A new study combines seismology, geomagnetism, and core-flow modeling to show gravitational torques between the inner core and mantle drive multidecadal changes in Earth's rotation, altering the length of day by a few milliseconds.

Over a 70-year cycle, the inner core is proposed to oscillate about 2.35 degrees relative to the mantle, with a perceptible shift around 2010 that makes the core appear to move backward from the mantle’s point of view.

The variation appears to originate from interactions among the inner core, outer core, and mantle, which slowly modulate rotational dynamics across decades.

Context is built on a Nature study, with prior work by Yang and Song (2023) and university communications to frame the discovery.

Researchers Huifeng Zhang and Mathieu Dumberry used statistical models and six decades of data to quantify the contributions of electromagnetic, topographic, and gravitational coupling.

Electromagnetic coupling involves a MAP estimate around 1.4×10^19 N·m, a relaxation time near 10 years, and Kem around 1.08; topographic coupling can reach up to 4.2×10^19 N·m with a relaxation time of roughly 1.7–21.5 years and Ktop up to about 0.83, all yielding similar long-term fits.

The work builds on a 2023 Peking University study that suggested a 70-year rhythm and provides a physical mechanism behind the slow inner-core motion.

Model constraints point to a relatively low degree-2 geoid/topography at the CMB and a shallow, low-viscosity interaction zone between outer and inner core, aligning with LLVPs and mantle pPv dynamics.

A Bayesian inversion over torque strength, core relaxation time, and CMB coupling parameters identifies best-fit values that reproduce the 1964–2019 multidecadal changes in length of day, with gravitational torque generally dominating.

Current models predict decadal-scale changes but struggle to resolve shorter intervals due to data and resolution limits.

Findings illuminate the geodynamo and geomagnetic field evolution, since core deformations influence outer-core fluid motion and magnetic behavior.

The multidecadal length-of-day changes arise from a balance where gravitational torque drives the trend and CMB torques resist, with gravity leading during the 1964–2019 window.