For practical purposes, we say that one day—the time it takes for Earth to rotate once on its axis—equates to 24 hours. In reality, different factors across different timescales influence how quickly our planet turns, which scientists have yet to fully understand.
But new findings, published today in Nature, may finally solve at least one of these outstanding mysteries, namely how Earth’s rotation changes every couple of decades. According to the study, the multidecadal changes in Earth’s behavior come from deep inside our planet. Three mechanisms—gravitational, electromagnetic, and topographic coupling—essentially compete with one another to speed up or slow down Earth’s rotation over several decades. Specifically, gravitational coupling, or a gravity “torque” between Earth’s inner core and mantle, creates tiny changes that are pushed back by the other two mechanisms, explained Huifeng Zhang, the study’s first author.
“What I find particularly exciting is that these different pieces of information can come together to provide a more coherent picture of Earth’s deep interior, a region that is extremely difficult to observe directly,” Zhang, a PhD 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 timescale they’re thinking about. For example, shorter-term variations are controlled by changes in the atmospheres and oceans, Zhang explained. On the other hand, tidal interactions with the Moon occur over millions of years. The relative mid-range timescales of several decades—which the new paper covers—remained a mystery for a long time, according to Zhang.
“Our main motivation was therefore to investigate the relative contributions of different core-mantle coupling mechanisms to the observed variations,” she added. To do so, Zhang and her co-author and supervisor, Mathieu Dumberry, referenced previous work from 1988 that demonstrated how exchanges of angular momentum between Earth’s core and mantle could influence Earth’s rotation.
The pair picked out three possible mechanisms that likely influenced this exchange. Gravitational coupling refers to the gravitational interactions between the inner core and the mantle and how quickly the former deforms. Electromagnetic coupling concerns the conductivity and thickness of a specific layer at the base of the mantle. Finally, topographic coupling depends on any irregularities that emerge along the core-mantle boundary.
These models go deep
For the study, Zhang and Dumberry ran statistical models to assess different combinations of these factors against real observations over the past six decades. As a result, they were able to find that gravitational coupling exerted the most influence on Earth’s rotation changes. Meanwhile, the other two mechanisms appeared to serve as a sort of buffer against these changes.
“This suggests that interactions deep inside Earth can 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 aren’t “yet well understood,” the findings also “provide valuable constraints on several properties of Earth’s deep interior,” she 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 tiny but tangible change. Overall, Earth’s days have been growing shorter over time.
Our fickle planet
Again, the latest work deals with variations on the scale of decades. Because so many factors contribute to Earth’s rotational changes, the new study alone is part of a bigger puzzle in understanding the behavior of our ever-changing planet. Currently, Zhang is investigating whether these mechanisms might also have a hand in another observation, or a six-year oscillation in the length of day.
“Even tiny changes in Earth’s rotation can provide valuable information about processes occurring thousands of kilometers beneath our feet,” she said. “By combining these improved observations with measurements of Earth’s rotation and inner core motion, we hope to develop a more detailed picture of how Earth’s deep layers interact and how motions thousands of kilometers beneath the surface subtly change the length of our day.”
