According to the researchers, the length of a day can vary by fractions of a second. A new study found "unambiguous evidence" that the Earth's inner core began to slow its rotation in 2010 when compared to the planet's surface. The Earth's inner core, a solid sphere composed of iron and nickel, is suspended within the liquid outer core (consisting of molten metals) and held in place by gravity. The inner and outer cores work together to produce one of the planet's three layers, along with the mantle and crust. Because the core is physically inaccessible, researchers typically examine it by analyzing seismograms, which are recordings of seismic waves. "When I first observed the seismograms that hinted at this alteration, I was stumped," remarked John Vidale, an Earth Sciences professor at the University of Southern California in the United States."

"But when we discovered two dozen more data that indicated the same trend, the conclusion was unavoidable. "The inner core had slowed down for the first time in many decades," said Vidale, who was also the corresponding author of the Nature article. The slowing of the inner core is a contentious issue in the scientific world, with some research claiming that it rotates faster than the Earth's surface. It is well understood that the magnetic field created in the outer core and gravitational effects beneath the Earth's mantle influence the rotation of the inner core. However, it is assumed that the inner core is reversing and retracing relative to the surface, due to rotating slower than the mantle for the first time in about 40 years. "Other scientists have recently argued for similar and different models, but our latest study provides the most convincing resolution," Vidale stated.

A study published earlier this year in the journal Nature discovered that climate change-induced melting of ice in Greenland and Antarctica was impacting global timekeeping by delaying the Earth's rotation. Duncan Agnew, a geophysicist at the University of California, San Diego, demonstrated that the Earth's liquid core was slowing down in its rotation. To counteract the impacts of this, Agnew stated that the solid Earth was revolving faster. According to Agnew, this has resulted in fewer 'leap seconds' needing to be added to Coordinated Universal Time (UTC) in recent decades.

Since 1972, a 'leap second' has been added once every few years to account for inconsistencies in the UTC caused by the fact that the Earth does not always rotate at the same pace. For the most recent study, the researchers examined seismic data from 121 repeated earthquakes in the South Sandwich Islands, a remote archipelago in the South Atlantic Ocean, between 1991 and 2023. The islands are susceptible to strong earthquakes. The investigation also includes data from twin Soviet nuclear tests conducted between 1971 and 1974, as well as various French and American nuclear tests conducted as part of other inner core studies.