Climate change is subtly altering one of the most fundamental rhythms of our planet: the length of a day. New research from scientists at the University of Vienna and ETH Zurich shows that rising sea levels caused by melting ice sheets are slowing Earth’s rotation, causing days to grow slightly longer. Although the changes are extremely small—measured in milliseconds—the rate at which they are occurring is unprecedented in at least the past 3.6 million years.
An exact 24-hour day might seem constant, but in reality the length of a day has always fluctuated. Natural forces influence Earth’s rotation, including gravitational interactions with the Moon, movements within Earth’s mantle and core, atmospheric circulation, and shifts in ocean mass. These processes redistribute mass around the planet, altering its rotational speed in subtle ways. Historically, these changes occurred gradually over long geological time scales.
In the modern era, however, climate change has introduced a new and powerful influence. As global temperatures rise, polar ice sheets and mountain glaciers are melting at accelerated rates. The resulting meltwater flows into the oceans, raising sea levels and redistributing mass from the continents toward the equator. This shift in mass slows Earth’s rotation through a process similar to the physics of a spinning figure skater. When a skater extends their arms outward, their rotation slows because mass moves farther from the center of rotation. Likewise, when melting ice spreads water across the oceans, the planet’s rotational speed decreases slightly, lengthening the day.
Previous research already suggested that climate-driven mass redistribution lengthened days between 2000 and 2020 at a rate of approximately 1.33 milliseconds per century. The new study set out to determine whether such rapid changes had occurred before in Earth’s recent geological past. To answer this question, the research team analyzed fossils of benthic foraminifera—microscopic single-celled marine organisms that live on the seafloor. The chemical composition of these fossils preserves information about past ocean conditions, including sea-level fluctuations.
By reconstructing ancient sea-level changes from foraminifera fossils, scientists were able to estimate corresponding variations in day length over millions of years. The researchers then used an advanced probabilistic deep-learning technique known as a physics-informed diffusion model to interpret the data. This approach allowed them to account for uncertainties in paleoclimate records while capturing the underlying physics of sea-level change.
The results revealed that modern changes stand out dramatically in the geological record. During the Quaternary period, which spans the past 2.6 million years, large ice sheets repeatedly expanded and melted during glacial cycles. These processes did affect day length through sea-level changes, but none of those variations matched the rapid pace observed in the early 21st century. Only once—around two million years ago—did day-length changes approach a similar rate, but even then they did not exceed modern values.
According to the researchers, this means the current increase in day length is the fastest observed since the late Pliocene, roughly 3.6 million years ago. In other words, the recent acceleration in changes to Earth’s rotation appears closely linked to human-driven climate change.
Although these changes amount to only milliseconds, they have practical implications. Precise measurements of Earth’s rotation are essential for satellite navigation, space missions, and global timekeeping systems. Even tiny variations must be carefully tracked to maintain accuracy in technologies that rely on precise positioning and orbital calculations.
The study highlights an unexpected connection between climate change and planetary physics. Human-driven warming is not only reshaping ecosystems and coastlines—it is subtly altering the fundamental mechanics of our planet, influencing the speed of Earth’s rotation itself.
https://phys.org/news/2026-03-climate-earth-unprecedented-million-years.html