Scientists have proven that the Earth’s inner core is backtracking – slowing down – in relation to the planet’s surface, as shown in new research published in Nature.

Movement of the inner core has been debated by the scientific community for almost three decades, with some research indicating that the inner core rotates faster than the planet’s surface. The new study, which was led by the University of Southern California and included Cornell University, provides unambiguous evidence that the inner core began to decrease its speed around 2008, moving slower than the Earth’s surface.

“These findings are exciting. Our findings provide additional constraints in inner core dynamics, inner core viscosity, and Earth's mantle density distribution.” said Guanning Pang, a postdoctoral researcher in the Department of Earth and Atmospheric Science at Cornell Engineering. “The findings have especially important implications for how the geomagnetic field generates and evolves while the geomagnetic field protects the Earth from solar winds, which is important for life to exist on Earth.”

Building on Pang's earlier work suggesting a differential rotation burst in the inner core from 2001 to 2003, and smaller rotations at other times, the new study’s expanded dataset and analysis of seismic rays revealed waveform changes between 2003–2008, which eventually realigned.

“When I first saw the seismograms that hinted at this change, I was stumped,” said John Vidale, Dean’s Professor of Earth Sciences at the USC Dornsife College of Letters, Arts and Sciences. “But when we found two dozen more observations signaling the same pattern, the result was inescapable. The inner core had slowed down for the first time in many decades. Other scientists have recently argued for similar and different models, but our latest study provides the most convincing resolution.”

In this study, the researchers compiled and analyzed seismic data recorded around the South Sandwich Islands from 121 repeating earthquakes that occurred between 1991 and 2023. They have also utilized data from twin Soviet nuclear tests between 1971 and 1974, as well as repeated French and American nuclear tests from other studies of the inner core.

Vidale said the inner core’s slowing speed was caused by the churning of the liquid iron outer core that surrounds it, which generates Earth’s magnetic field, as well as gravitational tugs from the dense regions of the overlying rocky mantle.

The implications of this change in the inner core’s movement for Earth’s surface can only be speculated. Vidale said the backtracking of the inner core may alter the length of a day by fractions of a second: “It’s very hard to notice, on the order of a thousandth of a second, almost lost in the noise of the churning oceans and atmosphere.”

The USC scientists’ future research aspires to chart the trajectory of the inner core in even greater detail to reveal exactly why it is shifting.

“Our study generated a predictable inner core rotation model,” Pang said, “so that people can make some predictions about the inner core movement in the next few years.”

This article was adapted from a version written by Will Kwong with permission from the University of Southern California.