There'll be little warning if Earth's magnetic field flips
Even if we knew precise details of Earth's core, we would not be able to predict a catastrophic flip in the polarity of its magnetic field more than a decade or two ahead.

Our planet's magnetic field has reversed polarity from time to time throughout its history. Some models suggest that a flip would be completed in a year or two, but if, as others predict, it lasted decades or longer we would be left exposed to space radiation. This could short-circuit satellites, pose a risk to aircraft passengers and play havoc with electrical equipment on the ground.

To test whether we would see a flip coming, Gauthier Hulot of Denis Diderot University in Paris, France, and colleagues ran computer simulations of Earth's magnetic dynamo based on a range of plausible values for inputs such as the viscosity, electrical and thermal conductivity of the outer core, and the temperature difference across it. The model's predictions remained consistent over this range of values for no more than a few decades, Hulot's team will report in Geophysical Research Letters. Their result implies that we can forecast a flip only this far in advance - and then only with data that is as precise as possible. "It's like predicting the weather," says Hulot.

The last polarity switch was around 800,000 years ago. Over the past few decades, the magnetic field has weakened rapidly enough to flip within a few thousand years, but this could also be part of a more limited variation.