to unprecedented levels not seen in 3.6 million years,
adding a planetary dimension to the climate crisis.
| Key Points |
|
For as long as civilisations have measured time, the 24-hour day has seemed immutable. It is not.
A study published in March 2026 in the Journal of Geophysical Research: Solid Earth finds that Earth's days are growing longer at a rate unmatched in 3.6 million years, and the primary driver is human-caused climate change.1
The finding connects two phenomena that seem, at first, to belong to entirely different realms: the melting of polar ice sheets and the rotation of the planet itself.
Yet researchers at the University of Vienna and ETH Zurich have shown they are inseparable.
As greenhouse gas emissions accelerate the melting of glaciers in Greenland and Antarctica, the resulting meltwater flows into the oceans and spreads toward the equator, making Earth slightly wider at its middle. That subtle redistribution of mass is enough to slow the planet's spin.2
A Millisecond That Matters
The change is not perceptible to a person going about their day. Between 2000 and 2020, climate-related factors lengthened each day by the equivalent of 1.33 milliseconds per century.1
A millisecond is one-thousandth of a second. But in the world of precision timekeeping, even that fraction carries consequences.
Coordinated Universal Time, or UTC, is set by atomic clocks and must be periodically adjusted to match Earth's actual spin. These adjustments, called leap seconds, are added or subtracted to keep clocks aligned with the planet's rotation. The global financial system, GPS navigation, power grids and internet infrastructure all depend on this synchronisation.5
Duncan Agnew, a geophysicist at the Scripps Institution of Oceanography at the University of California San Diego, documented in Nature in 2024 that ice melt had pushed back the need for a timekeeping adjustment by roughly three years, from 2026 to around 2029.8
Many computer systems can add a leap second, but far fewer are programmed to subtract one. That asymmetry, Agnew noted, represents a genuine technical risk.
The Physics of the Spin
The mechanism at work is a fundamental principle of physics: the conservation of angular momentum. Any spinning body resists changes to its rotation. When its mass is concentrated close to the axis of spin, it rotates quickly. When that mass spreads outward, the rotation slows.
Scientists at ETH Zurich describe it in terms every sports fan can visualise. A figure skater executing a pirouette pulls her arms tightly to her body to spin faster. When she extends them outward, her rotation slows immediately.4
Earth works the same way. Ice stored at the poles is positioned close to the planet's rotational axis. When it melts and flows into the oceans, that mass migrates toward the equator. Earth's waistline widens, and the spin decelerates.
Since 1993, global sea levels have risen by roughly 10 centimetres on average.4
Scientists project a rise of at least 60 centimetres by the end of this century under high-emissions scenarios. Each increment of sea level rise adds to the equatorial bulge. Each addition slows the rotation a fraction more.
Reading the Deep Past
To understand whether today's changes are unusual, the research team needed to compare them against millions of years of history. That required a novel form of detective work rooted in ocean sediment.
The scientists turned to benthic foraminifera, microscopic single-celled organisms that live on the seafloor and build calcium carbonate shells. When they die, those shells accumulate in ocean sediment, preserving a chemical record of the water conditions at the time of their formation.1
By analysing the oxygen isotope ratios locked inside fossil foraminifera shells, researchers can infer ancient sea levels, and from those sea levels, calculate how Earth's rotation must have changed.
"From the chemical composition of the foraminifera fossils, we can infer sea-level fluctuations and then mathematically derive the corresponding changes in day length," said Mostafa Kiani Shahvandi, the study's first author and a climate scientist at the University of Vienna.1
Paleoclimate data of this kind carries substantial uncertainty. Fossil records are incomplete, and the signals preserved in ancient shells can be distorted by diagenesis, the chemical alteration of sediment over geological time. To account for these complexities, the team applied a probabilistic deep learning algorithm, a physics-informed diffusion model designed to extract meaningful patterns from noisy data.2
The model was constrained by the known physical laws governing sea-level change, lending it greater credibility than a purely statistical approach.
Unprecedented in 3.6 Million Years
The results were stark. During the Quaternary period, spanning roughly the past 2.6 million years, natural ice ages caused Earth's ice sheets to advance and retreat in cycles driven by orbital variations in the planet's path around the Sun. These cycles produced measurable fluctuations in day length.2 They were, however, comparatively gradual.
The current rate of 1.33 milliseconds of additional day length per century stands outside that entire range of natural variation. Going back further, to the Late Pliocene some 3.6 million years ago, the researchers found no comparable period of change.2
The closest historical parallel involved deglaciation events after major ice ages, but even those transitions, driven entirely by natural orbital forcing, did not produce a rate of rotational change equal to what is occurring today.
"The current rapid rise in day length can thus be attributed primarily to human influences," said Benedikt Soja, Professor of Space Geodesy at ETH Zurich and a co-author of the study.6
A Planetary System Responding to Human Activity
Earth's rotation is not governed by a single force. The Moon's gravitational pull exerts tidal friction on the oceans, and that friction has been the dominant driver of rotational slowing over geological timescales.
Simultaneously, the slow rebound of Earth's crust following the last Ice Age, a process called glacial isostatic adjustment, shifts mass back toward the poles and tends to speed the planet's rotation. Both processes are predictable and relatively constant.4
A fourth factor, fluid motion within Earth's molten outer core, can temporarily accelerate or decelerate the spin over periods of 10 to 20 years. Right now, core dynamics are causing a slight countervailing speed-up that partially offsets the climate-driven slowing.6
The interplay between these forces demonstrates how tightly Earth's internal processes and its surface climate are linked.
NASA-funded research, published in separate papers in 2024 in Nature Geoscience and the Proceedings of the National Academy of Sciences, found that climate-related mass redistribution, including groundwater depletion and glacial melt, accounts for roughly 90 per cent of the periodic oscillations in Earth's polar motion since 1900.5
The same research documented that the lengthening of the day has been accelerating at a faster pace since 2000 than at any point in the preceding century.
The Coming Century
The implications extend well beyond timekeeping. If high-emissions trajectories continue, Soja's team projects that the climate-driven influence on Earth's rotation could surpass the Moon's gravitational effect before the end of the 21st century.6
That would mark a profound inversion: for billions of years, the Moon set the pace. Human industry would have overtaken it.
Research published in 2024 found that melting ice had already shifted Earth's rotational axis by roughly 10 metres since 1900, a phenomenon called polar motion.5
That shift in turn generates small perturbations in Earth's interior, feeding back into the dynamics of the molten core in ways scientists are only beginning to understand.
For satellite systems, the changes are not trivial. GPS and space navigation rely on precise models of Earth's rotation to calculate the position of objects on the ground and in orbit. Small but persistent errors in those models, compounded over time, can translate into navigational inaccuracies. As the rate of rotational change itself changes, those models must be continuously revised.5
Caveats and Open Questions
The researchers are transparent about the limitations of their methodology. Deriving day-length changes from foraminifera fossils requires a chain of inferences: from isotope ratios to sea levels, from sea levels to ice volumes, and from ice volumes to rotation rates. Each step carries uncertainty, and the deep-learning model, however sophisticated, is only as good as the data it processes.1
Christian Bizouard, an astrogeophysicist at the International Earth Rotation and Reference Systems Service in France, noted that Earth's core activity remains nearly impossible to predict. Any projection about future rotational change depends on assumptions about core dynamics that current science cannot fully resolve.7
Other unresolved questions include the long-term effect of groundwater depletion on rotational dynamics, the potential role of deep-ocean circulation changes, and whether atmospheric mass redistribution caused by shifting weather patterns contributes meaningfully at multi-decadal scales.
Future research using higher-resolution sediment cores and improved geodynamic models may refine these findings, or challenge some of their assumptions.
Beyond Temperature: Rethinking What Climate Change Does
Public understanding of climate change tends to focus on temperature records, sea-level projections and extreme weather events. This research points to a broader and more unsettling truth: human activity is now altering the physics of the planet itself.
Geophysicist Duncan Agnew put it plainly in comments following his 2024 Nature paper. Humanity, he said, had "done something that affects, measurably, the rotation rate of the entire Earth."8
That statement is not hyperbole. It is a finding published in peer-reviewed journals, supported by satellite geodesy, fossil chemistry and deep-learning modelling applied to 3.6 million years of data.
The interconnections are striking. Carbon emissions warm the atmosphere. A warmer atmosphere melts ice. Meltwater redistributes mass across the globe. That redistribution slows a planet four and a half billion years old. Each step in that chain is measurable. Together, they describe an influence that no previous civilisation has exerted.
Conclusion: Time, Physics and the Human Footprint
There is a particular quality to the claim that humans have altered Earth's rotation. It moves the climate crisis out of the domain of weather and economics and into the domain of planetary mechanics. The day itself, which for 3.6 million years changed only when natural forces of ice ages and orbital cycles compelled it, is now changing because of us.
That change remains invisible without instruments. Judah Levine, a physicist at the National Institute of Standards and Technology, has noted that everyday life is not sensitive at the millisecond level.5
The person catching the 7 am train will not notice. But the GPS receiver calculating where that train sits on the track will, eventually, need to account for a slightly longer day than the one its model assumed.
The deeper significance is not logistical. It is conceptual. When the Quaternary's great ice ages waxed and waned in response to orbital shifts, they did so over tens of thousands of years. The current rate of rotational change, driven over decades by the burning of fossil fuels, has already exceeded what those vast natural cycles produced. If emissions remain high, scientists project that rate will double by 2100.1
What remains unknown is whether there are thresholds, points at which the cascading feedbacks between ice loss, sea level, polar motion and Earth's interior dynamics produce changes that outpace current projections. The foraminifera in their ocean-floor sediment recorded every ice age for millions of years. They are now recording something that has no precedent in their long archive.
What that record will show to researchers a century from now depends, in no small part, on choices being made today.
References
1. University of Vienna: Climate change slows Earth's spin, day lengthening unprecedented in 3.6 million years (2026)
2. Phys.org: Climate change is slowing Earth's spin at unprecedented rate compared to past 3.6 million years (2026)
3. Smithsonian Magazine: Melting polar ice sheets are slowing Earth's rotation (2024)
4. Newsweek: Earth's rotation is changing at a speed not seen in 3.6 million years (2026)
5. NBC News: Melting ice is slowing Earth's spin, shifting its axis and influencing its inner core (2024)
6. Euronews: Unprecedented in the past 3.6 million years, how human-made climate change is making days longer (2026)
7. NBC News: Melting polar ice is slowing the Earth's rotation, with possible consequences for timekeeping (2024)
8. Nature: A global timekeeping problem postponed by global warming, Duncan Agnew (2024)
9. EurekAlert: Climate change slows Earth's spin, day lengthening unprecedented in 3.6 million years (2026)
10. Gizmodo: Earth's spin is slowing at a pace not seen in millions of years (2026)
No comments :
Post a Comment