Are the earth's magnetic poles shifting?

January 12, 2007

Dear Cecil:

News articles say the next magnetic pole shift is imminent. Magnetic fields in Africa are getting weak and they say in some places you can't get accurate compass readings. What will happen to us without protection from the sun's radiation?

Don't stress, kid. When talking about the magnetic poles flipping, we can say three things with reasonable confidence: it's going to happen, we don't know when, and it probably won't be as bad as some people think.

As far as scientists can tell, the earth has had a magnetic field for billions of years. The ancient Chinese were likely the first to discover the field's effect on lodestones, although Europeans may not have figured it out till the Middle Ages. In 1600 a physician in Queen Elizabeth's court named William Gilbert proposed that the earth itself generated the force that moved compass needles. Scientists began measuring the strength of the earth's magnetic field in the 1830s using a magnetometer invented by Carl Friedrich Gauss. Since then the magnetic field has been subject to intensive if somewhat inconclusive study, the upshot of which is that we're not certain why it's there, but we're lucky it is - it does a lot more than make compasses work. In combination with the atmosphere it shields us from many of the solar and other charged particles that would otherwise spatter the planet's surface and its inhabitants; it also protects the atmosphere itself from erosion by the solar wind.

While the experts are vague on the details, they're pretty sure the earth's magnetic field is a result of movement in the planet's liquid iron and nickel core (although Jupiter and other planets believed to lack such cores also have magnetic fields). They also know the field isn't stable and tends to shift in at least two different ways: the magnetic poles typically wander around slowly, but every so often flip completely, reversing the magnetic polarity of the planet.

Ever since the compass was invented, navigators have noticed that the direction indicated by the needle changes slightly each year. Over the last century, both the north and south magnetic poles have been drifting north (that is, toward the geographic north pole, which itself wanders somewhat, but let's not make this more complicated than it already is). The north magnetic pole historically has moved at a rate of about ten kilometers per year, but has lately accelerated, heading toward Russia at four times that rate.

The first evidence for a complete flip of the magnetic poles came in the 1950s, when studies of the tectonic plates in the mid-Atlantic revealed that the rock making up the ocean floor was arranged in stripes of alternating magnetic polarity. It turns out that as the iron-rich magma pushing up from beneath the plate boundary cools and spreads out, the mineral crystals align themselves with the magnetic field as it then exists, creating a tree-ring-style record of its direction and strength. The alternating stripes attest to frequent field flip-flops over geological time.

Intervals between pole flips vary greatly - sometimes things will be stable for tens of millions of years, other times the poles reverse after a few tens of thousands of years. The last flip was roughly 780,000 years ago, making some think we're overdue, although since no one really understands polar acrobatics, prediction seems futile. Circumstantial evidence suggests that a comet or large meteorite impact could trigger a magnetic pole flip, but proof is lacking. Recent reports about a decrease in the strength of the earth's magnetic field have led to anxiety that another reversal looms. However, while the field's strength has weakened by about 10 percent since the 1800s, it's still about double the average strength over the last million years.

When the magnetic field does reverse, it won't be like someone hit a light switch - the process can take thousands of years, and that's part of the problem. While the poles are in midflip, the earth will have a much weaker magnetic field than normal, allowing more solar and cosmic radiation to penetrate. Recent research suggests that a weak magnetic field could lead to severe damage of the ozone layer, letting in more ultraviolet radiation and sharply increasing skin cancer rates, resulting in perhaps a hundred thousand additional cancer deaths per year. Pole flips generally haven't coincided with mass wildlife extinctions, so most plants and animals (presumably including us) will likely survive. Still, during the flipping process not only would the north and south poles radically shift location, there might be several different small poles scattered around the planet. This could lead to disruption of electrical systems and satellites, auroras appearing at tropical latitudes, and widespread death (or at least confusion) among animals like the loggerhead turtle that rely upon the earth's magnetic field for navigation. So OK, things might get ugly for a bit. But my guess is we'll deal.

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