A new study has delivered “unambiguous evidence” that the Earth’s inner core has been slowing its rotation since 2010 relative to the planet’s surface. This deceleration could alter the length of a day on Earth by fractions of a second, according to researchers.
The inner core of the Earth, composed of a solid sphere of iron and nickel, is suspended within the liquid outer core and anchored by gravity. Together, the inner and outer core form one of the Earth’s three main layers, along with the mantle and crust. Due to its inaccessibility, scientists study the core through seismograms, which are recordings of waves generated by earthquakes.
“When I first saw the seismograms that hinted at this change, I was stumped,” said John Vidale, a professor of Earth Sciences at the University of Southern California. “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,” added Vidale, the corresponding author of the study published in the journal Nature.
The slowing of the inner core’s rotation is a topic of significant debate within the scientific community. Some studies even suggest that the inner core rotates faster than the Earth’s surface.
The spin of the inner core is influenced by the magnetic field generated in the outer core and gravitational effects within the Earth’s mantle. Recent findings indicate that the inner core is now rotating slower than the mantle for the first time in about 40 years, suggesting a relative backtracking compared to the surface.
“Other scientists have recently argued for similar and different models, but our latest study provides the most convincing resolution,” Vidale said.
Earlier this year, a study published in Nature by Duncan Agnew, a geophysicist at the University of California San Diego, revealed that climate change-driven melting of ice in Greenland and Antarctica was impacting global timekeeping by slowing down Earth’s rotation.
Agnew’s study showed that the Earth’s liquid core was slowing down in its rotation, causing the solid Earth to rotate faster. This change has led to fewer ‘leap seconds’ being needed to adjust Coordinated Universal Time (UTC) in recent decades.
Since 1972, leap seconds have been added every few years to account for irregularities in UTC caused by the Earth’s varying rotational speed.
For the latest study, researchers analyzed seismic data from 121 repeating earthquakes—multiple quakes occurring in the same location—recorded between 1991 and 2023 in the South Sandwich Islands, a remote archipelago in the South Atlantic Ocean known for violent earthquakes.
The analysis also included data from twin Soviet nuclear tests between 1971 and 1974, along with multiple French and American nuclear tests from other studies of the inner core.
These findings contribute to a growing understanding of the dynamic processes within the Earth and their impact on global timekeeping and rotational behavior.
