Is Earth's faltering magnetic field turning south?

SAN FRANCISCO -- In the recent movie "The Core," Earth's molten core stops spinning, with dire effects on the magnetic field that protects the planet from energy-charged particles from the sun. People with pacemakers fall dead in the street; the Golden Gate Bridge collapses.

Scientists have known for some time that the magnetic field is in fact collapsing -- at a rate faster than it would if flows of molten iron in the core had stopped completely.

And while the consequences would be nowhere near as catastrophic as those depicted in the movie, geophysicists increasingly wonder whether the magnetic field has begun one of its occasional reversals that in a few centuries will cause compasses to point south instead of north.

At a meeting of the American Geophysical Union here on Thursday, scientists presented research investigating the cause of the current decline, its possible effects on the planet and what might be learned from geological records of earlier reversals.

The decline, as measured by magnetometers on Earth's surface, is 10 percent in the past 150 years. "We're seeing it's actually decreasing at a fairly impressive rate," said John A. Tarduno, a professor of geophysics at the University of Rochester.

Still, long-term fluctuations in the magnetic field are normal, and the scientists said the magnetic field could just as easily reverse and strengthen again.

Most scientists think that the Earth's magnetic field is produced by electric current generated by chaotic eddies in the molten iron of the Earth's outer core and that random changes in the eddies cause the reversals.

The magnetic field last flipped 780,000 years ago, but the time between reversals has varied from a few thousand years to 35 million years.

At the current rate, the field would disappear in 1,500 to 2,000 years. Even if the iron flows came to a complete stop, the electric currents would persist for another 15,000 years, leading scientists to conclude that the changes in iron flow have produced kinks in the magnetic field that are weakening it.

"They're twisting the field the wrong way," said Dr. Peter L. Olson, a professor of earth and planetary sciences at Johns Hopkins University.

The effects of such a reversal are hard to predict. In the fossil record, no noticeable die-offs of species coincided with earlier magnetic reversals, although some scientists speculate that the reversals may have disrupted migratory animals that use the magnetic field to navigate. "Whatever is occurring, life can adapt to on that time scale," Tarduno said.

A shrinking magnetic shield would increase the radiation risk to astronauts in orbit, especially during solar storms that eject jets of high-speed protons. The protons could also short-circuit satellites or even induce electrical surges leading to widespread blackouts.

A weak spot in the field, 30 percent weaker than elsewhere, has already appeared over the southern Atlantic Ocean. Some satellites passing over this region have suffered electronic glitches from high- energy particles that were able to penetrate the magnetic bubble.

Protons hitting the atmosphere also produce nitrogen oxide, which break apart ozone molecules that protect Earth from harmful ultraviolet light. Now, the nitrogen oxide appears mainly over the polar regions. Dr. Charles H. Jackman, an atmospheric scientist at the NASA Goddard Space Flight Center, showed computer models indicating that with a weaker magnetic field, a series of strong solar storms could cause "significant but not catastrophic" ozone destruction over a much larger portion of the planet. The damage would naturally heal in two to three years, he said.

If the magnetic field continues toward reversal over the next several centuries, additional magnetic poles could appear in the weakening field. A compass then would point in different directions depending on the location. Then the magnetic field would almost completely disappear for several thousand years before re-emerging in the opposite direction.

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