The Earth’s magnetic poles might feel like a constant – north is north, south is south – but they turn out to be a lot more complicated than that, as we found out yesterday in our event at the British Science Festival in Aberdeen, where Kathy Whaler, Conall Mac Niocaill and Ciaran Beggan took our audience on a tour of the Earth’s magnetic field and what it does for us.
The expeditions which made the south pole famous in the early twentieth century, a race won by Amundson and lost so tragically by Scott, were striving to reach the geographical South pole, rather than the the magnetic pole. And while the geographical poles are more or less (though not entirely) fixed, the magnetic poles are much less predictable.
So what’s the difference? The geographical poles are the two points where the Earth’s axis of rotation intersects its surface. That sounds straightforward enough, but the axis does wobble slightly so its not completely fixed. Magnetic poles are a different story – they’re constantly moving across the surface – making them a lot less reliable for navigation than you might think.
In fact, in the last century magnetic north has migrated more than 1500 km, which sounds like more than just a wobble. Not only that, but the strength of the field has declined by around ten percent in the last 150 years.
That sounds dramatic, but its nothing compared to what the Earth has experienced in the past.
‘From longer term records of the fossil magnetism recorded in rocks, we know that the poles reverse on average every quarter of a million years’ says Conall.
A total reversal of the field – north is south, south is north – sounds like pretty bad news. Even worse news is that the last time this happened was around 780,000 years ago – so it sounds like we’re well overdue another one.
‘There’s no way to tell whether this is the beginning of a reversal or just normal fluctuations’ says Ciaran. ‘We won’t know if we’re in another reversal until it happens.’
No need to panic, disaster movie fans. Although we know it will happen eventually, reversal is a random process, so that 780,000 year time gap is no proof it’s about to happen. And if it is, reversal takes around 3-5 thousand years to complete. So it won’t be even remotely like this:
When it does happen, its hard to know what the effects will be for Earth’s inhabitants, says Ciaran, because we’ve never been around to see it before.
‘The most immediate effects will be to low orbit satellites, which will need to be protected against solar radiation. Large power lines will need to be redesigned. Technology moves very fast, so there’s no reason to think we’ll be caught out.’
In the few thousand years a reversal will take, there will be a period where the field strength is so low it will effectively disappear. Compasses will be useless, and animals which rely on the field to navigate might be in trouble. There could even be ‘multiple poles’ – several north and south poles distributed throughout the planet.
Whatever happens, it will be slow, and we’ll have time to adapt. In fact, we should be thankful for magnetic reversals – they’ve been a key part of the evidence used to reconstruct the Earth’s tectonic history, and provided the vital clue for Fred Vine and Drummond Matthews in their development of plate tectonics in the 1960s.
It was the ‘stripes’ of the differently magnetised oceanic crust, symmetrical on either side of oceanic ridges, that hinted at a spreading of the sea floor from the ridges, and led to one of the most revolutionary theories in the history of Earth science.
So, no exploding mobiles or birds crashing into windows – but for Earth scientists, and for our huge audience yesterday, magnetic reversals still provide plenty of reasons for excitement.