through North America.
In total, the dam-related drift is small relative to other forces in the so-called “budget” of polar wander, such as ice melt, sea level rise, or the roiling convections in Earth’s mantle. But it’s important to tease out the contributions of each factor to make projections about where, and how far, the poles might wander in the future.
“It is actually a pretty big debate—how important convection is, as opposed to ice melting and other phenomena, like dams,” Valencic said. “Dams are a smaller contribution to polar wander; maybe an order of magnitude smaller than ice melt. But still, it's important to close the budget.”
In addition to isolating each input on polar wander, researchers want to understand the head-spinning ways in which they all interact. Human-driven climate change is fueling the rapid loss of glaciers and ice sheets around the world and subsequent sea level rise. Those trends affect polar wander by shifting mass around Earth’s surface. Polar wander is, in turn, linked with regional differences in sea levels.
“When the axis moves in one direction, the hemisphere toward which it's moving sees a sea level fall and the same thing happens in the opposite hemisphere,” Valencic said (this effect is illustrated in figure 1 of this study). “So if it's moving toward the northeastern hemisphere, the southwestern one will also see a fall, and the other two will see a rise.”
Unraveling this whole tapestry of natural and anthropogenic inputs is no easy task, but it’s possible to tug on a few fascinating threads. For instance, scientists reported in 2024 that recent human-linked shifts in ice and groundwater are slowing Earth’s rotation at a rate of 1.33 milliseconds per century. This factor may eventually become a more forceful brake on Earth’s rotation than the Moon’s tidal influence.
Meanwhile, the largest dam on Earth, China’s Three Gorges, is estimated to single-handedly account for a rotational slowdown of 0.06 microseconds and a 0.8-inch polar shift, if its reservoir is filled to capacity, according to a 2005 study.
So while the overall effect of dams on polar wander is relatively small, “it's still a factor that we have to think about,” Valencic concluded. “If we want to fully understand polar wander, and what's going on with the axis, we need to pin everything down.”