Earth’s inner core may be reversing its rotation, study finds

In a study published Monday in the journal Nature Geoscience, researchers said the reversal of the inner core rotation would shorten the length of the day by a fraction of a millisecond over the course of a year, and might have a small effect on Earth’s magnetic field, but wouldn’t affect life on the surface. The Earth’s inner core is made of iron and nickel and is separated from the rest of the solid Earth by the liquid outer core, enabling it to rotate differently than the rest of the planet.

“In theory it has been going on for a long time but we only have observations over a few decades,” said Xiaodong Song, a seismologist at Peking University in Beijing, and a co-author of the new study.

The spinning of the inner core is caused by the magnetic field that is generated by the liquid outer core, Dr. Song said. Understanding how the inner core rotates can help scientists figure out how the different layers of the Earth interact with each other.

Dr. Song and co-author Yi Yang, also a seismologist at Peking University, analyzed seismic waves from similar earthquakes that have passed through the Earth along similar paths since the 1960s. They found that between 2009 and 2020, that rotation has stopped and might have reversed direction.

“We have those repeating earthquakes that happen at the same location,” Dr. Yang said. “It is like we are doing a CT scan for the Earth.”

John Vidale, a professor of earth sciences at the University of Southern California who wasn’t involved with the study, said there may be other interpretations of the seismic data.

“The changes they noticed are valid although what’s actually happening isn’t so clear,” Dr. Vidale said. “They have a very good analysis and the theory they put in the papers is probably as good as anything at the moment, but there are several competing ideas as well.”

Dr. Vidale says other scientists theorize that the inner core changes its rotation at shorter intervals than the 70-year cycle described by the new study’s authors. Other theories from scientists include that the inner core’s rotation actually stopped between 2001 and 2003, or that it isn’t really reversed at all, but just changed the way it rotates, Dr. Vidale said.

In a study published Monday in the journal Nature Geoscience, researchers said the reversal of the inner core rotation would shorten the length of the day by a fraction of a millisecond over the course of a year, and might have a small effect on Earth’s magnetic field, but wouldn’t affect life on the surface. The Earth’s inner core is made of iron and nickel and is separated from the rest of the solid Earth by the liquid outer core, enabling it to rotate differently than the rest of the planet.

“In theory it has been going on for a long time but we only have observations over a few decades,” said Xiaodong Song, a seismologist at Peking University in Beijing, and a co-author of the new study.

The spinning of the inner core is caused by the magnetic field that is generated by the liquid outer core, Dr. Song said. Understanding how the inner core rotates can help scientists figure out how the different layers of the Earth interact with each other.

Dr. Song and co-author Yi Yang, also a seismologist at Peking University, analyzed seismic waves from similar earthquakes that have passed through the Earth along similar paths since the 1960s. They found that between 2009 and 2020, that rotation has stopped and might have reversed direction.

“We have those repeating earthquakes that happen at the same location,” Dr. Yang said. “It is like we are doing a CT scan for the Earth.”

John Vidale, a professor of earth sciences at the University of Southern California who wasn’t involved with the study, said there may be other interpretations of the seismic data.

“The changes they noticed are valid although what’s actually happening isn’t so clear,” Dr. Vidale said. “They have a very good analysis and the theory they put in the papers is probably as good as anything at the moment, but there are several competing ideas as well.”

Dr. Vidale says other scientists theorize that the inner core changes its rotation at shorter intervals than the 70-year cycle described by the new study’s authors. Other theories from scientists include that the inner core’s rotation actually stopped between 2001 and 2003, or that it isn’t really reversed at all, but just changed the way it rotates, Dr. Vidale said.