A peer-reviewed study published in the Proceedings of the National Academy of Sciences finds that climate change is now lengthening Earth’s days at a rate that could soon exceed the slowing effect of the Moon’s gravitational pull. The research traces how melting polar ice and shifting water masses have accelerated the planet’s rotational slowdown since the turn of the century, with projections showing the trend may nearly match or surpass lunar tidal friction by 2100 under high-emissions conditions. The finding reframes a force long considered purely astronomical as one increasingly shaped by human activity.
How Ice Melt Slows Earth’s Spin
The basic physics is straightforward. When ice locked at the poles melts and flows toward the equator as liquid water, it redistributes mass away from Earth’s axis of rotation. The effect is the same as a figure skater extending their arms mid spin: the rotation slows. Between 1900 and 2000, this climate-driven lengthening of the day varied between about 0.3 and 1.0 milliseconds per century, a modest signal against the background of other geophysical processes. Since 2000, as ice loss in Greenland and Antarctica has accelerated, that rate has jumped to 1.33 plus or minus 0.03 milliseconds per century, according to the same PNAS study.
That acceleration matters because it is closing in on a benchmark that has governed day length for billions of years. The Moon’s tidal friction, which gradually slows Earth by dragging on the oceans and solid body, adds roughly 2.4 milliseconds per century to the length of the day. For most of human history, no other process came close to rivaling that figure. The new research shows climate change is on track to do so within decades, making human influence on Earth’s spin comparable to that of its only natural satellite.
Projections That Rival the Moon
Under a high-emissions scenario, the PNAS paper projects climate-driven day lengthening will reach approximately 2.62 plus or minus 0.79 milliseconds per century by 2100. That central estimate exceeds the Moon’s 2.4-millisecond contribution, meaning human-caused warming would become the dominant force reshaping how long a day lasts. The projection is not a certainty; it depends on the trajectory of greenhouse gas emissions, the stability of ice sheets, and how oceans and groundwater respond. The uncertainty range reflects those unknowns, but even its lower bound of roughly 1.8 milliseconds per century would mark a profound departure from the 20th-century baseline.
Researchers from the University of Vienna and ETH Zurich have described the current 1.33-millisecond-per-century rate as unprecedented in the last 3.6 million years, based on comparisons between modern observations and paleoclimate reconstructions. That geological context sharpens the stakes: the planet has not experienced this pace of rotational change since long before modern humans existed. A separate analysis of rotational dynamics, archived in biomedical and earth science databases, emphasizes that the present acceleration is tightly linked to anthropogenic warming rather than to slow, natural cycles.
To explore these trends in detail, the PNAS team drew on a broader body of geophysical work, including a technical report on the increasingly dominant role of climate change in length-of-day variations. That analysis integrates satellite gravimetry, sea-level observations, and models of the solid Earth to show how climate impacts are overtaking internal processes in shaping Earth’s spin. Together, these studies converge on the same conclusion: as emissions rise, the climate signal in rotational data is growing clearer, stronger, and harder to dismiss as background noise.
Satellite Evidence From GRACE
The latest conclusions rest heavily on observational data from NASA’s Gravity Recovery and Climate Experiment, known as GRACE. Since 2003, this twin-satellite mission has measured tiny variations in Earth’s gravitational field to track how mass moves across the planet’s surface, from shrinking ice sheets to changing groundwater reserves. The resulting gravity-based measurements provide the empirical backbone for connecting ice loss to changes in Earth’s moment of inertia and, by extension, its rotation rate.
The successor mission, GRACE Follow-On, has extended those observations, delivering a continuous two-decade record of mass redistribution. Without these data, attributing the rotational signal specifically to climate-driven processes, rather than to deep-Earth dynamics or random fluctuations, would be far more difficult. Instead, scientists can now correlate year-to-year changes in day length with observed mass loss from Greenland, Antarctica, and mountain glaciers, as well as with large shifts in terrestrial water storage caused by droughts, floods, and groundwater pumping.
Those correlations show that as more mass moves toward the equator (either as meltwater flowing into the oceans or as water redistributed across continents), Earth’s spin slows measurably. Conversely, episodes in which mass shifts poleward or is stored higher in the atmosphere can slightly speed the rotation. Over the last two decades, the net trend from ice and water has been firmly in the direction of a slower spin and longer days.
Polar Motion Tells a Parallel Story
Day length is not the only rotational property being reshaped. A companion study published in Nature Geoscience analyzed a 120-year record of polar motion, the slow wobble of Earth’s rotational axis relative to its surface. That research attributed much of the interannual and multidecadal variability in the axis position to surface mass redistribution from ice melt and changes in terrestrial water storage, while also quantifying contributions from the mantle and core.
NASA summarized the two papers together, noting that scientists examined polar motion across 12 decades and found that nearly all of the periodic oscillations in the axis position could be explained by climate-related processes. The same melting that slows Earth’s spin also nudges the axis, subtly shifting the geographic location of the poles by centimeters to meters over time. These shifts do not threaten everyday navigation, but they underscore how tightly linked Earth’s climate, oceans, and deep interior are to its rotational behavior.
To handle the growing complexity of these datasets, some researchers rely on personalized tools and profiles within large scientific repositories, such as curated research accounts that track publications and data products across geophysics, climate science, and related disciplines. The cross-disciplinary nature of the work, combining satellite geodesy, oceanography, glaciology, and timekeeping, reflects the breadth of systems now responding to human-driven warming.
Why Milliseconds Matter
A fraction of a millisecond sounds trivial. It is not perceptible to anyone glancing at a clock, and it has no direct bearing on daily routines. But modern infrastructure depends on timekeeping precision that operates at scales far smaller than human perception. GPS satellites, telecommunications networks, and financial trading systems all synchronize to atomic clocks that account for Earth’s rotation rate. When that rate shifts, even by tiny amounts, the adjustments required to keep those systems accurate become more frequent and more complex.
The most visible example is the leap second, an occasional one-second correction added to Coordinated Universal Time (UTC) to keep atomic clocks aligned with Earth’s actual rotation. Historically, leap seconds have been needed because Earth, on average, rotates slightly slower than the uniform time kept by atomic clocks. If climate change continues to lengthen the day more rapidly, it could alter how often such corrections are required, complicating efforts to maintain a stable global time standard.
Engineers and standards bodies already debate whether to phase out leap seconds because of the challenges they pose for software, navigation, and critical infrastructure. A climate-driven acceleration in day lengthening would add another layer of uncertainty to those discussions. It would not make clocks suddenly fail, but it would force more frequent recalibration of the relationship between atomic time and Earth’s rotation, with implications for any system that depends on ultra-precise timing.
A Planetary Feedback We Can Measure
The emerging picture is that climate change is no longer just altering temperatures, sea levels, and weather extremes; it is now measurably reshaping the basic parameters of Earth’s rotation. What was once a slow, predictable tug-of-war between the planet and the Moon has gained a new participant: human civilization. By burning fossil fuels and melting ice sheets, humanity is moving enough mass around the surface to change how fast the world turns.
For most people, the consequences will remain abstract, a reminder that the climate crisis reaches into domains once thought immutable. For scientists, however, Earth’s spin is becoming a sensitive diagnostic of planetary change, one that integrates signals from ice, oceans, and the solid Earth into a single, precisely measurable quantity. As observational records lengthen and models improve, the length of the day may become yet another vital sign of a warming world, reflecting not just the pull of the Moon but the sum of human choices on the planet below.
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*This article was researched with the help of AI, with human editors creating the final content.
