There’s a renewed interest right now in Earth’s magnetic poles – specifically, whether or not they’re about to flip, and what may happen. The consequences of this seemingly rapid geomagnetic backflip may sound a little ominous, but don’t worry: we’re not sure when the next reversal will happen, and even when it does, the risks aren’t likely to be as scary as you may think.
Let's start with the basics.
As Earth's liquid, iron-rich outer core gradually cools, it sloshes around through colossal convection currents, which are also somewhat warped by Earth’s own rotation. Thanks to a quirk of physics known as the dynamo theory, this generates a powerful magnetic field, with a north and south end.
Although 99 percent of the magnetic energy remains within the core, the slithers that escape extend into space, and spends most of its time deflecting potentially deadly, atmosphere-stripping solar wind.
Right now, the magnetic north pole is exactly where you suspect it is; the same goes for the magnetic south pole. Both represent locales in which the planet’s magnetic field is vertical, and at which point your compass needle tries to point upwards.
Throughout geological time, these magnetic poles have switched sides – a phenomenon known as a “geomagnetic reversal”. Although there are several hypotheses that attempt to explain this, geophysicists are still a little unsure as to why it happens. It’s clearly something to do with turbulence and chaos within the metallic outer core, but the specifics haven’t been nailed down yet.
Either way, the last time a complete reversal happened was 781,000 years ago; dubbed the Brunhes-Matuyama reversal after its discoverers, its path could be traced through volcanic rocks that, upon forming, “froze” a record of the planet’s magnetic field arrangement in place. Prior to this point, today's magnetic north pole was at the south pole, and vice versa.
There was a temporary changing of the guard 41,000 years ago, but this only caused a reversal of 250 years or so before “normality” was restored. In any case, over the last 20 million years, the poles have flipped once every 20,000-30,000 years.
NASA is at pains to stress that reversals are the norm, not the exception. They’ve always happened, and always will.
The current bemusement stems from the fact that we’re 20,000 years or so “overdue” for a reversal, and it’s true that Earth’s magnetic field has been (rapidly) weakening by about 5 percent per decade in recent times – a sign that a reversal is perhaps on its way. This, however, doesn’t mean a flip is “imminent” or “soon” in human lifetimes.
Even if a flip is approaching, it won’t happen overnight. “Paleomagnetic evidence suggests reversals take around 1,000-5,000 years or so,” Associate Professor Phil Livermore, an expert on Earth’s geomagnetic field at the University of Leeds, told IFLScience.