Melting ice sheets and rising seas are redistributing enough mass across the planet to measurably slow Earth’s rotation, stretching the length of each day at a rate not seen in 3.6 million years. A peer-reviewed study published in the Journal of Geophysical Research: Solid Earth by researchers at the University of Vienna found that since roughly the year 2000, climate-driven changes have been adding approximately 1.33 milliseconds per century to the length of a day. That figure may sound trivial, but it is already outpacing natural geological forces that have governed Earth’s spin for millions of years, and projections suggest the effect could roughly double by the end of this century under high-emissions scenarios.
How Melting Ice Reshapes Earth’s Spin
The physics behind the slowdown is straightforward. When ice locked at the poles in Greenland and Antarctica melts and flows into the ocean, water mass shifts toward the equator. That outward redistribution increases Earth’s moment of inertia, the rotational equivalent of a figure skater extending their arms to slow a spin. The effect is small on any given day but accumulates over decades into a measurable signal that scientists can now isolate from other forces acting on the planet’s rotation.
The University of Vienna team, led by researcher Mostafa Kiani Shahvandi, reconstructed how the length of a day has varied over the past 3.6 million years by combining geological records with modern geodetic observations. Their analysis, published in the journal’s solid Earth section, concluded that the current rate of day lengthening stands out sharply against that deep-time backdrop. Before the industrial era, tidal friction from the Moon was the dominant long-term driver of rotational slowing. Climate-related mass redistribution has now emerged as a competing force on timescales that matter for precision timekeeping and satellite navigation.
A Post-2000 Acceleration
Multiple independent research teams have converged on the same finding from different angles. A study in the PNAS journal documented a clear post-2000 acceleration in length-of-day variability, attributing the shift to the growing influence of anthropogenic climate change. The researchers found that human-caused factors are increasingly dominating the signal, overtaking shorter-term natural cycles such as ocean circulation shifts and atmospheric wind patterns that have historically driven year-to-year fluctuations.
The convergence of evidence matters because it rules out the possibility that any single dataset or methodology is producing an artifact. Satellite gravity measurements, tide-gauge records, and geodetic time series all point in the same direction: the planet’s spin is responding to the massive transfer of frozen water from high latitudes into the global ocean. That agreement across independent lines of evidence is what allows scientists to speak with growing confidence about the role of climate change in altering Earth’s rotation.
Projections Point to a Doubling Effect
NASA-funded work has put concrete numbers on where this trend is heading. Under scenarios with continued high greenhouse gas emissions, the rate of day lengthening could reach approximately 2.62 milliseconds per century by 2100, nearly double the current pace. Under lower-emissions pathways, the acceleration would be more modest, but the direction of change remains the same regardless of scenario.
The difference between those projections hinges largely on how much additional ice Greenland and Antarctica lose in the coming decades. Groundwater depletion, another consequence of human activity, also contributes to mass redistribution away from continents and toward the oceans, adding a secondary but measurable push on the rotational budget. The NASA analysis links these combined climate-driven shifts not only to the length of the day but also to polar motion, the slow wobble of Earth’s rotational axis relative to its surface geography, which can subtly shift the locations of the geographic poles over time.
Real Consequences for Global Timekeeping
A millisecond per century sounds abstract, but it creates a concrete engineering problem. Coordinated Universal Time, or UTC, the global standard that synchronizes everything from financial transactions to GPS satellites, is periodically adjusted with “leap seconds” to keep atomic clocks aligned with Earth’s actual rotation. A separate study published in Nature found that polar ice melt is altering Earth’s angular velocity enough to affect the timing of a potential first “negative leap second,” an unprecedented subtraction of a second from UTC that timekeeping authorities had been anticipating.
The practical stakes are not trivial. Telecommunications networks, stock exchanges, and distributed computing systems all depend on sub-second synchronization. A news article in Nature examining the timekeeping implications noted that the interaction between Earth’s liquid core, which has its own rotational dynamics, and the climate-driven surface changes creates competing influences on rotation that complicate predictions about exactly when such adjustments will be needed. The core has been slightly speeding up Earth’s spin in recent decades, partially offsetting the climate-driven slowdown, but that internal process operates on its own timeline and cannot be counted on as a permanent counterweight.
Timekeeping agencies and standards bodies are already debating how to handle these emerging complexities. Proposals under discussion include redefining how leap seconds are implemented or even allowing a larger drift between atomic time and Earth rotation before corrections are made. As the Nature coverage and its associated access portal make clear, the technical community is treating the prospect of a negative leap second as more than a curiosity: it is a stress test for the infrastructure that underpins the digital economy.
What Most Coverage Gets Wrong
Much of the public discussion around this research frames the slowdown as a curiosity, a quirky side effect of warming that has no bearing on daily life. That framing misses the deeper signal. The fact that a single species’ activity is now altering a planetary-scale geophysical parameter, one that has been governed by gravitational and geological forces for billions of years, represents a qualitative shift in the relationship between human civilization and the Earth system. It is not that anyone will notice a longer day. It is that the same mass redistribution driving the rotational change is simultaneously raising coastlines, shifting ocean circulation, and altering weather patterns in ways that affect hundreds of millions of people.
The rotational signal also serves as an independent check on climate models. If models predict a certain amount of ice loss and sea-level rise, the resulting change in Earth’s spin provides a separate, physically distinct way to verify whether those predictions match reality. When teams supported by NASA and partner universities find that observed rotational changes align with modeled expectations, it strengthens confidence in projections of future sea-level rise and polar ice stability. Conversely, if the observed slowdown diverged from model-based predictions, it would be a red flag that key processes in ice dynamics or ocean circulation were being misrepresented.
There is also a communication challenge. Headlines that emphasize “longer days” risk trivializing what is fundamentally a story about mass, momentum, and the scale of human influence. A more accurate framing would highlight that the Earth system is now so tightly coupled to human behavior that burning fossil fuels and pumping groundwater can be detected not just in atmospheric chemistry or sea-level gauges, but in the very rate at which the planet turns beneath our feet. That realization underscores the extent to which industrial activity has become a geophysical force in its own right.
A Planet Responding to Human Choices
For most of Earth’s history, changes in rotation were driven by slow astronomical and tectonic processes far beyond the reach of any organism. The new research shows that this is no longer entirely true. Human choices about energy, land use, and water management are now part of the rotational ledger, nudging the length of the day in ways that can be quantified and forecast. Those nudges are small compared with the ancient tug of the Moon, but they are large enough to matter for the technologies and systems modern societies rely on.
Seen in that light, the millisecond-per-century slowdown is less a curiosity than a symptom. It is one more metric, alongside global temperature and sea level, that tracks how rapidly and profoundly the climate system is changing. And like those other metrics, it points to the same conclusion: the physical foundations of our world, from coastlines to clocks, are now in motion in response to human activity, and will continue to shift depending on the choices made over the coming decades.
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*This article was researched with the help of AI, with human editors creating the final content.
