At the International Earth Rotation and Reference Systems Service in Paris, technicians regularly check the world’s atomic clocks, which are highly accurate, against the actual rotation of Earth. Sometimes, the clocks get ahead, leading to the addition of a leap second to global timekeeping.
The first leap second was added in 1972, and since then, 27 more have been included. For many years, scientists understood these additions based on the predictable effects of the Moon’s gravity, which slows down Earth’s spin. However, in the last decade, this pattern has become more erratic. The slowdown of Earth’s rotation is now influenced by changes on its surface, primarily the movement of water.
When water moves away from the poles into the oceans, it’s similar to how a figure skater spins faster by pulling in her arms and slows by extending them. This shift causes a bulge at the equator, altering the planet’s moment of inertia and leading to a slower rotation. Mostafa Kiani Shahvandi from the University of Vienna highlights how unusual this period of change is, indicating a significant increase in sea levels from 2000 to 2020.
Currently, climate-driven factors are making days lengthen by about 1.33 milliseconds per century. This change, while small, adds up over time and affects modern technology that relies on precise measurements.
To assess whether this slowdown was remarkable, researchers from ETH Zurich and the University of Vienna analyzed fossils from tiny sea creatures called benthic foraminifera, which provide clues about past sea levels. Their findings, published in the Journal of Geophysical Research, show that today’s rate of increased day length is unprecedented in the last 3.6 million years.
Benedikt Soja, a geodesist at ETH Zurich, emphasized that this rapid change signals something unique about current climate dynamics. Unlike the slow, steady drag of the Moon’s gravity, the impact of climate change on Earth’s rotation is accelerating. In fact, studies estimate that by the end of this century, the influence of climate change on day length could surpass that of the Moon. This has serious implications for satellite navigation systems like GPS, which rely on exact timing.
Researchers are using advanced technology to blend ancient climate data with modern analytical methods to provide a clearer picture of these shifts. Their studies reveal that the current rapid changes in day length do not resemble any natural cycles from the past.
Since 1972, leap seconds have been added to keep atomic clocks in sync with Earth’s rotation, but the increasing unpredictability of these changes makes it more challenging. The impact may seem small—a millisecond here or there—but it poses real concerns for precision in navigation and other technologies.
Kiani Shahvandi and his team continue to study these trends, showing that human activity has increasingly influenced how our planet spins. Their 2024 research indicates that climate factors now play a leading role in variations in day length and Earth’s polar motion.
In the grand scheme, these shifts reveal a profound truth: for the first time in millions of years, Earth’s rotation is being altered not by celestial mechanics, but by the movements of water, dramatically influenced by human actions.
For further reading on the implications of these findings, check out the Journal of Geophysical Research here.
