On page 24 of the current (November) issue of Scientific American is an interesting "news scan" article about reversals of Earth's magnetic pole.
Apparently the Earth's "global dipole has been diminishing precipitously for the past 150 years and at this rate could disappear entirely sometime early in the next millennium" -- disabling the magnetic radiation shield against cosmic and solar particles.
As you might expect, Hollywood will soon be releasing a disaster flick named The Core (Paramount Pictures), about a world with frequent radiation alerts and about a heroic attempt to restart the magnetic field by setting off nuclear explosions thousands of kilometres underground.
The article concludes with the statement that "no major species extinctions correlate with past polarity reversals," quoting California Institute of Technology geophysicist Joseph L. Kirschvink as saying, "If there is a biological effect, we're evolved for it."
Jaro