NASA’s Suomi NPP satellite captured this composite image of southern Africa and the surrounding oceans on April 9, 2015. As sea levels in these oceans rise due to climate change, Earth’s rotation gradually slows — lengthening our days by milliseconds each century. Credit: Ocean Biology Processing Group at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
As we age, it starts to feel like our days are getting shorter and shorter, each year passing more quickly than the last. But according to a new study, the opposite is true. Thanks to climate change and rising sea levels, days on Earth are actually lengthening at a rate of about 1.33 milliseconds per century.
The study, published March 10 in the Journal of Geophysical Research: Solid Earth, used microscopic marine fossils and a deep-learning algorithm toreveal not only that the Earth’s spin is slowing, but that it is slowing at a faster rate than at any point in the last 3.6 million years —- imperceptible to us, but nearly unprecedented in Earth’s recent geological past.
The speed of the change is what alarmed researchers ”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,” said Benedikt Soja, study co-author and professor of space geodesy at ETH Zurich, in a press release.
How melting ice slows a spinning planet
The authors already knew from previous studies that melting glaciers and ice sheets were causing sea levels to rise and, therefore, days to lengthen. As water from melting ice spreads across the world’s oceans, Earth’s mass redistributes away from its rotational axis — increasing what physicists call the moment of inertia and, by the law of conservation of angular momentum, slowing the planet’s spin. Mostafa Kiani Shahvandi, a geophysicist at the University of Vienna and the study’s lead author, put it simply: It’s like a figure skater “who spins more slowly once they stretch their arms, and more rapidly once they keep their hands close to their body.” What the authors didn’t know was whether this observed increase in day length had a historic precedent.
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Days on Earth have never been exactly 24 hours long. Numerous factors influence day length, from the effects of the Moon’s gravity to Earth’s orbital dynamics, along with geological and atmospheric processes. These same forces can even cause days to shorten. In fact, since 1972, Earth has actually been speeding up, making days slightly shorter than 24 hours. Despite these short-term fluctuations, however, the dominant trend is a gradual slowing caused primarily by tidal effects from the Moon. This study specifically isolates the climate-driven component of that slowing, asking whether the pace of change caused by rising seas has any precedent in recent history.
Reading the ocean’s fossil record
The researchers analyzed the fossilized remains of benthic foraminifera, a single-celled marine organism that evolved during the Cambrian Period some 540 million years ago. Foraminifera, which live on the seafloor across the full range of marine environments — from the shallowest coastal waters to the deepest ocean trenches — are enclosed in a shell called a test, which is very sensitive to changes in the marine environment. For this reason, scientists can analyze fossilized foraminifera tests to determine information about past changes to the climate, such as sea level or temperature changes. “From the chemical composition of the foraminifera fossils, we can infer sea-level fluctuations and then mathematically derive the corresponding changes in day length,” said Shahvandi.
Reconstructing the past
Next, the team took their work a step further, developing the so-called Physics-Informed Diffusion Model (PIDM), a probabilistic deep-learning algorithm designed to account for the large uncertainties inherent in paleoclimate data. Diffusion models are the same class of AI that powers image generators like DALL-E: They learn to reconstruct a clear signal from noise. What makes this model different is the “physics-informed” part — unlike a standard diffusion model, a PIDM is penalized during training when its outputs violate physical laws. The result, in this case, is a model that reconstructs plausible climate histories from incomplete fossil records while remaining grounded in the known physics.
The study used the PIDM to reconstruct changes in day length over the past 3.6 million years. Previous work in the field has only reconstructed day-length changes as far back as the Common Era (starting in the year 0 C.E.). What the study found is that while day length has always fluctuated, the recent human-influenced rate of increase is unprecedented across the last 3.6 million years. Day length is currently growing at a rate of 1.33 milliseconds per century. “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,” Shahvandi said.
Ultimately, the study suggests that the rapid increase in length of day rate change is direct evidence that modern climate change is without equal in at least 3.6 million years — and that humans are primarily responsible. By the end of the 21st century, that influence is expected to grow stronger yet, surpassing even the Moon’s effect on Earth’s rotation and potentially interfering with critical infrastructure like GPS, which requires precise data on the Earth’s position relative to its satellites. ”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 said.
