A new study published in the scientific journal Nature has finally found the answer to a mystery that has puzzled scientists for decades about the changes in the speed of Earth's rotation every few decades.
In practical terms, we say that one day equals 24 hours, but in reality, the speed of the Earth's rotation is affected by different factors over varying periods of time.
The new study suggests that these multi-decade changes come from deep within our planet, where three mechanisms – gravitational, electromagnetic, and topographic coupling – compete with each other to speed up or slow down the Earth's rotation.
Specifically, the gravitational coupling, or gravitational "momentum," between Earth's inner core and mantle creates small changes that are then pushed back by the other two mechanisms, explained Huifeng Zhang, the study's first author and a doctoral student at the University of Alberta in Canada.
"What I find particularly exciting is that this diverse information can be put together to provide a more coherent picture of the deep interior of the Earth, an area that is extremely difficult to observe directly," Zhang says.
Scientists studying Earth's rotation rely on different factors depending on the timescale. Short-term changes are controlled by atmospheric and oceanic variations, while tidal interactions with the Moon occur over millions of years. The medium-term scale (a few decades) has long remained a mystery. To investigate this, Chang and her supervisor, Mathieu Dampri, revisited earlier work from 1988 that showed how exchanges of angular momentum (a physical quantity that expresses an object's ability to continue rotating) between Earth's core and mantle can affect the Earth's rotation. They selected three possible mechanisms: gravitational coupling, which refers to the gravitational interactions between the inner core and mantle and how quickly the former deforms; electromagnetic coupling, which relates to the conductivity and thickness of a particular layer at the base of the mantle; and topographic coupling, which depends on any irregularities along the core-mantle boundary.
The researchers ran statistical models to evaluate different combinations of these factors against real observations over the past six decades, and found that gravitational coupling had the most impact on changes in Earth's rotation, while the other two mechanisms appeared to act as a barrier against these changes.
Zhang says, "This suggests that interactions deep within the Earth could play an important role in speeding up or slowing down its rotation over several decades."
Observational data shows that between the early 1970s and 2021, core-driven mechanisms altered the length of the day very slightly, by a few milliseconds—a tiny but noticeable change. Overall, Earth's days have been getting shorter over time.
Zhang is currently investigating whether these mechanisms also influence the six-year oscillation in the length of the day. She says, "Even tiny changes in the Earth's rotation can provide valuable information about processes happening thousands of kilometers beneath our feet."
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