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The study's authors, Xiaodong Song and Yi Yang of China's Peking University, said they found that the inner core's rotation "came to near halt around 2009 and then turned in an opposite direction".
"We believe the inner core rotates, relative to the Earth's surface, back and forth, like a swing," they told AFP.
"One cycle of the swing is about seven decades", meaning it changes direction roughly every 35 years, they added.
They said it previously changed direction in the early 1970s, and predicted the next about-face would be in the mid-2040s.
Earth's inner core may have started spinning other way: study
Far below our feet, a giant may have started moving against us. - Geophysicists 'divided' - Another theory -- which Vidale said has some good evidence supporting it -- is that the inner core only moved significantly between 2001 to 2013 and has stayed put since.
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