Forces operating deep within Earth are subtly speeding up and slowing down the planet’s spin, causing the length of a day to vary by a few milliseconds over the span of decades, according to a study published in Nature. The research, led by University of Alberta doctoral candidate Huifeng Zhang and co‑author Mathieu Dumberry, examines why Earth’s rotation does not remain perfectly constant.
Competing mechanisms inside the planet
The authors propose three interacting mechanisms that can exchange angular momentum between Earth’s inner core and its mantle: gravitational coupling, electromagnetic coupling, and topographic coupling. By fitting statistical models to observational records that cover roughly sixty years, they determined that gravitational coupling between the solid inner core and the overlying mantle accounts for the majority of the observed shift. The electromagnetic and topographic mechanisms act in the opposite direction, partially offsetting the gravitational effect.
Observational data cited in the paper indicate that between the early 1970s and 2021, processes originating in the core altered the length of a day by only a few milliseconds. While such a change is imperceptible in daily life, it provides a rare window into the dynamics of Earth’s deep interior, a region that cannot be observed directly.
Context within broader rotation studies
Scientists have long recognized that Earth’s rotation varies on different timescales. Over short periods, atmospheric winds and ocean currents dominate the variability. Over geological eras, tidal friction with the Moon gradually lengthens days. The intermediate, multidecadal changes examined in this study have been more difficult to explain.
“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 told Gizmodo. The study builds on earlier work from 1988 that linked angular momentum exchanges between core and mantle to rotation changes, narrowing down which interactions are most relevant on decadal timescales.
Implications and future directions
The findings give geophysicists new constraints for modeling Earth’s interior. Because each of the three mechanisms depends on physical properties that remain only partially understood, the results help prioritize which aspects of core‑mantle coupling should be investigated further.
Zhang is also exploring whether the same deep‑Earth processes could explain a separate, six‑year oscillation observed in day length. “Even tiny changes in Earth’s rotation can provide valuable information about processes occurring thousands of kilometers beneath our feet,” she said. By combining improved observations of rotation, inner‑core motion, and the mechanisms outlined in the study, researchers hope to develop a more detailed picture of how the planet’s inner layers interact.
Other forces—such as melting Antarctic ice, sea‑level rise, and large‑scale groundwater extraction—also influence Earth’s rotation by redistributing mass. However, the new research isolates internal dynamical processes as a distinct contributor to the subtle, multidecadal variations in day length.
Steve Lopez is a Senior Editorial Columnist and Health & Public Policy reporter for News Raise. Steve focuses on healthcare advancements, medical technologies, and public health policies.
