I’m a bit behind with Horizon as there’s a new episode showing right now, but today I got to watch the previous one, “The Core”. Once again I’m impressed.

The programme is all about the innards of the planet we call home. The center of the Earth is a lump of nickel-iron alloy almost as big as the Moon. But it is not a spherical lump of rock. Experiments putting the material under high temperature and pressure in a laboratory show that under those conditions it forms rod-like crystals. The Earth’s inner core, therefore, is a giant cluster of linear crystals. Some of the properties of the core are defined by its structure including, of course, its magnetic field. The north-south alignment follows the alignment of the crystals.

But things are not that simple. Surrounding the inner core is a sea of molten metal about the same diameter as Mars. Like all fluids, it is dynamic, and can be turbulent. The “weather” in this outer core affects how the magnetic field behaves.

We have known for a long time that the Earth’s magnetic field sometimes flips. What is now the north pole has spent time as magnetic south, and vice versa. These changes have happened as often as five times in a million years, but there hasn’t been one for almost 800,000 years. It looks like we may be due.

What geologists have discovered, however, is that the flip is not a sudden event. Indeed, there are parts of the outer core where the field is already reversed. One of the most notable is an area currently centered under Uruguay which causes the South Atlantic Anomaly. We don’t notice this much of the time because the magnetic field that we experience is the sum of the fields generated by all parts of the core. If you were to stand in Montevideo with a compass it would not point south. However, if you measured the strength of the field it would be a lot less than you would find at other points on the globe.

Where this can matter is out in space. The Earth’s magnetic field protects us from all of the charged particles thrown out by the sun. Where the field is weaker, the Van Allen Belts, where those particles are confined, are closer to the surface. That’s still a long way up, but the Hubble telescope has to switch off its instruments when it passes over the Anomaly because of the interference. Astronauts have been affected as well, though as yet none of them have become superheroes.

So, are we due for a flip? If so, how soon, and what will the effects be?

Of course we don’t know. This will be the first time that human civilization has experienced a flip. We do know, however, that the South Atlantic Anomaly has been getting more pronounced of late. We know that the overall strength of the magnetic field has been falling constantly since regular measurements began in the 19th Century. And archaeological evidence suggests that the field was twice as strong in Roman times as it is now.

The good news is that no major extinction events have been correlated with past flips. There may be particular problems for species that depend on the magnetic field for things such as navigation on migrations, but it seems like the field still protects us from most of what the sun throws at us, and life will continue to be possible during a flip.

What a flip will do to our increasingly electronics-based technology is another matter.

Those of you outside the UK who are blocked from using the iPlayer might like to investigate TunnelBear. I have not used it, but Neil Gaiman has been tweeting happily about it while he has been in the UK so it may work the other way around too.