After hearing a lecture and watching a Nova special on them, polarity reversals were an obsession of mine for a while.  Briefly, a polarity reversal is an occasional switch in the Earth’s magnetic field, where the positions of magnetic north and south are interchanged.  The last reversal was 780 thousand years ago, so unfortunately we don’t really know what effects a polarity reversal will have, but there could for a time be several more magnetic poles than our usual two, and the Earth’s magnetic field will be severely weakened, which could lead to either the end of all life as we know it or a mild increase in cases of skin cancer and borealis across the globe.  For a more detailed explanation, click here.