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We take it for granted that a day lasts 24 hours – but in reality, the length of a day is not constant. New research from the University of Vienna and ETH Zurich now shows that climate change is currently contributing to the lengthening of the day more strongly than at any time in at least 3.6 million years.
At present, a day is getting longer by about 1.33 milliseconds per century. That may sound negligible, but from a geophysical perspective it is remarkable – and, according to the researchers, a clear signal of human‑driven climate change. The study was published in the Journal of Geophysical Research: Solid Earth.
When Earth Becomes a “Figure Skater”
Earth’s rotation is influenced by several factors: the gravitational pull of the Moon, processes inside the planet, and changes on its surface. A key factor is how mass is distributed across the globe.
“You can imagine Earth as a figure skater,” explains lead author Mostafa Kiani Shahvandi from the University of Vienna’s Department of Meteorology and Geophysics. “When the skater stretches out their arms, they spin more slowly – when they pull them in, they rotate faster.”
As polar ice sheets and mountain glaciers melt, sea levels rise. Large amounts of water shift from the continents into the oceans. This redistribution changes Earth’s moment of inertia – and slows its rotation. The result: days become longer.
Earlier studies had already shown that climate change in the 21st century has measurable effects on day length. But until now, it was unclear whether there were periods in Earth’s history when day length changed at a similar rate.
Fossils as a Climate Archive
To answer this question, the research team looked deep into Earth’s past. They analyzed fossil remains of benthic foraminifera – single‑celled marine organisms that lived on the seafloor.
The chemical composition of their calcareous shells provides clues about past sea levels. Using these data, the scientists reconstructed fluctuations in global water masses and mathematically derived the resulting changes in Earth’s rotation.
Their analysis was supported by a “physics‑informed diffusion model,” a probabilistic deep‑learning algorithm that combines physical knowledge about sea‑level changes with statistical methods and is robust against uncertainties in paleoclimate data.
Rapid Increase in Day Length: Unmatched for 3.6 Million Years
The results show that during the Quaternary Ice Age – the last 2.6 million years – day length fluctuated significantly due to the growth and melting of large ice sheets.
But today’s rate of change stands out clearly. Only once, around two million years ago, was the rate even roughly comparable. “Never before or after has the planet’s ‘figure skater’ raised her arms and sea level as quickly as between 2000 and 2020,” says Kiani Shahvandi.
According to Benedikt Soja, Professor of Space Geodesy at ETH Zurich, the current development is unprecedented at least since the late Pliocene, 3.6 million years ago. The rapid increase in day length can therefore be attributed primarily to human influence.
Climate Change Leads to Milliseconds That Matter
By the end of the 21st century, the influence of climate change on day length could even surpass that of the Moon, which has traditionally been considered the most important external factor affecting Earth’s rotation.
Even though we are talking about mere milliseconds, these changes are not trivial. Precision‑critical systems – such as satellite navigation or space research – rely on exact information about Earth’s rotation. Even tiny deviations can have significant consequences.
This new study is the first to systematically use fossil archives to reconstruct climate‑related changes in day length over millions of years. It bridges Earth’s past and present – and shows that climate change affects not only temperature and sea level, but even the rhythm of our planet.
- Hector Pascua with reperences for vienna.at/picture: pixabay.com
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