I had a small quarrel about this on the physics site. A question was asked about what could cause a change in the rotation speed of the Earth. In addition to the answers given I proposed the possibility that if the liquid core would solidify this would result in an increasing rotation speed.
I thought the same could happen if the matter currents giving rise to Earth's magnetic field changed direction, this would cause an increase in speed too. That is, if the currents have the right direction. The reversal of the currents' momentum (if such currents exist but I don't see why not) could also decrease the speed.
So, are these scenarios realistic?