A study published in Nature reveals that Earth’s rotation changes over decades due to gravitational, electromagnetic, and topographic coupling between the planet’s core and mantle, with gravitational torque playing the dominant role.
Scientists have long puzzled over why Earth’s day length fluctuates by milliseconds over decades, but a new study in Nature offers an explanation rooted deep within the planet. The research, led by University of Alberta PhD student Huifeng Zhang and her supervisor Mathieu Dumberry, identifies three mechanisms—gravitational, electromagnetic, and topographic coupling—that interact to alter Earth’s rotation. These processes, driven by forces in the core-mantle boundary, create subtle shifts in the planet’s spin, with gravitational torque emerging as the most influential factor.
How Deep Forces Shape Earth’s Rotation
Electromagnetic and topographic coupling act as counterbalances, moderating the effects of gravitational torque. Electromagnetic interactions depend on the conductivity of the mantle’s lower layer, while topographic coupling arises from irregularities at the core-mantle boundary. 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.
Implications for Understanding Earth’s Interior
These mechanisms also help explain a six-year oscillation in day length, a phenomenon Zhang is now investigating. By combining rotational data with inner core motion, the team aims to refine models of how Earth’s layers interact. 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,
she said.
A Puzzle of Interconnected Forces
While the study focuses on mid-range timescales—decades rather than milliseconds or millions of years—it highlights the complexity of Earth’s rotational dynamics. Shorter-term variations, like those caused by atmospheric and oceanic shifts, remain distinct from the core-driven changes identified here. The research highlights how even small forces deep within the planet can cause ripples that affect the length of a day, altering it by fractions of a second.
The study’s authors stress that these findings are part of a broader effort to understand Earth’s ever-changing behavior. Our main motivation was therefore to investigate the relative contributions of different core-mantle coupling mechanisms to the observed variations,
Zhang said. The work not only advances geophysics but also demonstrates how interdisciplinary approaches—combining observational data, statistical modeling, and theoretical frameworks—can unravel Earth’s hidden rhythms.