A new Nature study says long-term changes in Earth’s day length stem partly from a gravitational tug-of-war between the inner core and mantle, causing tiny accelerations and slowdowns in planetary rotation.
The mechanism hinges on the inner core rotating at a different speed from the rest of Earth and becoming slightly offset from mantle mass irregularities, with gravity pulling it back toward a stable position.
Using seismic data on inner-core rotation and models of liquid-metal flow in the 2,250-kilometer-thick outer core, researchers reconstructed rotation changes from 1964 to 2019 and matched both their timing and size.
The authors say atmospheric and oceanic shifts, earthquakes and glacier melt still affect day length, but those factors explain only part of the observed variation over longer periods.
Researchers added that the new mechanism is probably not the sole driver, leaving room for other deep-Earth processes in future work on how the planet’s interior shapes rotation.