The collapse of the Earth's magnetic field, which both guards the planet and guides many of its creatures, appears to have started in earnest about 150 years ago. The field's strength has waned 10 to 15 percent, and the deterioration has accelerated of late, increasing debate over whether it portends a reversal of the lines of magnetic force that normally envelop the Earth.
During a reversal, the main field weakens, almost vanishes, then reappears with opposite polarity. Afterward, compass needles that normally point north would point south, and during the thousands of years of transition, much in the heavens and Earth would go askew.
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On a planetary scale, the magnetic field helps shield the Earth from solar winds and storms of deadly particles. Its so-called magnetosphere extends out 37,000 miles from Earth's sunlit side and much farther behind the planet, forming a cometlike tail.
Among other things, the field's collapse, scientists say, could let in bursts of radiation, causing a variety of disruptions.
Dr. Charles H. Jackman, an atmospheric scientist at NASA's Goddard Space Flight Center in Greenbelt, Md., has worked with European colleagues on a computer model that mimics the repercussions. A weak field, they reported in December, could let solar storms pummel the atmosphere with enough radiation to destroy significant amounts of the ozone that protects the Earth from harmful ultraviolet light.
http://www.nytimes.com/2004/07/13/science/13magn.html