Earth’s Inner Core Slows and Reverses Rotation, Study Finds

Seismic analyses of earthquakes recorded between 1991 and 2023 reveal that the solid inner core of the Earth has slowed its spin and begun rotating opposite to the mantle. Researchers examined 121 quakes near the South Sandwich Islands, comparing 143 pairs of nearly identical seismic events to map how waves travel through the planet’s centre. The data show that the inner core, a iron‑nickel sphere about the size of the Moon, rotated faster than the mantle until the early 2000s, then decelerated and, around 2010, started lagging behind, giving the appearance of a reverse spin.

The phenomenon is part of a natural 70‑year geophysical cycle and alters the length of the day by only a few thousandths of a second—far too small to be noticed in everyday life. Scientists stress that the change has no direct impact on human activity, but it is valuable for understanding the dynamics that generate Earth’s magnetic field. The findings were published in the journal Nature by a team led by John Vidal of the University of Southern California, using seismic records collected since the 1960s.