Fears that a localized weakness in the Earth's magnetic field indicates an imminent flip in the field's direction are baseless, according to a new study. The direction of the geomagnetic field will flip eventually, but local anomalies like this are normal and occur whether such a change is coming soon or not.

When certain rocks are laid down, they are magnetized in the direction of the Earth's magnetic field. This simple fact has transformed geology, helping us unravel ancient continental movements. It has also revealed the field has fairly suddenly swapped direction many times over the planet's existence so the north and south magnetic poles swapped places.

We don't know why this happens, let alone when it will occur next, but every time something unusual is observed in relation to the field some people jump to the conclusion it's a harbinger of the next flip. A paper in Proceedings of the National Academy of Sciences shows the center of the latest kerfuffle, known as the South Atlantic Anomaly isn't actually unusual. If flips occurred every time an area of the planet experienced an unusually weak field, they'd be happening far more often.

A 10 percent decrease in magnetic field strength over the last 180 years could be a random fluctuation, but some worry it is a sign of things to come. Alarm rose further with the discovery that satellites sometimes malfunction over the south Atlantic because the field there isn't strong enough to protect against cosmic rays.

However, Dr Andreas Nilsson of Lund University says we simply lacked the awareness of previous versions of the anomaly. "We have mapped changes in the Earth's magnetic field over the past 9,000 years, and anomalies like the one in the South Atlantic are probably recurring phenomena linked to corresponding variations in the strength of the Earth's magnetic field," Nilsson said in a statement. Dips in field strength and local anomalies occurred frequently throughout the period studied, and tended to coincide, before reversion to the mean. A period around 2,600 years ago looks like a particularly good match for what is occurring today.

To establish this, Nilsson and co-authors investigated evidence of field strength from around the world over that period, including volcanic samples, sediment deposits, and clay pots fired at 580 °C (1070 °F) or higher. All of these, if they have enough magnetic elements in their composition, can record the magnetic field at the time they cooled or solidified. Although some such records already existed, Nilsson's team needed to develop new modeling techniques to reconstruct field strength from the patchy data.

Although the authors are not able to tell us when the next flip will come, they do make one prediction. "Based on similarities with the recreated anomalies, we predict that the South Atlantic Anomaly will probably disappear within the next 300 years,” Nilsson said. Might be a bit late for satellite operators, however.

On average, the Earth's magnetic field flips every 200-300,000 years, so with it (probably) being 780,000 years, it's certainly plausible one could happen soon, but the odds are against one in anybody's lifetime. We don't know if past flips did any harm – there has been speculation of increased exposure to radiation from space during a weak period during the flip, but no one knows for sure. Claims extinction rates rise during flips are debated.

For humanity, however, the dangers are probably greater. Quite what would happen, aside from compasses suddenly pointing in the wrong direction, is hard to tell, but it's plausible the damage from solar storms to electricity grids would rise dramatically during the flip. Stone tools, the closest thing we had to technological infrastructure during the last flip, were presumably much less susceptible.