Climate change is lengthening our days because rising sea levels slow Earth's rotation. Researchers from the University of Vienna and ETH Zurich now show that the current increase in day length — 1.33 milliseconds per century— is unprecedented in the past 3.6 million years. The team reconstructed ancient day-length fluctuations using the fossil remains of single-celled marine organisms known as benthic foraminifera. The study has just been published in the Journal of Geophysical Research: Solid Earth.

An exact 24-hour day is not a given—day length changes due to gravitational effects of the Moon, as well as various geophysical processes acting within the Earth's interior, at its surface, and in the atmosphere. Today's climate change also affects day length: prior work showed that from 2000 to 2020 our days lengthened by a rate equivalent to 1.33 milliseconds per century due to climate-related factors, especially the continental-ocean mass redistribution due to the melting of polar ice sheets and mountain glaciers.

In a new study, now appearing in the Journal of Geophysical Research: Solid Earth, Mostafa Kiani Shahvandi (University of Vienna) and Benedikt Soja (ETH Zurich) demonstrate that this rapid rise in day length is unparalleled over the last 3.6 million years.

Earth as a figure skater

"In our earlier work, we showed that the accelerated melting of polar ice sheets and mountain glaciers in the 21st century is raising sea levels, which slows Earth's rotation and therefore lengthens the day — similar to a figure skater who spins more slowly once they stretch their arms, and more rapidly once they keep their hands close to their body," explains Mostafa Kiani Shahvandi of the University of Vienna’s Department of Meteorology and Geophysics. "What remained unclear was whether there were earlier periods when climate increased day length at a similarly rapid pace."

Foraminifera as sea-level and day-time indicators

To answer this question, the researchers used the fossilized remains of single-celled marine organisms known as benthic foraminifera. "From the chemical composition of the foraminifera fossils, we can infer sea-level fluctuations and then mathematically derive the corresponding changes in day length," says first author Kiani Shahvandi from the University of Vienna. To draw more robust conclusions, the team employed a probabilistic deep learning algorithm— a physics-informed diffusion model: "This model captures the physics of sea-level change, while remaining robust to the large uncertainties inherent in paleoclimate data," adds the climate scientist and geophysicist.

The result: During the Quaternary (2.6 million years), the growth and melting of large continental ice sheets repeatedly caused significant day-length variations via sea-level changes. Compared with values from the 21st century, however, it is clear that today's increase in day length stands out in the climate history of the past 3.6 million years. "Only one time—around 2 million years ago—the rate of change in length of day was nearly comparable, but never before or after that has the planetary 'figure skater' raised her arms and sea-levels so quickly as in 2000 to 2020", says Kiani Shahvandi.

Rate of day-length change since the late Pliocene is unprecedented

"This rapid increase in day length implies that the rate of modern climate change has been unprecedented at least since the late Pliocene, 3.6 million years ago. The current rapid rise in day length can thus be attributed primarily to human influences," says Benedikt Soja, Professor of Space Geodesy at ETH Zurich. By the end of the 21st century, climate change is expected to affect day length even more strongly than the Moon. Even though the changes are only milliseconds, they can cause problems in many areas, for example in precise space navigation, which requires accurate information on Earth's rotation," Soja notes.

This study by Mostafa Kiani Shahvandi (University of Vienna) and Benedikt Soja (ETH Zurich) is the first to examine fossil archives for research into the history of climate-induced changes in the length of the day. It thus provides a link between past and future climatic effects on the Earth's rotation.

Summary:

• Earth’s day is currently lengthening at about 1.33 ms/century, mainly because climate-driven sea-level rise from melting ice redistributes mass and slows Earth's rotation.
• This rate is unprecedented over the past 3.6 million years.
• Researchers reconstructed past day-length changes by inferring sea-level fluctuations from benthic foraminifera fossils and applying a physics-informed probabilistic diffusion model.
• The findings imply modern climate change has been unprecedented at least since the late Pliocene and that the current day-length rise is primarily human-driven.
• By late 21st century, climate's impact on day length may surpass the Moon’s effect.
• Despite being millisecond-scale, these changes potentially affect precision-dependent systems such as space navigation.

About the University of Vienna:

At the University of Vienna, curiosity has been the core principle of academic life for more than 650 years. For over 650 years the University of Vienna has stood for education, research and innovation. Today, it is ranked among the top 100 and thus the top four per cent of all universities worldwide and is globally connected. With degree programmes covering over 180 disciplines, and more than 10,000 employees we are one of the largest academic institutions in Europe. Here, people from a broad spectrum of disciplines come together to carry out research at the highest level and develop solutions for current and future challenges. Its students and graduates develop reflected and sustainable solutions to complex challenges using innovative spirit and curiosity.