Earth’s rotation is subtly shifting, changing the length of a day by milliseconds over decades as forces deep inside the planet speed up and slow down its spin. According to a study published Wednesday in Nature, geophysicists have found that a long-running question about multidecadal changes in Earth’s day length can be explained by a gravitational tug-of-war inside the core and mantle.
How Gravitational Coupling Drives Multidecadal Day-Length Variations
The planet’s rate of rotation changes gradually over time, resulting in variations in day length on the order of a few milliseconds between the early 1970s and 2021. Researchers Huifeng Zhang, a University of Alberta doctoral candidate, and Mathieu Dumberry, a geophysicist at the same university, analyzed these shifts by comparing statistical models with observational records spanning roughly six decades. Their findings show that gravitational coupling between the solid inner core and the rocky mantle produces most of the shift in day length.
The Geophysical Tug-of-War Inside Earth’s Metallic Core
While gravity pulls dense regions of the inner core toward dense regions of the mantle, two opposing mechanisms push back. The study identifies electromagnetic coupling—where the magnetic field generated by the outer core attracts iron-rich parts of the mantle—and topographic or mechanical coupling—where the flow of the molten outer core pushes against bumps and bulges at the mantle boundary—as forces driving in the opposite direction. Dumberry noted that researchers previously knew the core was responsible for these decadal changes, but the exact mechanical forces involved remained unclear until now.
Comparing Short-Term Surface Forces With Deep-Earth Mechanics
Scientists have long understood that Earth’s rotation varies depending on the timescale being studied. Over short periods, the atmosphere and oceans play a major role in altering day length, while millions of years of tidal interactions with the moon drive long-term changes. Middle-range shifts unfolding over several decades, however, proved much harder to isolate. Zhang pointed out that combining these improved observations with measurements of Earth’s rotation and inner core motion provides a clearer picture of a region thousands of kilometers beneath the surface that is otherwise impossible to observe directly.
Frequently Asked Questions About Earth’s Rotating Core
Will milliseconds of change affect daily life?
A difference of a few milliseconds will not impact daily life, but it gives scientists valuable insight into how the planet’s deep interior layers interact over time.
How do researchers measure changes in Earth’s rotation?
Geophysicists track these small fluctuations by observing Earth’s position relative to distant cosmic objects and by utilizing seismic waves from earthquakes to measure the inner core’s rate of rotation.
What forces are competing inside the planet?
According to the University of Alberta study, gravitational coupling drives the main shift in day length, while electromagnetic and mechanical torque push in the opposite direction.