The Rigorous Study of the Ancient Mariners

Measuring changes in the Earth's magnetic field.

By Stephen Ornes|Monday, August 14, 2006

"Fall in Earth's Magnetic Field Is Erratic," published in the journal Science. The goal is to measure changes in Earth's magnetic field, generated by churning currents of molten iron that lie more than 3,000 miles below the surface.

Earth's magnetic field sculpts the dancing lights of the aurora borealis, aims compass needles, and most crucially, protects us from potentially lethal particles spewed by the sun. But it cannot be counted on. From time to time the field flips, fading away completely before turning north to south. About 60 such reversals have occurred in the last 20 million years. And because the last reversal occurred 780,000 years ago, geophysicist David Gubbins of the University of Leeds in Britain wanted to know if it was time to prepare for the next one.

To forecast flips of Earth's magnetic field, Gubbins needed to know what it had done in the past. Scientists have been taking direct measurements of the field only since 1837, but mariners have been watching magnetic north far longer. So Gubbins scrutinized more than 150,000 measurements of the inclination and the intensity of Earth's magnetic field taken from old ships' logs, including more than 50,000 notes from the East India Company and 7,000 surveys from the voyages of Captain James Cook.

After correcting the logs for errors in location, Gubbins compared the directions of captains' compass needles with the magnetic properties of rocks from the same period, allowing him to construct a time line of movements in the magnetic field. Gubbins noticed that around 1770, an excursion—a small, localized reversal in the magnetic field—bloomed near the equator and began migrating south. This excursion distorted the compasses of mariners, throwing more than one unlucky captain off course.

It might have also signaled the beginning of the end of our current magnetic field. When Gubbins aligned his historical data with current measurements from more than 200 magnetic observatories scattered across the globe, he came to a clear, if unsettling, conclusion: In the last 140 years, the magnetic field has been fading at an accelerating rate.

The speed-up began around 1860—shortly after the 1770 excursion vanished. Despite that one anomaly, the overall field stayed nearly constant in strength from 1590 to 1860. From 1860 to 1990, however, it decayed by about 5 percent per century. If the field weakens enough, the poles will disappear altogether. Compasses will then be useless for a few years to a few thousand years until the new, reversed field emerges. At the current rate, the field will vanish in 2,000 years. While a depleted field will increase our exposure to solar radiation, that doesn't necessarily spell disaster. "I don't think huge ozone holes will open up," Gubbins tells doomsayers. "We've had lots of these, and the human race has survived."

David Gubbins is familiar with the problem of marine navigation—he sails a Sadler 29 from Whithorn, his home port in southern Scotland. A bit old-fashioned, he still plots his course with a needle and sextant. He also couldn't help noticing details in the ships' logs about life on board—including records of who had been flogged, when, and why. "I acquired enormous respect for early explorers," he says.

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