Climate Change Is Slowing Earth’s Spin, Study Finds—Lengthening Days by Milliseconds

The days are getting longer — not just in how they feel, but in how long Earth actually takes to turn once on its axis.

A new peer-reviewed study finds that human-driven climate change is now measurably slowing the planet’s rotation by shifting vast amounts of water from ice sheets into the oceans. The effect is tiny on human timescales, adding roughly 1.3 milliseconds to the length of a day every century. But researchers say the rate of change is faster than anything seen in at least 3.6 million years and large enough to matter for the world’s most precise clocks, navigation systems and plans for keeping civil time.

“This rapid rise in day length is unparalleled over the last 3.6 million years,” said Mostafa Kiani Shahvandi, a geophysicist at the University of Vienna and lead author of the study, in a statement released March 12. “The current rapid rise in day length can thus be attributed primarily to human influences.”

Published this month in Journal of Geophysical Research: Solid Earth, the study links modern satellite-era measurements with a new reconstruction of how climate has affected Earth’s spin since the late Pliocene, about 3.6 million years ago. The work builds on a 2024 paper in Proceedings of the National Academy of Sciences that first quantified how melting ice and shifting water, driven by global warming, have increasingly dominated recent changes in the length of day.

Why Earth’s day length changes

Scientists have long known that Earth’s rotation is not perfectly steady. One “mean solar day” is defined as 86,400 seconds, but in reality the length of day fluctuates by milliseconds over days to decades and slowly drifts over longer periods.

The main long-term brake is the Moon, whose tides dissipate energy and gradually lengthen the day by about 1.7 to 2.4 milliseconds per century.

Other processes, however, can speed Earth’s spin. Since the last ice age, the crust and mantle have been slowly rebounding as thick ice sheets melted — a process called glacial isostatic adjustment — which pulls mass closer to the poles and shortens day length by around 0.8 milliseconds per century.

How warming slows the planet

Climate change adds another term to that balance sheet. As Greenland, Antarctica and mountain glaciers lose ice, the water flows into the oceans and redistributes toward lower latitudes. That moves mass away from Earth’s spin axis, like a figure skater extending their arms.

“Rising sea levels slow Earth’s rotation and therefore lengthen the day — similar to a figure skater who spins more slowly once they stretch their arms,” the University of Vienna said in its announcement of the findings.

To put numbers on that effect, Shahvandi and co-author Benedikt Soja, a professor of space geodesy at ETH Zurich, used a chain of climate and geophysical models.

Their earlier PNAS work drew on satellite gravimetry, sea-level data and reconstructions of ice loss and water storage back to 1900. It found that climate-related mass shifts kept the length of day increasing by about 0.3 to 1.0 milliseconds per century through the 20th century, then accelerated to 1.33 milliseconds per century since about 2000. Under a high-emissions pathway, they projected the climate-driven contribution could reach roughly 2.6 milliseconds per century by 2100, rivaling or exceeding the Moon’s influence on long-term trends.

Looking back 3.6 million years

In the new study, the team turned to the distant past to find out whether such a rapid climate-related change had happened before.

They used chemical signatures preserved in the shells of benthic foraminifera — tiny single-celled organisms that live on the seafloor — drilled from deep-ocean sediments. The ratio of oxygen isotopes in these fossils records changes in global ice volume and sea level over millions of years. From that, the researchers reconstructed how much mass shifted between continents and oceans during past glacial and interglacial cycles.

Translating those shifts into changes in Earth’s rotation required a sophisticated statistical approach. The team employed a physics-informed diffusion model, a type of probabilistic deep learning algorithm that incorporates the equations governing sea-level change and the solid Earth’s response, while accounting for uncertainties in the fossil record.

The analysis suggests that over the past 3.6 million years — a period when large ice sheets waxed and waned repeatedly — natural climate swings produced substantial ups and downs in day length. But the present human-driven increase stands out. Only one interval, about 2 million years ago in the early Pleistocene, shows a roughly comparable rate of climate-induced change, and its estimated peak is similar to or slightly above today within wide error bars.

“This study is the first to examine fossil archives for research into the history of climate-induced changes in the length of the day,” the University of Vienna said.

Why milliseconds matter: leap seconds and modern systems

Independent experts say the numbers are small but scientifically significant.

“Many factors influence Earth’s spin speed. The moon’s pull … is the most significant over the long term,” climate scientist Michael Mann, now at the University of Pennsylvania, said in comments to other media about the broader body of work on length of day. Climate’s growing role, he and others argue, reinforces evidence that modern warming is unusually rapid by geologic standards.

Duncan Agnew, a geophysicist at the Scripps Institution of Oceanography who was not involved in the new study, has reached similar conclusions about climate’s emerging impact on timekeeping. In a 2024 paper in the journal Nature, he calculated that changes in Earth’s interior and atmosphere had sped up the planet’s rotation enough that, without ice-melt effects, the world would likely have needed to introduce the first-ever “negative leap second” — deleting a second from civil time — around 2026. Ongoing polar ice loss, by slowing the rotation, has delayed that moment to about 2029.

“That climate change has been able to change how fast the whole Earth spins is yet another indication that we are having an effect unlike anything seen before,” Agnew said in a statement at the time.

Leap seconds are the practical interface between planetary dynamics and daily life. Atomic clocks underpin International Atomic Time (TAI), which ticks onward at a uniform rate. Universal Time (UT1) tracks Earth’s actual rotation. Coordinated Universal Time (UTC), the basis for civil time worldwide, is kept within 0.9 seconds of UT1 by occasionally inserting an extra second at the end of June or December. Since 1972, 27 positive leap seconds have been added. None has ever been subtracted.

The International Earth Rotation and Reference Systems Service, based in Paris, decides when to add leap seconds by analyzing data from very long baseline interferometry — networks of radio telescopes that time signals from distant quasars — satellite laser ranging and global navigation satellite systems.

Even though the shift in day length is measured in milliseconds, it matters for technologies that depend on precise timing and positioning.

“Even though the changes are only milliseconds, they can cause problems … for example in precise space navigation, which requires accurate information on Earth’s rotation,” Shahvandi said in the Vienna release.

Past leap seconds have already caused glitches in some computer systems, including temporary outages for major websites and software crashes in financial and airline networks. Engineers are particularly wary of a negative leap second, which would require clocks to skip from, say, 23:59:58 directly to 00:00:00.

Recognizing the difficulty, member states of the International Telecommunication Union voted in 2022 to phase out frequent leap seconds by around 2035 and allow a larger gap between atomic time and Earth-rotation time. Details of how that change will be implemented are still being worked out with national metrology institutes and timekeeping labs.

A quiet marker of the Anthropocene

Beyond its technical implications, the new study adds a striking data point to discussions of the Anthropocene, the proposed epoch in which human activity is the dominant influence on climate and the environment.

“We can see our impact as humans on the whole Earth system,” Soja has said in earlier interviews about related work, “fundamentally altering how it moves in space and rotates.”

No one will feel an extra 1.3 milliseconds tagged onto the day. But as ice continues to melt and seas creep higher, the planet’s great mechanical clock is quietly ticking at a slightly different pace — and the systems built to keep time with it are already taking notice.