The last reversal happened about 780,000 years ago, over a period of several thousand years. Now Gauthier Hulot from the Institute of Earth Sciences in Paris and his colleagues think they have spotted early signs of another reversal.

They used data from the Ørsted satellite to study strange variations in the Earth's magnetic field. In particular, one large patch under South Africa is pointing in the opposite direction from the rest of the Earth's field and has been growing for hundreds of years.

The anomalies have already reduced the overall strength of the planet's magnetic field by about 10 per cent. If they continue to grow at the same rate, the Earth's dipole will disappear within just two millennia

There are other satellites which will be making a more detailed study, so expect revised guesstimates within the next few years. Article at New Scientist