Gravitational torque drives multidecadal variations in length of day.

Gravitational torque from the Earth's inner core primarily drives multidecadal variations in the length of day, outweighing electromagnetic and topographic effects.

• Why it matters: Understanding the mechanisms behind LOD fluctuations is crucial for insights into Earth's internal dynamics and core-mantle interactions, which influence planetary rotation and geophysical processes.
• What they did: The study used seismically reconstructed inner core rotation and magnetic field-derived core flows to analyze torque histories, highlighting gravitational torque as the main driver over decades.
• The result: Findings support models of a low-viscosity inner core and a thermochemically complex lowermost mantle, advancing knowledge of Earth's deep interior and its influence on planetary rotation.