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If all the compasses in the world started pointing south rather than north,
many people might think something very strange, very unusual, and possibly very
dangerous was going on. Doomsayers would have a field day proclaiming the end
is nigh, while more rational persons might head straight to scientists for an
explanation.
Fortunately, those scientists in the know—paleomagnetists, to be exact—would have a ready answer. Such reversals in the Earth's magnetic field, they'd
tell you, are, roughly speaking, as common as ice ages. That is, they're
terrifically infrequent by human standards, but in geologic terms they
happen all the time. As the time line at right shows, hundreds of times in our
planet's history the polarity of the magnetic shield ensheathing the globe has
gone from "normal," our current orientation to the north, to "reversed," and
back again.
The Earth is not alone in this fickleness: The sun's magnetic shield appears to
reverse its polarity approximately every 11 years. Even our Milky Way galaxy is
magnetized, and experts say it probably reverses its polarity as well.
Moreover, while a severe weakening or disappearance of the magnetic field would
lay us open to harmful radiation from the sun, there's little evidence to date
that "flips" per se inflict any lasting damage (see Impact on Animals).
It might sound as if scientists have all the answers regarding magnetic
reversals. But actually they know very little about them. Basic questions haunt
researchers: What physical processes within the Earth trigger reversals? Why do
the durations and frequencies of both normal and reversed states seem random?
Why is there such a disproportionately long normal period between about 121 and
83 million years ago? Why does the reversal rate, at least during the past 160
million years, appear to peak around 12 million years ago?
All these questions remain unanswered, though experts like Dennis Kent, the
Rutgers University geologist who supplied NOVA with updated figures for the
time line, are hard at work trying to answer them. In the meantime, not to
worry. Reversals happen on average only about once every 250,000 years, and
they take hundreds if not thousands of years to complete.
Even the weakening currently under way may be a false alarm. The field often
gets very weak, then bounces back, never having flipped. As Ron Merrill, a
magnetic-field specialist at the University of Washington remarked when asked
whether we're in for a reversal: "Ask me in 10,000 years, I'll give you a
better answer." So hang on to your compass. For the foreseeable future, it
should work as advertised.—Peter Tyson
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