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A weakening magnetic field could interrupt power grids and radio communication,

This does not sound good. I saw once a documentation what might happen when a huge EMP destroys the infrastructure of the modern western world. In the documentation one possible source of such an EMP was an extra-terrestrial gamma-ray burst. So compared to such an event, how does the effects measure to the weakening magnetic field? Might be that such a polarity change never before caused a serious catastrophe. Before the mid-20th century it most likely would not have been much of a problem either. However, the predicted gamma-ray burst EMP scenarios for today's tech/power dependencies came quite close to an extinction level event for humans.