“We’ve been arguing about this for 20 years, and I think this nails it,” said Dr. John Vidale, Dean’s Professor of Earth Sciences at the University of Southern California. Those are his words, a rare moment of agreement in a decades-long scientific argument: Earth’s solid inner core, a sphere of iron and nickel buried some 3,220 miles deep beneath the surface, has not only slowed down but has now reversed direction relative to the mantle.
The new findings, built on meticulous analysis of seismic waves from 121 repeating earthquakes in the South Sandwich Islands between 1991 and 2023, confirm that the inner core’s spin changes follow a roughly 70-year oscillation cycle. A similar reversal appears to have occurred in the early 1970s. By comparing “earthquake twins”, very similar-seeming seismic events years apart scientists were able to register slight changes in wave arrival times as they travelled through the core. These measurements indicated a deceleration around 2008, a pause, and afterward a reversal in direction.
The mechanism of this movement lies in the interaction between the solid inner core and the liquid outer core that surrounds it. The turbulent, metal-enriched fluid of the outer core produces electrical currents that power Earth’s magnetic field, and gravitational coupling to the mantle and electromagnetic torques at the core–mantle boundary push and pull on the inner sphere. Over periods of decades, these forces alternately speed up and slow down the rotation of the core. The most recent reversal is asymmetric: the currently ongoing westward sub-rotation is around 2.5 times slower than the previous eastward super-rotation.
Seismic tomography, employing pressure (P) and shear (S) waves as probes, has been at the heart of these findings. P waves, which can pass through solids and liquids, adjust speed and trajectory based on the material’s density and elasticity they travel through. As scientists monitor their travel times through the inner core along identical paths over several decades, they can deduce changes in rotation with unprecedented accuracy. In a few instances, coupled earthquakes separated by decades created waveforms that were so similar they showed the core had rotated back to an earlier position, a clear signature of oscillation.
Other than rotation, recent research has also shown that the surface itself is deforming. Seismic waves running along its edge have revealed changes missing in waves that travel deeper, indicating viscous deformation due to turbulence in the outer core. These are the first times such structural changes have been seen on human timescales. Churning flow of the molten outer core can reform the inner core’s surface, much as molten metal distorts under pressure, with implications for geomagnetic field stability.
These Earth’s internal dynamics are not mere isolated curiosities. The magnetic field that they assist in maintaining protects the planet from deleterious solar and cosmic radiation, supports navigation systems, and affects everything from animal migrations to satellite operations. Changes in core motion can slightly modify the field strength and geometry, and have been associated with geomagnetic “jerks”, sudden field behavior changes that can interfere with technology.
The rotation of the core also engages with Earth’s total spin, generating tiny but detectable variations in the duration of a day. Though these changes total at most thousandths of a second, they’re inextricably linked with other mechanisms that redistribute mass on the planet. Global warming melting of ice caps and glaciers and groundwater pumping pump mass towards the equator, flattening the figure of the Earth and lengthening the day still more. NASA-supported research has determined that since 2000, such surface mass changes have added day lengthening to 1.33 milliseconds per century, the largest rate in more than a century a rate that could increase further if greenhouse gas emissions keep on rising.
Long-term polar motion, the gradual drift of the rotation axis of the Earth, also carries the signature of core dynamics. Machine learning models combining mantle convection, glacial isostatic adjustment, and hydrological cycles have revealed a substantial core flow-induced torque contribution at the core–mantle boundary. Such torques can produce multidecadal wobbles of order several milliarcseconds, linking deep-Earth process with climate-driven mass redistribution at the surface. Paradoxically, some analyses indicate a feedback loop: surface water storage perturbations can impact core dynamics, and vice versa.
What comes into view is a picture of the inner core as neither fixed nor monolithic, but as an active partner in a planetary dance in which deep metal flows, mantle density heterogeneities, and surface climate processes are integrated. The reversal currently in progress is merely one step in a long, rhythmic dance that subtly forces the magnetic shield, the rotation of the planet, and, indirectly, the surface environment.
