Flips have happened as close together as 50,000 years, though the last one was 780,000 years ago. But, as discussed at the Greenland Space Science Symposium, held in Kangerlussuaq this week, the signs are that another flip is coming soon.
One of those signs is that the strength of the field has been falling by 5% a century recently. A similar (though more rapid) diminution accompanies the reversing of the sun's magnetic field, which happens every 11 years or so. Other evidence comes from old navigation records. Researchers such as Nils Olsen, of the Danish National Space Centre, have used such records to chart the growth of patches of abnormal magnetism. They are able to do so because these records use both compass bearings and astronomical observations to locate a vessel. The changing relationship between the two shows that patches of abnormal magnetism have been growing off south-east Africa and in the South Atlantic.
Just when the magnetic field will flip is impossible to predict from what is known at the moment; the best guess is that there are still several centuries to go. Nor is it clear how long its protective shield will be down. (The record in the rocks is little help, since a geological eyeblink represents many human lifetimes.) But understanding how the magnetosphere works now should help to deal with the consequences if and when it vanishes.
I'm wondering if all the recent exposure (Economist, USA Today, History Channel, Hollywood movies) is a good thing, because some people with money or influence might be convinced to do something, or bad because it is over-popularizing, dumbing-down and making it all sound a bit crackpot.
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