SAN FRANCISCO--Earth's magnetic field is rapidly getting weaker, and geophysicists don't know why. The decrease in strength--a startling 10% in the last 160 years--could signal that the magnetic field is starting one of its sporadic flip-flops. But even if it's just a temporary blip, Earth's atmosphere may sustain some damage, according to reports here 11 December at a meeting of the American Geophysical Union (AGU).
Swirling liquid iron in Earth's core generates a magnetic field that loops far into space in a dipole pattern similar to that formed by iron filings around a bar magnet. By studying ancient magnetic fields locked in the metallic grains of volcanic rocks and sediments, geologists know that the field occasionally reverses: The north magnetic pole becomes the south, and vice versa. Before and during that transition, the strength of the field plunges. Today, the dipole is weakening so quickly that it would vanish within 2000 years if the current rate continues. Some scientists have wondered whether this is the early stage of a reversal, because the field has been stable for an unusually long 780,000 years.
It's clear that some process in the core is actively destroying part of the dipole, says geophysicist Jeremy Bloxham of Harvard University. Most destruction is happening in one spot: the "South Atlantic Anomaly," a patch of reversed magnetic field lines that emerge into space near the southern parts of Africa and South America. Bloxham's simulations of circulation in the core show that such patches sometimes grow into planet-wide reversals. However, most of them peter out within a few centuries as the core restores its normal patterns.
The geologic findings agree, says paleomagnetist Robert Coe of the University of California, Santa Cruz. Records preserved in rocks show that the magnetic field commonly weakens, wanders, and then reestablishes itself--"excursions" about 10 times more common than full-fledged flip-flops. "In the last 50,000 years, there were many times when the field strength was a lot lower [than it is today] without reversing," Coe says. "The dipole may be stumbling, but it's far from a collapse."
Still, meeting speakers noted that a weaker field leaves Earth vulnerable to high-energy particles from the sun and space. More satellites may suffer damage as solar ions penetrate deeper into the planet's weakened magnetic shield. Computer models also suggest that if the dipole keeps dropping, blasts of protons from major solar storms could destroy up to 40% of Earth's ozone at high latitudes for months to years at a time, says atmospheric physicist Charles Jackman of NASA's Goddard Space Flight Center in Greenbelt, Maryland.