Gravitational torque drives multidecadal variations in length of day

Fluctuations in the length of day (LOD) on decadal timescales are caused primarily by an exchange of angular momentum between the Earth's mantle and core^1-3. Several mechanisms have been proposed to explain this exchange, including electromagnetic^4-7 and topographic^8-10 coupling at the core-mantle boundary (CMB) and a gravitational torque by the inner core^11,12. However, the precise nature of the core-mantle torque remains unknown. Here we show that the seismically reconstructed differential rotation of the inner core^13-15 and core flows derived from magnetic field changes^16,17 suggest that the multidecadal LOD changes are driven primarily by the gravitational torque and resisted by electromagnetic and topographic torques, consistent with results from Earth-like dynamo models¹⁸. Our reconstructed torque histories, although tied to the accuracy of the inner core rotation and core flow models, support a lowermost mantle that features near-neutrally buoyant thermochemical piles^19-21, a post-perovskite (pPv) phase with a low viscosity²² and a highly conducting^23,24 iron-enriched layer a few kilometres thick at its base²⁵. Our results also suggest a low-viscosity inner core deforming in only a few years^26-28 and yield an upper limit on the stratification at the top of the fluid core. Altogether, our study contributes to bringing into focus an emerging picture of the deepest regions of our planet.