THE epic voyages of Captain James Cook and other mariners tell the story of the Earth鈥檚 magnetic field. It seems that the gradual weakening is a relatively recent phenomenon, so the Earth may not be on track for a reversal of the north and south poles after all.
It was common practice for captains to use the magnetic field to calibrate their ship鈥檚 compasses relative to true north. 鈥淭heir lives depended on it,鈥� says David Gubbins, an expert in geomagnetism at the University of Leeds, UK. Mariners also sometimes measured the steepness at which magnetic field lines entered the Earth鈥檚 surface. By sifting through ships鈥� logs recorded by Cook and others dating back to 1590, Gubbins鈥檚 team has greatly extended the period over which the behaviour of the magnetic field can be studied. They have found that the decline in Earth鈥檚 magnetism was virtually negligible before 1860, but has accelerated since then.
The field鈥檚 strength is now declining at a rate that suggests it could virtually disappear in about 2000 years. This may be the prelude to a magnetic reversal, when the north and south magnetic pole switch. Alternatively, the weakening could be down to a magnetic anomaly in the southern Atlantic Ocean (Science, vol 312, p 900).
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