NASA warns China could slow Earth’s rotation with one simple move

In a world where human projects reshape rivers and coastlines, it’s startling to realize they can nudge Earth’s rotation, too. China’s Three Gorges Dam is a case in point—its vast reservoir subtly shifts mass on the planet and, with it, the clockwork of a day.

When Engineering Reshapes The Planet

The Three Gorges Dam is often hailed as a modern engineering wonder. But NASA’s modeling shows the reservoir’s water load has lengthened the day by about 0.06 microseconds and shifted Earth’s pole by ~2 cm—minute, but measurable effects linked to changes in the planet’s moment of inertia.¹

Did you know?
When the reservoir is full, it holds ~40 km³ (≈10 trillion gallons) of water—enough redistributed mass to explain those tiny rotational tweaks.

The Physics Behind A Slowing Earth

Think of a figure skater: arms out, the spin slows; arms in, it quickens. The same conservation of angular momentum applies globally. By concentrating tens of cubic kilometers of water in one place, the dam increases Earth’s moment of inertia and slightly reduces its spin rate—effects far too small to notice when you set an alarm, but detectable with modern geodesy.

Back in 2005, after the 2004 Indian Ocean earthquake, NASA documented a comparable mechanism in nature: the quake’s mass redistribution shortened the day by a few microseconds and nudged the pole by ~2.5 cm. These comparisons help put the dam’s modeled signal in perspective.

What Does It Mean For Daily Life?

For everyday schedules: essentially nothing. But as a signal, it matters. Large-scale mass shifts—melting ice, changing groundwater, major quakes—continually push and pull on length of day and polar motion. The Three Gorges contribution is tiny, yet it illustrates how human systems now register alongside geophysical ones. ²

Did you know?
Over billions of years, lunar tidal friction has been the dominant force slowing Earth’s rotation—lengthening the day by milliseconds per century.³

Could We Ever Adjust Time Itself?

Because Earth’s spin isn’t perfectly steady, timekeepers occasionally tweak Coordinated Universal Time with leap seconds. A 2024 Nature analysis suggests an unprecedented negative leap second (removing one second) could be needed in the late 2020s, though ongoing climate-driven mass shifts may push the date back (and the governance of leap seconds is itself changing).⁴

A Double-Edged Achievement

The Three Gorges Dam delivers renewable power, navigation gains and flood control—even as it has well-documented social and ecological costs. Layer onto that a poetic footnote: this infrastructure project is large enough to register in Earth’s rotation records. The effect is microscopic, but the message is outsized—our engineering now lives in the same equations as celestial mechanics.

Footnotes

1. NASA/JPL — “NASA Details Earthquake Effects on the Earth” (dam comparison, 0.06 µs & ~2 cm; ~40 km³): https://www.jpl.nasa.gov/news/nasa-details-earthquake-effects-on-the-earth/
2. NASA/JPL — “NASA Details Earthquake Effects on the Earth” (2004 quake polar shift & microseconds): https://www.jpl.nasa.gov/news/nasa-details-earthquake-effects-on-the-earth/
3. NASA — “NASA-Funded Studies Explain How Climate Is Changing Earth’s Rotation” (lunar tidal friction context): https://www.nasa.gov/science-research/earth-science/nasa-funded-studies-explain-how-climate-is-changing-earths-rotation/
4. Nature — “A global timekeeping problem postponed by global warming” (Agnew, 2024): https://www.nature.com/articles/s41586-024-07170-0