The earth’s magnetic field is created by an internal dynamo of swirling magnetic material in its outer core. Heat convection through this liquid mass of mostly iron is thought to create an electric current and an associated magnetic field which extends into space surrounding us. This then interferes with the magnetic solar wind, protecting us from all manner of evil.
The liquid outer core is about 15% of the earth’s volume. In comparison our oceans are about 0.1% of the earth’s volume and the solid inner core is about 0.7%. A combination of gravity and nuclear decay in the core create the heat.
The whole of the earth’s mass is made up of about 32% iron, the most stable element according to its binding energy ( the end result of nuclear fission or fusion reactions) . Luckily the earth’s crust has a different composition to the core, like a floating scum of essential minerals spewed out from the centre, otherwise life would have little chance of being created or surviving.
The earth’s rotation controls the rotating liquid iron, creating an incredibly stable magnetic field. That said it does flip on an irregular basis, the last measured time being 780 000 years ago (from the rock record). Or so it was thought until recently when it has been suggested a possible reversal, or at least a loss of field (Laschamps Excursion) also occurred about 42 000 years ago, albeit a very temporary event of a few hundred years.
So how do we know that this happens? Mostly from the setting of magmatic flows about the mid-Atlantic Ridge. The Americas and Europe / Africa are on separate tectonic plates that have been moving apart for about 150 million years. Where they split new magma is continually flowing to the undersea surface and setting solid in place (think Iceland as a surface example). When cooled to solid this magma has a magnetic signature that locks in place a NS line, telling us when the magnetic field was one way or the other. A magnetometer can then tell the polarisation for any particular flow.
And what causes a flip? That is the big question and fundamental to understanding the timing of such flips. A better question might be ‘why don’t we have flips more often?’ It is thought that if the core is completely liquid, we would have regular flips every few thousand years, but the faster rotation speed of the solid inner core (also mostly iron) prevents this from happening. Magnetohydrodynamic computer models have shown that flips occur naturally based on our knowledge of the earth so far.
Is a reversal due? It is probably overdue and signs that the magnetic field appears to be currently weakening increase geophysicist’s fears. It has also increased its annual drift over recent decades.
Why is this a worry? The major concern is the time it takes to flip (another unknown) and whether the field diminishes substantially during this time. Earth’s magnetic field protects us from harmful particles ejected continuously by the sun as well as cosmic rays (high energy particles coming from wherever). If we had no magnetic field, we would be severely bombarded by cancer causing rays that would slowly strip off the atmosphere. Sounds pretty nasty.
If it happens, let’s hope it is quick. If not, we are in for a tough time. CMEs (Coronal Mass Ejections) occur regularly in the sun and if pointed directly at earth can cause what is called a Carrington event. The last big one was in 1859 with a few smaller ones since. Carrington events can fry the world’s electrical circuits by inducing currents in closed conducting loops (eg electric wires). In 1859 there were few of these, while today we have millions. If a Carrington event occurred during a reversal caused, magnetic minimum, the modern world would be in serious trouble. It has been suggested that a large enough CME may actually cause a flip!
Reversals have obviously occurred throughout the history of life on the planet, with unknown effect on the resident fauna and flora. Some animals depend on the magnetic field for navigation but have obviously survived previous reversals.
Recent studies of tree rings in ancient Kauri trees in New Zealand have suggested a major climate event with ecosystem collapses at the Laschamps Excursion. It is suggested that the magnetic field faded for a few hundred years creating severe weather events and a collapse of the ozone layer and may have been responsible for megafauna extinctions.
So science tells us that an event is likely (in the next few thousand years), possibly soon (due to mobility of current poles) and is likely to be catastrophic for modern humans. With any luck it is slow enough for us to recognise the event in advance and take some precautions. We have a precarious existence over geologic time.