Iron-age arsonists’ leftovers hint at how Earth’s poles may flip

When Earth’s magnetic field will flip remains uncertain (Image: G. Glatzmaier, Los Alamos National Laboratory/ P. Roberts, UCLA/ SPL)

Flipping heck! Deposits from fires set by farmers centuries ago reveal that Earth’s magnetic field dramatically weakened in the past without actually flipping – suggesting that current field weakening might not necessarily lead to a pole swap either.

Earth’s poles have swapped in the past, but without a regular pattern, says Rory Cottrell of the University of Rochester in New York. This means we don’t know when they will flip again – but many suspect it might be soon: the field has been weakening since about 1840. A flip may affect our power grids and communications systems.

Cottrell’s team examined magnetic minerals that had their magnetism orientated when South Africa’s farmers lit fires between 500 and 1000 years ago.

This captures the size and direction of Earth’s magnetic field. “It fixes the magnetic field at that time,” says Cottrell.

The analysis shows that around the year 1370, field strength was falling by 0.054 microteslas a year – substantially faster than today’s drop of 0.036 microteslas.

Until now, we had a poor record of magnetic field changes in the southern hemisphere.