Rising sea levels are slowing Earth’s rotation, lengthening how long an average day lasts. And the current rate of increase to a single average day—1.33 additional milliseconds per century—is unprecedented for at least the past 3.6 million years, a new study finds.
Most people take for granted that a single day will last for 24 hours, not a second more or less. But in reality, the length of any given day varies for different reasons, including the moon’s gravitational pull on the planet and geophysical processes in Earth’s interior and at its surface, as well as atmospheric conditions.
Incredibly, climate change is expected to have more influence over day length than the moon by the end of this century, according to Benedikt Soja, the study’s senior author and a geophysicist at the Swiss Federal Institute of Technology Zurich.
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“Even though the changes are only milliseconds, they can cause problems in many areas, for example in precise space navigation, which requires accurate information on Earth’s rotation,” he said in a statement.
The researchers behind the new study had previously found that climate change affects day length, as rising ocean levels—caused by the melting glaciers and the dwindling polar ice sheets—affect Earth’s spin.
The slowdown is linked to the distribution of mass once held at the poles toward the planet’s midsection. In the statement, lead study author and University of Vienna geoscientist Mostafa Kiani Shahvandi compared the effect 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.”
In the new paper, published in the Journal of Geophysical Research: Solid Earth, the researchers show that the rate of increase in day length is higher now than it has been for millions of years. The researchers analyzed the chemical composition of fossils to infer sea-level changes over the past 3.6 million years. Then they calculated the corresponding changes in the length of a day. Using a probabilistic deep-learning algorithm to better model the physics of sea-level change, they found that, as the planet’s ice has formed and melted, day length has fluctuated in lockstep. But today’s rate of increase in day length is an outlier, according to the study.
“Only one time—around 2 million years ago—the rate of change in length of day was nearly comparable, but never before or after that has the planetary ‘figure skater’ raised her arms and sea-levels so quickly as in 2000 to 2020,” Kiani Shahvandi said.