Why would a warming climate change the length of a day? Earth still turns once every 24 hours by the clocks people live by, but the planet’s spin is not perfectly fixed. It speeds up and slows down by tiny amounts as the Moon pulls on the oceans, the atmosphere shifts, and mass moves through ice, water, and rock. What once seemed like an abstract quirk of planetary physics is now tied to a modern force with global reach: melting land ice is subtly reshaping the planet’s rotation.
Researchers from the University of Vienna and ETH Zurich found that Earth’s day is now lengthening at about 1.33 milliseconds per century, a pace they concluded is unmatched for at least 3.6 million years. Their study, published in the past 3.6 million years, traced ancient sea-level shifts and linked them to changes in the length of the day. In effect, the research turns deep-time climate archives into a record of how Earth’s spin has responded to rising and falling oceans.
The mechanism is simple to picture. Ice stored on Greenland, Antarctica, and mountain glaciers sits relatively close to Earth’s rotational axis. As that ice melts and the water spreads into the oceans, more mass shifts outward toward the equator. A spinning body rotates more slowly when its mass is distributed farther from its center, which is why the researchers leaned on a familiar image. “In our earlier work, we showed that the accelerated melting of polar ice sheets and mountain glaciers in the 21st century is raising sea levels, which slows Earth’s rotation and therefore lengthens the day similar to a figure skater who spins more slowly once they stretch their arms, and more rapidly once they keep their hands close to their body,” Mostafa Kiani Shahvandi said.
The Moon still matters. Its gravitational pull has long acted as a brake on Earth’s spin through ocean tides, and the Moon continues to move away from Earth at about 4 centimeters per year as tidal forces exchange energy between the two bodies. Day length also shifts because of motions in Earth’s core, crust, oceans, and atmosphere. On short timescales, even the Moon’s position can make a given day run slightly long or short. But the new work highlights a larger pattern: climate-driven sea-level rise is no longer a minor background influence.
To reconstruct that longer history, the team used benthic foraminifera fossils, tiny marine organisms whose chemical signatures preserve evidence of past sea levels. They then applied a physics-informed probabilistic model to convert those sea-level changes into likely shifts in Earth’s rotation. Across the Quaternary, ice-sheet growth and retreat repeatedly altered day length, but the present trend stands apart. One interval around 2 million years ago came close, yet the modern rate still emerges as the stronger signal.
The change is too small for human senses. It is not too small for infrastructure built on precision. Satellite navigation, space operations, telecom systems, and parts of high-speed finance depend on exact knowledge of Earth’s orientation and rotation. Benedikt Soja warned that “By the end of the 21st century, climate change is expected to affect day length even more strongly than the Moon.” That does not mean longer mornings or noticeable evenings. It means the planet’s warming is now measurable in the timing of its spin, written not in hours but in milliseconds.
