Sea level rise has been reshaping coastlines and stressing infrastructure for decades, and scientists have long acknowledged its physical effects on the planet. A new study now quantifies one of those effects in terms that extend far beyond geography.
Human-driven climate change is slowing Earth’s rotation at a rate not seen in 3.6 million years, according to the study. Rising seas are adding 1.33 milliseconds to the length of each day per century — a figure researchers describe as anomalous when measured against millions of years of planetary history.
The mechanics follow a principle familiar from figure skating.
Earth spins faster when its mass is concentrated toward its center, and slower when that mass spreads outward. Sea level rise redistributes water across the planet’s surface, pushing mass away from the poles and toward the equator, which drags on the planet’s rotational speed. Mostafa Kiani Shahvandi, a geoscientist at ETH Zurich and co-author of the study, told Live Science that he set out to determine whether this pattern was historically unusual. “As it turned out, it is quite anomalous,” he said. “The effect is therefore anthropogenic.”
To reach that conclusion, Shahvandi and co-author Benedikt Soja, a professor of space geodesy at ETH Zurich, examined fossils of foraminifera — shelled single-cell organisms whose oxygen content records ancient sea levels. From those sea-level readings, the researchers extrapolated day lengths across millions of years of Earth’s history.
Several forces already shape how fast the planet spins, and the study places the new climate-linked figure within that broader accounting. The moon’s gravitational pull lengthens the day by approximately 2.4 milliseconds per century. Glacial isostatic adjustment — the slow rebound of Earth’s crust following the retreat of ancient ice sheets — shortens it by roughly 0.8 milliseconds per century. El Niño-driven wind patterns add further variation, slowing rotation by about a millisecond per century. Michael Mann, a climatologist at the University of Pennsylvania who was not involved in the study, provided that context to Live Science. The combined background rate of day lengthening sits at approximately 1.71 milliseconds per century, with about 0.1 milliseconds of observational uncertainty. The newly documented climate signal sits within that range but is driven by an entirely different cause.
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One historical parallel exists, though it offers limited reassurance. Around 2 million years ago, during the Early Pleistocene, a period of rising carbon dioxide and temperatures produced a day-length increase of approximately 2.1 milliseconds per century. Uncertainty in that historical estimate means the two periods may have been comparable — or that today’s rate may already exceed it.
Shahvandi says the current trend is not expected to stabilize. “This is expected to get even larger and even bigger than the effect of the moon,” he told Live Science.
The study’s findings identify the present rate of rotational slowing as among the fastest observed across 3.6 billion years of Earth’s history.
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