A new study published in Nature suggests that the core is backtracking relative to the surface, similar to how a car that slows down appears to move backwards to a driver moving at a constant speed.
The study provides "unambiguous evidence" that the inner core began to slow down around 2010, moving slower than the Earth's surface. This creates the appearance of the core moving backwards relative to the surface.
If correct, this would be the first observed slowdown in 40 years and supports the theory that the core's speed changes in 70-year cycles.
To uncover these changes, the team studied repetitive earthquakes, specifically 121 quakes in the South Sandwich Islands between 1991 and 2023, using seismographs in Canada and Alaska. They also used data from Soviet-era nuclear tests.
The team searched for matching seismic waveforms from different times. If the inner core rotates independently of the rest of the Earth, waves from recurring quakes would pass through different regions of the core, creating unique waveforms due to its uneven structure.
Conversely, if the core had reversed its rotation, some waveforms should match those from before the reversal, indicating the core had realigned with its previous path.
The effects of the inner core's movement on the Earth's surface are uncertain, but they could slightly alter the length of our day by about a thousandth of a second, almost imperceptible amid the noise of the churning oceans and atmosphere.
Future work for the team includes measuring more waveforms from different locations and paths through the planet. "Part of it is just waiting. The core is moving back through some positions where it did some weird things around 2001, so we'll try figuring out what that was," Vidale concluded.