Normally, the earth's core is spinning faster than the mantle and crust, but lately it has slowed down to match speed, and now may be spinning slower. People are saying this happens every 70 years or so, and that it's not a big deal, but we didn't really start studying the interior workings of the planet until about 50-70 years ago, so how do we know it changes speed on a regular basis? And will it have any effect on the magnetosphere, earth's rotation, climate, tectonics, or anything else?
https://www.sciencenews.org/article/earth-inner-core-reverse-rotation
https://www.sciencenews.org/article/earth-inner-core-reverse-rotation