Home Science & Tech Down to Earth: Has Earth’s Inner Core Stopped Spinning?

Down to Earth: Has Earth’s Inner Core Stopped Spinning?

Down to Earth: Has Earth’s Inner Core Stopped Spinning?
Illustration by Diane Kim

Erick Aragon Alvarado 

News Editor 

Illustration by Diane Kim

A recent study published on Jan. 23 in “Nature Geoscience” attracted a lot of attention with its conclusion that Earth’s inner core has stopped spinning and is now actually spinning in the opposite direction.

To be clear, Earth’s solid inner core never stops rotating, but if it were to rotate at the same speed as the rest of the Earth then it would appear to have stopped. This is because the same sides of the core would always be facing the same “faces” of the Earth’s surface. This is based on the concept that two or more objects moving at the same speed appear still to each other — like passengers in a car appearing still to each other while actually moving at 50 miles per hour or so. 

Thus, if the inner core is spinning faster than the rest of the Earth, it appears to be rotating counterclockwise (when viewed from the North Pole); if it is spinning at the same speed, it appears to be stopped; and if it is spinning slower than the rest of Earth, it looks as if it’s spinning clockwise. 

Currently, it is widely believed that the inner core spins slightly faster than the rest of the Earth — a phenomenon called super-rotation. The overarching differences in rotation between the inner core and the mantle is what the authors refer to as their differential rotation

Yi Yang and Xiaodong Song’s study claims that the inner core stopped its differential rotation around 2009 and is now seemingly rotating in the opposite direction. Looking at decades of seismic data from earthquake waves traveling through the core, they’ve concluded that the inner core goes through this oscillation cycle of speeding up and slowing down every 70 years, which means that the core looks as if it’s changing direction every 35 years. 

The rotation of the inner core is the consequence of two geophysical phenomena: electromagnetic torque and the gravitational coupling between the inner core and the mantle.  

Electromagnetic torque is a product of the generation of Earth’s electromagnetic field through convection currents in the liquid metal outer core. As the less dense and hotter material in the liquid outer core rises, the denser and cooler material falls, solidifies, and adds to the solid inner core in a process named the dynamo theory. These currents cause the rotation of the inner core, while the mantle and its gravitational relationship with the inner core is thought to be the reason the inner core appears to oscillate from one direction to another. 

The authors corroborated what they perceived as the theoretical effects of their findings with similar 70-year oscillations present in other natural phenomena. These include fluctuations in global temperature, greenhouse gas concentrations, sea levels, and the length of the day. 

Now, while the slight variations of Earth’s inner core rotation are not considered to be a direct threat to surface life, its indirect effects, mainly concerning the ocean-atmosphere system, could turn out to be especially important in the context of today’s worsening climate crisis.

If this study proves accurate, the length of a day is increasing by a fraction of a millisecond every year since 2020. However, this phenomenon is not as relevant to humanity as its other potential consequences. These can include increasing sea levels, global temperatures, and greenhouse gasses. Again, if true, this possible cyclical exacerbation of the climate crisis could be reliably predicted, serving as a sort of early warning system for people. 

That said, although it seems this study was done using an abundance of data, the scientific community remains skeptical. The authors of the study themselves acknowledged what they deemed as a “less sound” interpretation of the data. This interpretation would explain the decadal patterns in the seismic data as a consequence of the complicated topography of Earth’s inner core.

Other scientists simply disagree about the length of the oscillation cycle. For example, John Vidale, a seismologist at the University of Southern California, used seismic data from two nuclear explosions to conclude that the inner core changes direction every six years. 

However, considering the extent of the data used in Yang and Song’s study, the soundness and professionalism of their methods, and the other 70-year oscillations in some geophysical phenomena, this report seems to carry great weight. Luckily, their findings will be put to the test relatively soon as we continue to collect seismic data over the years.

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