The vast sheets of ice at the planet’s poles are melting at such a scale that they are nudging the Earth’s spin, a shift that will push back the next adjustment to global timekeeping by three years. A study published in the journal Nature on Wednesday found that the redistribution of meltwater is slowing the planet’s rotation, delaying the need for a “negative leap second” until 2029.
Geophysicist Duncan Agnew of the University of California, San Diego, who led the research, used satellite data to track changes in the Earth’s mass distribution. As polar ice melts into the oceans, mass moves from the poles toward the equator, making the planet slightly more oblate and causing its spin to decelerate—a phenomenon Agnew compares to a figure skater slowing down by extending her arms.
“If you have a skater who starts spinning, if she lowers her arms or stretches out her legs, she will slow down,” Agnew told NBC News, explaining the physics behind the slowdown.
The implications extend far beyond the length of a day. Global timekeeping relies on Coordinated Universal Time (UTC), which is based on atomic clocks but is occasionally adjusted to stay in sync with the Earth’s rotation. These adjustments, known as leap seconds, have been added 27 times since 1972, but the current trend of slowing rotation means a negative leap second—subtracting a second—may soon be required for the first time.
Without the influence of polar ice melt, Agnew’s calculations show that a negative leap second would have been needed by 2026. However, because the melting ice is slowing the Earth’s rotation, that deadline is now pushed to 2029. “It’s kind of impressive, even to me, we’ve done something that measurably changes how fast the Earth rotates,” Agnew said. “Things are happening that are unprecedented.”
Why a Second Matters
The potential shift in timekeeping is not just a matter of adjusting our clocks. Accurate time is fundamental to modern infrastructure, including satellite navigation, computer networks, and financial transactions, all of which depend on precise synchronization. A negative leap second could pose challenges for systems designed to handle only positive leap seconds, requiring updates to software and protocols.
The study highlights how human-induced climate change is now influencing even the most fundamental aspects of our planet’s physics. While the effect on rotation is minuscule—amounting to milliseconds over decades—the need to adjust our timekeeping systems is a tangible consequence of a warming world.
Agnew’s research underscores the interconnectedness of Earth’s systems and the far-reaching impacts of climate change, from sea-level rise to the very measurement of time.
A new study in Nature reveals that melting polar ice is slowing Earth's rotation, which will delay the need for a negative leap second by three years. This adjustment is crucial for global timekeeping systems that rely on precise atomic time.
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