For practical purposes, we say that one day (the time it takes for the Earth to rotate once on its axis) is equivalent to 24 hours. In reality, different factors on different time scales influence how fast our planet spins, something scientists haven’t fully understood yet. But new findings, published today in Nature, may finally solve at least one of these outstanding mysteries: how the Earth’s rotation changes every two decades. According to the study, multi-decade changes in Earth’s behavior come from within our planet. Three mechanisms (gravitational, electromagnetic, and topographical coupling) essentially compete with each other to speed up or slow down the Earth’s rotation over several decades. Specifically, gravitational coupling, or a gravitational “torque” between Earth’s inner core and the mantle, creates small changes that are rejected by the other two mechanisms, explained Huifeng Zhang, first author of the study. “What I find particularly exciting is that these different pieces of information can come together to provide a more coherent picture of the Earth’s deep interior, a region that is extremely difficult to observe directly,” Zhang, a doctoral student at the University of Alberta in Canada, told Gizmodo. Timely Changes Scientists studying Earth’s rotation will likely investigate different factors, depending on the time scale they are thinking about. For example, short-term variations are controlled by changes in atmospheres and oceans, Zhang explained. On the other hand, tidal interactions with the Moon occur over millions of years. According to Zhang, the relatively average multi-decade time scales, which the new paper covers, remained a mystery for a long time. “Therefore, our main motivation was to investigate the relative contributions of different core-mantle coupling mechanisms to the observed variations,” he added. To do so, Zhang and his co-author and supervisor, Mathieu Dumberry, referenced earlier work from 1988 that demonstrated how angular momentum exchanges between Earth’s core and mantle could influence Earth’s rotation. The pair chose three possible mechanisms that likely influenced this exchange. Gravitational coupling refers to the gravitational interactions between the inner core and the mantle and the speed with which the former deforms. Electromagnetic coupling refers to the conductivity and thickness of a specific layer at the base of the mantle. Finally, topographic coupling depends on any irregularities that arise along the core-mantle boundary. These models run deep For the study, Zhang and Dumberry ran statistical models to evaluate different combinations of these factors against real observations over the past six decades. As a result, they were able to discover that gravitational coupling exerted the greatest influence on Earth’s rotation changes. Meanwhile, the other two mechanisms seemed to serve as a kind of buffer against these changes. “This suggests that interactions deep within the Earth may play an important role in speeding up or slowing down its rotation over several decades,” Zhang explained. Because each mechanism depends on physical properties that are “not yet well understood,” the findings also “provide valuable constraints on several properties of the Earth’s deep interior,” he added. Observational data show that between the early 1970s and 2021, core-driven mechanisms very slightly altered the length of the day by several milliseconds, a small but tangible change. In general, Earth’s days have been getting shorter over time. Our fickle planet Once again, the latest work addresses variations on the scale of decades. Because there are so many factors that contribute to Earth’s rotation changes, the new study alone is part of a larger puzzle in understanding the behavior of our ever-changing planet. Zhang is currently investigating whether these mechanisms could also influence another observation, or a six-year oscillation in day length. “Even the smallest changes in the Earth’s rotation can provide valuable information about processes occurring thousands of kilometers beneath our feet,” he said. “By combining these improved observations with measurements of Earth’s rotation and the motion of the inner core, we hope to develop a more detailed picture of how Earth’s deep layers interact and how motions thousands of kilometers below the surface subtly change the length of our day.”
