A study conducted by the University of Vienna and ETH Zurich reveals that the sea-level rise instigated by global warming is slowing Earth’s rotation at a pace not witnessed since the Late Pliocene.
The length of a day is not static. Various influences, ranging from the Moon’s gravitational pull to complex internal planetary movements, continuously modify the time required for the Earth to complete one full turn on its axis.
Now, research published in the Journal of Geophysical Research: Solid Earth introduces a novel and concerning element into this equation: human-induced climate alteration is affecting the planet’s spin at a rate unparalleled in the last 3.6 million years.
Authored by Mostafa Kiani Shahvand of the University of Vienna’s Department of Meteorology and Geophysics and Benedikt Soja, Professor of Space Geodesy at ETH Zurich, the investigation confirms that the present lengthening of the day—estimated at 1.33 milliseconds per century—has no historical precedent in recent geological history.
To arrive at this conclusion, the scientists had to reconstruct past variations utilizing an indirect yet remarkably dependable technique: analyzing the fossilized remains of single-celled marine organisms known as benthic foraminifera.
The physical reason behind this phenomenon is quite straightforward, with researchers drawing an analogy to the movement of an ice skater. When a skater extends their arms, their spin slows down; when they pull them in, they spin faster. In Earth’s case, the accelerated melting of polar ice sheets and mountain glaciers transfers enormous quantities of water from the continents into the oceans.
This mass redistribution, which results in rising sea levels, alters the planet’s weight distribution and acts like the skater extending their arms, slightly decelerating the rotation and consequently increasing the duration of our days.
The team led by Kiani Shahvand and Soja sought to determine if a similar, climate-related event of comparable magnitude had ever occurred before. To answer this, they turned to benthic foraminifera.
The chemical makeup preserved in the shells of these bottom-dwelling organisms serves as a historical record of ocean levels. Based on these sea-level fluctuations, the scientists could mathematically ascertain the corresponding changes in day length over millions of years.
To account for the considerable uncertainty inherent in paleoclimatic data, the team employed a probabilistic deep-learning algorithm—a physics-informed diffusion model capable of reliably simulating the dynamics of sea-level change. The model’s findings are compelling: throughout the Quaternary period, the last 2.6 million years, the cycles of growth and melt of the great continental ice sheets caused substantial variations in day length. However, comparing this historical data with modern measurements leads to the conclusion that the rates of day length increase observed between 2000 and 2020 are exceptional.
During the period under review, there was only one instance, roughly two million years ago, where the rate of day length change was nearly comparable, explains Kiani Shahvand. Yet, neither before nor after that occasion did the planetary “skater” expand their “arms”—meaning sea levels had not risen as rapidly as they have in the first two decades of the 21st century. The research frames this event as unique over the last 3.6 million years, dating back to the Late Pliocene.
This unprecedented acceleration leads the researchers to point directly to human activity as the principal cause. The speed of the lengthening day indicates that the current pace of climate change is unparalleled, at least since the Late Pliocene era. Therefore, the current, rapid increase in day length can primarily be attributed to human influence, states Benedikt Soja.
Although this deviation might seem minor—we are talking about milliseconds—its practical significance is quite substantial, particularly in a world that is highly interconnected and technologically dependent. For instance, high-precision space navigation systems require exact knowledge of Earth’s rotation for proper functionality. Even a small timing discrepancy can lead to errors in positioning and synchronization.
The study’s projections suggest that by the close of the 21st century, the impact of climate change on day length could surpass the gravitational effect of the Moon itself. This work, which opens a new avenue of research utilizing fossil archives to explore the history of climate-driven changes in Earth’s rotation, establishes a direct link between past and future climatic consequences, confirming that human activities are altering not only the atmosphere or the oceans but the very motion of the planet.
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