Researchers have found that Earth’s inner core – a solid metal ball more than a quarter the size of Mars that drives the motion of the entire planet – has dramatically slowed its rotation and might even be reversing polarity. The study, appearing in the journal Nature Geoscience this week, is the first to show the results of mysterious processes occurring deep in the planet that could force our weather patterns to shift in unforeseen ways.

It is essentially an iron-nickel ball, with a radius about four-fifths that of the Earth, and it rotates a bit faster than the rest of the planet’s mass, responding to the magnetic field that is generated in the liquid outer core. A few years ago, however, some seismologists came across a curious issue. They analysed seismic waves from distant earthquakes that passed through the inner core and made an unexpected observation.

‘We were quite shocked that we found this result,’ says Xiaodong Song, a seismologist at Peking University and co- author of the paper. ‘We found that the inner core might not slow down but might completely stop, and now is rotating very slowly in the opposite direction.

Nor is this super-rotation unique. Past changes in the inner core’s rotation rate, both speeding up and slowing down, have been documented, but this latest deceleration is special. The most recent set of accelerations and decelerations ended with this slowdown, and it begs the question of what mechanism would give rise to such an oddball cycle.

The scientists think the drag from the mantle, the region that lies between the Earth’s core and the crust that surrounds it, and the magnetic torque of the outer core, the part that lies between the mantle and the crust, are the most important tidal forces acting on the inner core, determining whether it spins faster or slower. The combined effect of those powers creates a sluggish waxing and waning, in which the inner core spins back and forth at a pace of one cycle every 70 years.

Its implications are still unclear, however. Yet, scientists conjecture that this slowdown might yield myriad — if somewhat suble — relate that the inner core’s rotation helps to generate Earth’s magnetic field, an invisible shield that protects us from the Sun’s harmful radiation. A shift in the rotation of the inner core should, at least in theory, weaken Earth’s magnetic field, thereby exposing us to solar storms and other space weather.

And finally, the rotation of the inner core changes the length of our days. As the inner core slows down, it would have the potential to cause minuscule differences in the Earth’s day – thousands of a second, which wouldn’t be noticeable over the course of a day, but would change the length of a day by a few thousandths of a second.

But, over the long term, even tiny shifts in the Earth’s axis can alter our climate and landscape. The inner core’s rotation, for example, could alter the balance of energy within the planet, altering ocean currents and thus, patterns of weather.

‘We are just beginning to scratch the surface on what this slowdown means,’ said Yang, a seismologist at Peking University who co‑wrote the paper. ‘But clearly, the inner core has important climate implications.’

Its identification has opened a new range of experimental options for probing Earth’s interior and the intricate mechanisms at work there. Now, scientists will continue to chase down how exactly the inner core accelerates, and what it means about Earth’s long-term future.

The slowing of the inner core might seem abstract and distant, but it reminds us of how interconnected Earth’s vital systems truly are. What drives the Earth is a planet in constant flux and motion, and it is vitally important for our future to better understand these processes.

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