Birds and compasses know north from south because, like a giant bar magnet, the earth's magnetic field has two poles that line up closely with the planet's axis of rotation. That's simple physics.

Less widely known is that this global dipole has been diminishing precipitously for the past 150 years and at this rate could disappear entirely sometime early in the next millenium. With the world's protective magnetic shield severely disabled, intensified doses of cosmic and solar particles could knock out satellites--the least of humanity's concerns under this deadly shower of radiation.

The good news is that any disappearance of the dipole will be temporary, the halfway point along a southward swing that would leave compass needles pointing toward Antarctica rather than the frozen North. Magnetic minerals trapped inside ancient rocks have recorded hundreds of these so-called polarity reversals in the past 500 million years. But no known pattern exists in the timing or duration of these events, making them impossible to predict.

Most geophysicists have long assumed that a 2,200-kilometer-thick layer of molten iron swirling deep inside the core creates the planets self-sustaining field. But until about six years ago, no one had written computer code sufficiently complex to simulate core motion and its magnetic effects. Now several programs can simulate not only motion but even polarity reversals, some of which require only 1,200 years--a wink of geologic time.

Other investigators have seen real-world hints of why the reversals might occur. Earlier this year Gauthier Hulot of the Paris Geophysical Institute and his colleagues used satellite measurements to track changes in the fields behavior near the top of the core. Far below the southern tip of Africa they found a small region where the magnetic field lines point peculiarly toward the center of the earth instead of toward the surface, as do the dominant lines in that region. A clump of similar patches exists near the North Pole.

Hulots team argues that the growth of these reversed patches, presumably eddies that are working against the primary motion of the core, can explain the current decline in the dipole field. What is more, the rampant growth of such patches has caused full-blown reversals in some computer simulations.

As for what life would be like at a time of flip-flopping polarity, Paramount Picturess new geophysical thriller The Core suggests that birds will lose their way and that humans will live under frequent radiation alerts. In the movie, world governments unite to build a manned craft that can burrow through 2,900 kilometers of solid mantle rock and survive the cores scorching heat--comparable to that at the surface of the sun. The mission: to set off nuclear explosions that could revive the cores natural flow and fight the magnetic fields tendency to reverse.

With current technology falling far short of this Jules Verne-esque solution, scientists can offer other reassurances: The shrinking dipole doesnt guarantee an imminent reversal. Only a random few of the fields myriad natural fluctuations actually mushroom into an all-out switch. Recent computer simulations also indicate that the planets peripheral magnetic fields, which constitute only 10 percent of the total, may get stronger as the dominant dipole field weakens.

Most comforting of all may be that no major species extinctions correlate with past polarity reversals. As geophysicist Joseph L. Kirschvink of the California Institute of Technology says, "If there is a biological effect, were evolved for it."

Originally published in the November 2002 issue of Scientific American.