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A Sailor's Magnetism

11 May 2006 (All day)
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Science

Waning attraction.
The average intensity of Earth's magnetism between 1840 and 1990 (above) is far weaker (more purple in north, more orange in south) than between 1590 and 1840 (below).

Earth's magnetic field has been steadily weakening over the last century and a half, which could herald a field reversal. If that happens, compasses would point south instead of north and loss of protection from solar winds could subject Earth to intense solar radiation. But a new study may give less cause for concern. According to historical data, Earth's field was relatively stable from 1590 to 1840, which suggests that the recent decline is more likely a mere wobble in the field's strength.

No direct measurements of the intensity of Earth's magnetic field exist before 1837, when mathematician Carl Friedrich Gauss invented a method to gauge it. But thanks to the rigorously kept logs of sailors who used the sun and stars to judge the accuracy of their compasses, researchers can glean magnetic data points from further back in history. Archaeological finds are another rich source of magnetic data. Items such as pottery and bricks contain minerals which preserve the direction of the magnetic field that existed at the time they were heated. These artifacts give scientists a rough idea of Earth's magnetic field strength and direction at the time they were used.

Taking all the human artifact and ship log data together, researchers at the University of Leeds in the United Kingdom led by geophysicist David Gubbins sought to reduce the uncertainty of these measurements. They fit a line to the hundreds of data points and found almost no change in Earth's magnetic field strength between 1590 to 1840--in contrast to the 10% decline that's happened since 1840. Previous field reversals (inferred from examining paleontological and archaeological remains) have been preceded by sharp dips in magnetic intensity, says Gubbins, whose team reports its findings tomorrow in Science. The new model seems to fit this pattern, he says. But he notes that the overall level of field strength is higher now than it has been leading into past flips, so an imminent reversal is unlikely.

Geophysicist Peter Olson of Johns Hopkins University in Baltimore, Maryland, agrees we shouldn't throw out those compasses just yet. He says that he wouldn't be surprised if the change in field strength doesn't undergo a straight and steady decline but rather wobbles around. In that case, the current weakening might not lead directly to a field reversal. "It's very likely we're just witnessing the downside of an oscillation," he says.

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