Earth’s days are getting longer, and human-driven climate change is now a primary driver—new AI-powered climate models show that since the Late Pliocene, 3.6 million years ago, each century has added 1.5 milliseconds to the day, a rate accelerating in lockstep with polar ice loss and sea level rise, with profound implications for timekeeping, climate systems, and planetary stability.
The length of a day on Earth is not fixed. While we organize our lives around 86,400 seconds, the actual time it takes for Earth to complete one full rotation varies—and it has been growing imperceptibly longer over millennia. For centuries, this slowdown was attributed almost entirely to the Moon’s gravitational pull, a process known as tidal braking. But new research reveals that climate change has become a significant, and potentially accelerating, force in altering the planet’s spin.
A groundbreaking study by researchers Mostafa Kiani Shahvandi from the University of Vienna and Benedikt Soja from the Institute of Geodesy and Photogrammetry in Zurich has used an AI algorithm to re-analyze paleoclimate data, concluding that since 3.6 million years ago, Earth’s rotation has slowed enough to add 1.5 milliseconds to each day per century. This rate, derived from climate-driven mass redistribution, now rivals or exceeds traditional tidal factors—and if current ice melt trends continue, the effect could become dramatic within centuries.
The Moon Isn’t the Only Brake Anymore
The conventional explanation for Earth’s slowing rotation is well-established: the Moon’s gravity pulls on Earth’s oceans, creating tidal friction that gradually drains rotational energy. This process has been lengthening the day by about 1.72 milliseconds per century, according to astronomical models based on precise measurements.
But the new research, published in the Journal of Geophysical Research: Solid Earth, identifies a second, now-dominant mechanism: barystatic effects from melting ice sheets. As glaciers and polar ice sheets lose mass, that water redistributes from land to ocean, shifting Earth’s mass distribution. Because angular momentum must be conserved, this redistribution slows the planet’s spin—like a figure skater extending their arms to reduce rotation speed.
The AI model developed by Shahvandi and Soja integrates paleoclimate reconstructions with physical laws of motion to estimate past day lengths with reduced uncertainty. “The main advantage of employing our algorithm is that it analyzes [day length] in a precise probabilistic framework, providing uncertainty estimates by spanning the model and data spaces,” the authors noted in their peer-reviewed study. This approach allows for more reliable historical reconstruction than previous statistical methods.
The Late Pliocene Warning: A Preview of the Future?
The pivotal moment in the new analysis is the Late Pliocene, approximately 3.6 million years ago. During that epoch, global temperatures were 4 to 5 degrees Fahrenheit warmer than today—levels comparable to projections for 2100 if emissions continue unchecked. Massive ice sheets melted, raising sea levels by about 98 feet (30 meters) according to paleogeological records.
This melt event coincided with a measurable jump in day length. The researchers’ data suggests that the redistribution of mass from continental ice to the oceans significantly altered Earth’s moment of inertia, directly impacting rotation. The rate of lengthening since that period—1.5 milliseconds per century—now serves as a potential analog for what could happen this century under continued climate change.
Notably, atmospheric CO₂ levels during the Late Pliocene, while not anthropogenic, reached concentrations similar to those projected for 2100 under high-emission scenarios. This parallel is not lost on the researchers: if the climate system crosses similar thresholds, the rotational response could mirror past changes.
How Scientists Reconstruct Deep-Time Day Length
Determining day length millions of years ago requires indirect but highly precise methods. The study relies on records from fossilized foraminifera—single-celled marine organisms whose growth layers reflect seasonal and tidal cycles. By analyzing the thickness and chemistry of these layers, scientists can infer the number of days in a year during a given period.
For example, if 365 growth layers are found in a shell formed one year, but the orbital year (Earth’s revolution around the Sun) remains constant, the difference reveals how many hours each day contained. Coral reefs similarly record tidal and daily cycles, offering complementary data.
The AI algorithm used in this study synthesizes these proxy records with sea level and ice sheet reconstructions, filtering out noise and providing probabilistic estimates of day length with quantified confidence intervals—a major improvement over earlier deterministic models.
What This Means for Time, Technology, and the Planet
A 1.5-millisecond-per-century increase sounds trivial. But over geological timescales, it accumulates. Over 3.6 million years, that rate amounts to 54 seconds of additional day length. While still imperceptible in human terms, the rate itself is the concern—because it is now linked to a nonlinear, accelerating process: ice sheet collapse.
Should sea levels rise by meters over the next few centuries, as some models suggest, the corresponding mass redistribution could shorten or lengthen days by milliseconds more rapidly. This has practical implications for high-precision systems like atomic timekeeping, GPS, and astronomical observations, which rely on precise models of Earth’s rotation (UT1). The International Earth Rotation and Reference Systems Service (IERS) already accounts for small variations, but a sustained upward trend would require more frequent adjustments.
More critically, the lengthening of days is not an isolated curiosity—it is a symptom of a planet in climatic flux. The same processes that slow Earth’s spin also disrupt ocean currents, weather patterns, and sea level stability. The rotational signal is a new indicator of planetary-scale energy imbalance.
Key Takeaways
- Earth’s day has lengthened by ~1.5 milliseconds per century over the past 3.6 million years, primarily due to climate-driven ice melt.
- The Late Pliocene, withits 30-meter sea level rise and warm climate, serves as a potential analog for this century’s climate trajectory.
- AI-enhanced paleoclimate modeling now allows more accurate reconstruction of past day lengths, reducing uncertainty in historical climate-rotation links.
- If current ice loss accelerates, day-length changes could become a measurable, disruptive factor for precision timekeeping systems within centuries.
- The rotational slowdown is a secondary but telling symptom of the same mass redistribution driving sea level rise—making it another metric of climate change’s planetary reach.
The researchers emphasize that their findings are not about adjusting clocks tomorrow, but about recognizing a new feedback loop in the Earth system. “Future studies could focus on these aspects of climate dynamics and their impact on [day length],” they stated. As ice sheets continue to retreat, the planet’s spin will respond—a silent, mechanical echo of a warming world.
For continuous, expert breakdowns of how climate change is reshaping the fundamental systems of our planet—from ocean currents to Earth’s rotation—follow onlytrustedinfo.com’s climate science desk. We deliver the fastest, most authoritative analysis of the research that matters, translating complex data into clear, actionable insight.
