Patrick Barry in New Scientist:
The voyages of Captain Cook have just yielded a new discovery: the gradual weakening of Earth’s magnetic field is a relatively recent phenomenon. The discovery has led experts to question whether the Earth is on track towards a polarity reversal.
By sifting through ships’ logs recorded by Cook and other mariners dating back to 1590, researchers have greatly extended the period over which the behaviour of the magnetic field can be studied. The data show that the current decline in Earth’s magnetism was virtually negligible before 1860, but has accelerated since then.
Until now, scientists had only been able to trace the magnetic field’s behaviour back to 1837, when Carl Friedrich Gauss invented the first device for measuring the field directly.
The field’s strength is now declining at a rate that suggests it could virtually disappear in about 2000 years. Researchers have speculated that this ongoing change may be the prelude to a magnetic reversal, during which the north and south magnetic pole swap places.
But the weakening trend could also be explained by a growing magnetic anomaly in the southern Atlantic Ocean, and may not be the sign of a large scale polarity reversal, the researchers suggest.