Climate change is making our days longer by slowing the Earth’s rotation. This climate-related braking effect is currently around 1.33 milliseconds per century – and is therefore stronger than at any time in the last 3.6 million years, as researchers have determined. Accordingly, even the climatic capriciousness of the Ice Age changed the length of the earth’s day less quickly than today’s ice melting and the redistribution of water masses it caused.

Earth’s rotation changes

The rotation of the earth forms the basis for our universal time and our 24-hour day. But this rotation does not remain constant: it changes, among other things, due to the influence of the moon, currents in the Earth’s core and mass shifts on the Earth’s surface. For example, the day was 30 minutes shorter in the time of the dinosaurs and has gradually become longer since then. This is primarily due to the braking effect of the moon, whose tidal forces slow down the Earth’s rotation by around 2.4 milliseconds per century.

In parallel, there are other processes that counteract the braking effect of the moon. These include the springing back of the earth’s crust after the end of the last ice age, but also a slower rotation of the earth’s inner core. This paradoxically causes the earth’s outer layers to rotate faster, thereby shortening the length of the day.

Why climate influences the Earth’s rotation

But there is another important influencing factor: the climate-related distribution of water and ice on the earth’s surface. When the ice sheets in the polar regions thaw and the meltwater spreads into the oceans, the mass balance of our planet shifts. Rising sea levels slow down the Earth’s rotation. “This is comparable to a figure skater who turns more slowly with her arms outstretched when she places them close to her body,” explains lead author Mostafa Kiani Shahvandi from ETH Zurich.

Because climate change is causing a particularly large amount of polar ice to melt, this climate-related influence is currently extending the length of the Earth’s day by around 1.33 milliseconds per century – and thus increasing the braking effect of the moon. “However, it remained unclear whether there were previous phases in which the climate increased the length of the day at a similarly rapid rate,” says Kiani Shahvandi. Because there are no precise rotation measurements from hundreds of thousands or millions of years ago.

Review of day lengths over the last 3.6 million years

But there is another way: In order to find out what role the climate played in the rotation of the earth in past times, Kiani Shahvandi and his colleague Benedikt Soja evaluated fossil witnesses from marine sediments. “Using the chemical composition of foraminiferal shells, you can understand the fluctuations in sea levels and mathematically derive the fluctuations in day length,” explains.

For their analyzes of this data, the researchers developed and trained a special AI system, the so-called physics-informed diffusion model: “This deep learning algorithm captures the physics of sea level change, but at the same time is robust to the large uncertainties in the paleoclimate data,” says the team. This made it possible to reconstruct the climate-related share of the Earth’s ration over the last 3.6 million years.

Rate of change has never been higher than today

The result: The strong climate fluctuations in the Ice Age not only caused glaciers to grow and shrink, they also influenced day lengths. This fluctuated by 10 to 30 milliseconds in the rhythm of warm and cold periods – far more than today. However, these fluctuations occurred over periods of tens of thousands of years, as reported by Shahvandi and Soja. Calculated over the course of a century, the climate-related proportion of day lengths during the Ice Age did not change more than today – on the contrary:

“The climate-related rate of change in Earth’s rotation is higher today than at any time in the last 3.6 million years,” the researchers write. Only once, around two million years ago, was the rate of change in day length almost comparable to today. “But never before or since has the planetary ‘figure skater’ raised her arms as quickly as she did between 2000 and 2020,” says Kiani Shahvandi.

What does this mean for the future?

According to the researchers, these results confirm that even during the capriciousness of the Ice Age, the Earth did not warm as drastically in such a short period of time as it does now. “The rapid increase in day length implies that the rate of modern climate change is unprecedented since at least the late Pliocene 3.6 million years ago,” says Soja. Humans are not only heating up the Earth’s climate through their greenhouse gas emissions, they are also slowing down the Earth’s rotation – and thus influencing the length of the day.

Unless processes in the Earth’s core compensate for this, the length of the day could slow even further in the future. “By the end of the 21st century, climate change will have an even greater impact on day length than the moon,” explains Soja. The combined effect of both could cause the Earth’s rotation and atomic clock-based universal time to drift further apart – and need to be adjusted more frequently.