Topline
Earth’s rotation is gradually slowing as climate change melts glaciers and polar ice sheets, redistributing water across the planet and subtly lengthening the day. According to new research from the University of Vienna and ETH Zürich, the current increase in day length — 1.33 milliseconds per century — is unprecedented over at least the past 3.6 million years. It’s a new measure of how profoundly human-driven warming is affecting the Earth system, even as only 48% of Americans believe climate change is the result of human activity.
Key Facts
Earth’s day is currently lengthening at a rate of about 1.33 milliseconds per century due largely to climate-driven sea-level rise. Even though the changes are minuscule, they can cause problems in satellite navigation, space missions and global timekeeping systems, which require accurate information on Earth’s rotation.
Melting ice sheets and glaciers move mass from the poles toward the oceans, slowing the planet’s rotation.
Researchers reconstructed changes in day length over the past 3.6 million years using fossil remains of benthic foraminifera — single-celled marine microorganisms on the seafloor — and advanced machine-learning techniques.
The study, published in the Journal of Geophysical Research: Solid Earth, found that no period in the past 3.6 million years experienced a climate-driven increase in day length as rapid as that observed between 2000 and 2020.
The moon remains the dominant long-term influence on Earth’s rotation rate, over time slowing Earth’s spin and making days longer, according to the Institute of Physics. However, climate change could surpass its effects by the end of this century under some scenarios.
Reading Earth’s Ancient Clock
To investigate how climate has influenced Earth’s rotation through deep time, researchers turned to benthic foraminifera, microscopic marine organisms whose fossilized shells preserve evidence of ancient sea-level changes. They then reconstructed fluctuations in the length of day since the late Pliocene, around 3.6 million years ago. The study’s most striking finding is not the size of past fluctuations but the speed of the current change. During the 21st century, accelerated melting of the Greenland and Antarctic ice sheets, as well as mountain glaciers, has increased sea levels and shifted mass away from the poles. It’s making Earth bulge slightly at the equator, causing the planet to rotate slightly more slowly. The findings suggest that contemporary climate change is unprecedented in its impact on Earth’s rotation over the past 3.6 million years.
“This rapid increase in day length implies that the rate of modern climate change has been unprecedented at least since the late Pliocene, 3.6 million years ago,” said Benedikt Soja, Professor of Space Geodesy at ETH Zurich. “The current rapid rise in day length can thus be attributed primarily to human influences.”
Why Earth’s Rotation Is So Complex
If climate change is slowing Earth’s rotation, why have we also heard that Earth has recently been spinning faster and producing some of the shortest days ever recorded? While climate change is contributing to a long-term slowing of Earth’s rotation, scientists note that many other factors can temporarily speed it up or slow it down. Movements within Earth’s liquid outer core, shifts in atmospheric circulation, ocean currents and subtle changes in the planet’s shape can all alter the length of a day by fractions of a millisecond. Since 2020, Earth has recorded some of its shortest days since modern measurements began, indicating a short-term acceleration in rotation that researchers do not yet fully understand. These fluctuations occur against a much longer-term backdrop in which the moon’s gravitational pull steadily slows Earth’s spin, while climate-driven melting of ice sheets increasingly adds to that deceleration. According to the new research, by the end of the 21st century, climate change is expected to affect day length even more strongly than the moon — if greenhouse gas emissions remain high.
