Does The Three Gorges Dam Actually Slow Down Earth?

A photo of the Three Gorges Dam.

The Three Gorges Dam in China is pretty high up among the world's largest man-made structures. Located on the Yangtze River in the Hubei province of China, this giant hydroelectric dam generates power, controls flooding, and improves navigation. But what scientists have also discovered is that it has a very minute but measurable impact on the rotation of the planet. Believe it or not, the dam actually makes every day longer by 0.06 microseconds! In this article, we will look into the dam’s design, its purpose, the science behind how it affects Earth’s spin, and the broader implications of this phenomenon.

What is the Three Gorges Dam?

The Three Gorges Dam spans the Yangtze River, which is the third longest river on Earth and flows over 3,900 miles through China. The construction process began in 1994, and the dam was officially completed in 2012, though the core structure of the dam had already been finished in 2006. The dam measures 7,661 feet long and 607 feet tall, and its reservoir holds 39.3 cubic kilometers of water, or around 10 trillion gallons. This reservoir runs 410 miles upstream, covering an area of 403 square miles. The power plant of the dam, with 34 turbines, produces 22,500 megawatts of electricity, which is enough to power millions of homes. It set a world record in 2020 when it generated 112 terawatt-hours of electricity in just one year. To put it into perspective, that’s more than the generation capacity of many smaller countries.

The dam has three main purposes. Firstly, it prevents flooding, which has caused headaches for those living along the Yangtze River, creating disastrous floods virtually every decade. During the 20th century alone, the floods had killed roughly 300,000 people. Secondly, it offers clean energy and lowers China’s reliance on coal, thereby reducing its carbon footprint. Thirdly, it promotes navigation by making it possible to push bigger vessels against the flow, boosting business. However, the dam also came with its downsides, as it displaced over 1.3 million people, flooded 156,000 acres of land, and raised fears about environmental devastation, including habitat loss and water pollution. Despite all of these problems, the dam remains a cornerstone of China’s energy and flood-control policy.

How Does the Dam Make Earth Move More Slowly?

The idea that a dam is capable of slowing Earth down might surprise you, but it is based on basic physics. The planet spins on its axis approximately every 24 hours, completing the entire cycle of a day. The rate at which the Earth spins is set by the distribution of mass across the planet. The farther away from the axis of rotation—the imaginary straight line from the North Pole to the South Pole about which the Earth spins—that the mass is, the slower the planet spins. In other words, shifting the mass in, towards the axis, will make rotation speed up. This idea, called moment of inertia, is what causes the Three Gorges Dam to have such an effect, though minimal, on Earth’s rotation.

The dam's reservoir holds 42 billion tons of water, distributed, on average, 175 meters above the sea level. By displacing this vast amount of water, the dam shifts the Earth’s mass slightly away from the axis of rotation. NASA researchers, including geophysicist Benjamin Fong Chao, calculated that this tilt contributes to the moment of inertia of the planet, slowing down its rotation by 0.06 microseconds a day. A microsecond is a millionth of a second, and the difference is so infinitesimal that it has no effect on daily life. Added up every day over a lifetime, the additional time is approximately 0.03 seconds. The dam also shifts the position of the North and South Poles by approximately 2 centimeters, or 0.8 inches, and makes the Earth slightly rounder at the equator and flatter at the poles.

The Science Behind the Moment of Inertia

Moment of inertia, also known as rotational inertia, is a way to measure the difficulty of changing the rotation of an object. For the Earth, this is determined by the planet’s mass and its distribution around the axis of rotation. The larger the amount of mass at the equator, further from the axis of rotation, the larger the moment of inertia and the slower the rotation. When the mass is close to the poles, the moment of inertia decreases and the rotation speeds up. The reservoir behind the Three Gorges Dam, located at a latitude of about 30°N, holds water farther from the axis than it would be if it just naturally flowed out to the sea. This shift in mass, though very small in comparison to the Earth’s mass, is enough to make a measurable difference.

Data from NASA, in a 2005 report, estimate that the reservoir behind the dam makes the length of a day approximately, as mentioned before, 0.06 microseconds longer. This is based on precise models that account for the volume of water, its height above sea level, and how close to the axis of rotation it is. The effect of the dam is tiny because in the grand scheme of things, Earth’s moment of inertia is enormous, driven by 5.97 sextillion tons of mass. To put this into perspective, the 42 billion tons of water in the reservoir is not even 0.000000001% of the total mass of Earth. Nevertheless, today’s equipment can measure even these small changes, demonstrating just how sensitive science measurements have become.

The effect of the dam on Earth’s shape and pole position has a direct relationship to the same concept (moment of inertia). By redistributing mass, the dam creates a slight bulge at the equator and a flattening at the poles, making the planet slightly less spherical. The two-centimeter change in the pole’s position is due to the fact that the shifted mass unbalances the planet, shifting the axis of rotation. These effects, while scientifically significant, have no practical impact on daily life or global systems like weather or tides.

Broader Implications and Context

The Three Gorges Dam effect on Earth’s rotation leads to the question of how human civilization influences the world. Skyscrapers, big dams, and even draining groundwater can reallocate mass, slightly altering the Earth’s rotation. Melting of the polar ice caps due to global warming, for example, shifts water from the poles to the oceans and therefore contributes to slowing the Earth’s rotation. These cumulative impacts, though subtle in themselves, can add up over time, especially as humans build increasingly massive structures. Scientists are still tracking how these changes might affect satellite orbits, tide cycles, or global climate patterns in the long run, though no major perturbations have been observed.

The environmental and social impacts of the dam are closer to home. The reservoir flooded 13 cities, 140 towns, and 1,350 villages and displaced over 1.3 million individuals. It entombed ancient sites and severely impacted ecosystems, causing the Chinese paddlefish to become extinct and putting species like the Yangtze sturgeon in danger. Water pollution and reduced river flow have also harmed aquatic populations and contaminated drinking water in certain areas. The dam, in addition, has been linked to increased seismic activity, with research in 2022 showing that changes in reservoir water levels affect the gravity field in western Sichuan and can lead to earthquakes. These issues highlight the trade-offs between the benefits of the dam and its costs.