It seems that every few hundred thousand years or so, the Earths magnetic field dwindles to practically nothing and then gradually reappears with the north and south poles flipped. Now this flipping of the magnetic poles—which appears to have last happened about 780,000 years ago—isnt particularly dangerous, but this brief period—about a century or so in duration—of decreased magnetic fields could threaten life on the planet, for without magnetic protection, particle storms and cosmic rays from the sun, as well as even more energetic subatomic particles from deep space, would strike
Earths atmosphere, eroding the already beleaguered ozone layer and causing all sorts of problems to both man and beast (especially among those creatures that navigate by magnetic reckoning). Further, scientists estimate that we are overdue for such an event and have also noticed that the strength of our magnetic field has decreased about 5 percent in the past century, possibly signaling that such an event may be in our immediate future—within a few centuries if not sooner.
However, in being so gradual, should scientists in the future discover that such a shift is in the works, there should be plenty of time to take the necessary precautions to avoid the most destructive effects by moving underground or off planet, or perhaps strengthening the planets atmospheric defenses through the use of exotic, futuristic technologies. In any case, it isnt something we need to worry about in the short term—though it could be a concern for those living a few hundred or even thousands of years from now.