Many of us don’t think much about the dams we see, but they profoundly impact our planet. Dams block rivers, create lakes, and even provide electricity to towns and cities. They help irrigate crops, manage floods, and store drinking water, making them seemingly practical and essential.
However, when we zoom out and consider the broader implications, the water stored behind these dams alters Earth’s landscape significantly. By trapping rivers, we’re essentially moving water from the oceans to large reservoirs on land. Over time, this has changed how water is distributed across the planet.
According to a recent study from Harvard University, the construction of thousands of reservoirs has shifted the location of Earth’s geographic poles by about one meter in the last 200 years. This subtle change may seem small, but it demonstrates our influence on Earth’s rotation.
Earth spins every 24 hours, and where mass is located affects that spin. When we concentrate mass—like water in reservoirs—we affect the balance. This results in what scientists call true polar wander, which is different from movements caused by tectonic plates.
In the last two centuries, we have built over 7,000 dams, creating extensive reservoirs. When water is moved from oceans to these reservoirs, we change its mass distribution on the planet. This is similar to how a spinning top might wobble if its weight is unevenly distributed.
The research team used a model of Earth’s interior to understand how this mass redistribution impacts our planet. They treated Earth like a spinning elastic ball, allowing them to see how reservoirs adjust the crust’s position slightly as well as how oceans respond. When water is trapped in reservoirs, sea levels fall just a bit, and the ocean surface rebalances as gravity shifts.
The researchers ran their model from 1835 to 2011. They found that constructing reservoirs has nudged the geographic pole about 3.7 feet (or 1.13 meters) over time, particularly during the 20th century as dam-building accelerated. Early on, dams in North America and Europe shifted the North Pole slightly toward Asia. As dam construction spread to Asia and Africa after 1954, the pole began drifting in a new direction, impacting how we understand Earth’s shifting geography.
Interestingly, while scientists often report that global sea levels are rising by a few millimeters every year, these changes are uneven worldwide. Dams alter this balance by locking water into specific areas, which has slightly masked part of the sea-level rise related to ice melting and warming oceans. Natasha Valencic, a graduate student at Harvard and lead author of the study, noted, “We’re not going to drop into a new ice age… but it does have implications for sea level.”
Despite these changes being small relative to Earth’s size, understanding them helps scientists grasp broader environmental shifts. This research shows that human activities, particularly in engineering and water management, have become significant enough to affect planetary systems. As Valencic pointed out, the placement of reservoirs can greatly impact sea-level changes, prompting further considerations in climate science.
The complete study is shared in the journal Geophysical Research Letters.
