The Earth’s rotation has never been perfectly stable; the spin has changed significantly throughout history. Even slight changes on the planet, from melting ice sheets to flux in the Moon’s gravitational effects, can make days longer or shorter. But for most of Earth’s history, the changes were tiny and driven mostly by natural forces. Now, a new study shows that modern, human-induced climate change is accelerating this effect, lengthening days at “unprecedented” rates.
“The length of day is actually increasing because sea level is rising.” Mostafa Kiani Shahvandi, a geoscientist at the University of Vienna, told Refractor in an interview. “Sea level rises, and because of this, Earth’s rotation slows down, and the length of the day increases.”
Shahvandi said that recent studies have shown how different geological processes affect Earth’s rotation. “I noticed that climate is increasingly affecting Earth’s rotation,” he adds. Basically, the warmer the planet gets thanks to climate change accelerated by human activities, the more its ice sheets melt, and the higher the sea levels climb.
To check whether the effect has been constant over time or become more pronounced in the 21st century, Shahvandi and his colleagues used paleoclimatic data from fossils and coral reefs to train a new deep learning model. The scientist told us that this method can handle noisy paleoclimatic data and reconstruct the history of climate-induced changes in the Earth’s rotation rate.
The reconstructions showed that the formation and the melting of the continental ice sheets caused notable fluctuations in the Length of Day (LOD), ranging from 10 to 30 milliseconds. Shahvandi explains that this amplitude (the total volume of the change) is large, but it happened over millions of years — meaning the rate of the change was slow.
Once in many millions
However, in the 21st century, the rate at which our days are lengthening is the highest seen in the past 3.6 million years, even if the amplitude is small. This effectively means that the LOD is increasing very rapidly in the modern world, he told us. In fact, the rate has reached 1.33 milliseconds per century, a rate not seen over the past 3.6 million years.
Shahvandi says that when ice sheets and mountain glaciers melt, the sea level increases, and the mass is redistributed towards the equator. This causes the Earth to oblate — flatten at the poles and bulge at the equator. With this escalated oblateness, the rotation gets slower, and the length of the day increases. Think of an ice skater who throws their arms out to slow their rotation during a spin versus bringing their arms in tightly to the body to increase speed.
“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 Shahvandi.
If the Earth’s rotation slows down significantly, the mismatch between astronomical time and atomic time will grow, requiring us to add or subtract leap seconds from our clocks more frequently to keep them aligned. This could be detrimental to computer networks, says Shahvandi, as they are all based on a 24 hour LOD. He further adds that this variation may bring along many uncertainties in space navigation because it requires precise knowledge of the planet’s rotation.
As for the impact on the planet’s significant bodies of water, “the amplitude is very small; it still cannot affect the ocean circulation,” Shahvandi concludes.
The study has been published in the Journal of Geophysical Research: Solid Earth.