When a reversal does eventually happen, we have a good idea of what it will and won't do — both to us and the planet …The idea that Earth's magnetic field will flip — switching the north and south magnetic poles — isn't anything new. It's even been the topic of choice for some doomsday prophets, who say that it will destroy the planet or at the very least turn it on its side, putting an end to us in either case.
It's well known that the polarity of Earth's magnetic field has changed over the course of history. Geologists have seen this change by examining the floor of the Atlantic Ocean. When magma wells up at the Mid-Atlantic Ridge and is cooled by the ocean water, crystals in the magma will orient themselves in the direction of the Earth's magnetic field. Thus, bands of cooled magma along the ocean floor are a geologic record of what direction the magnetic field pointed in during Earth's history, and show that the magnetic field has reversed many times in the past, and these reversals last anywhere from 100,000 to 50 million years or so.
Within the last 5 million years, the reversals have been characterized by long periods of stability lasting between 450,000 and 590,000 years, each separated by several shorter-term reversals. The last reversal happened around 780,000 years ago, so by the reckoning of some, we are long overdue.
The Earth's magnetic field is generated by electric currents in the liquid-iron outer core that surrounds the planet's solid-iron inner core. As the planet rotates, the motion stirs the liquid outer core into roughly helical-shaped patterns from poles to equator, and like copper wire wrapped around an iron bar, the electric currents flowing in these patterns generates the magnetic field. Reversals are thought to occur when large areas of the outer core become oppositely-aligned to their surroundings. If enough areas of the outer core do this, the entire magnetic field will reverse. This does not happen suddenly, though. It takes from thousands to tens of thousands of years for the polarity to completely reverse. It's what will happen during the reversal that is the cause for concern.
When a reversal does eventually happen, we have a good idea of what it will and won't do — both to us and the planet.
Speculation by some — mainly the doomsday prophets — is that when the magnetic field flips, it will be because the solid-iron inner core of the planet flips 180 degrees, and that this will cause the surface of the planet to shift at least 90 degrees, effectively switching our lines of latitude and longitude, and destroying us in the process. However, that just isn't true. Only the size of the inner core affects the magnetic field, with a growing core making a stronger field and a shrinking core making a weaker field, and a shift in the magnetic field will not effect the orientation of the solid core at all. Even if it did, it would take tens of millions of years for such a change to spread out from the core, through the liquid outer core, and through the mantle, before it reached the Earth's surface and did us in.
What a magnetic reversal would do is cause a weak period in the magnetic field strength for a thousand or more years, as the various conflicting magnetic fields in the liquid outer core sort themselves out to all orient in the same direction. A weaker magnetic field would expose us to heightened amounts of solar radiation, and solar particles could even punch large, although temporary, holes in the ozone layer. Our technology would bear the greatest brunt of this kind of event, though. Even now, solar storms and coronal mass ejections can cause havoc with computers, communications systems and power grids. This would be much worse with a weakened magnetic field. We would likely lose every satellite in orbit, including GPS and communications, and our power grids would suffer regular disruptions or complete failure. Additionally, there would be several magnetic poles during this time, making travel by compass much more difficult. What's certain is that it wouldn't be the end of us, but the loss of technology would be the end of our current way of life.
There have been some signs that the magnetic field may be on the move, since within the last century, Earth's magnetic field has shifted and has lost some of its strength.
"Magnetic north has migrated more than 1,500 kilometres over the past century," said Conall Mac Niocaill, a geology and geophysics professor at Oxford University. "In the past 150 years, the strength of the magnetic field has lessened by 10 percent, which could indicate a reversal is on the cards."
Or it may not. The field changes, both in direction and strength, are well within the normal range that we've seen over the past million years or so. Also, just because something has happened in the past is no guarantee that it will happen in the future.
On the other hand, a large area underneath South America and the South Atlantic — called the South Atlantic Anomaly — was identified in the 1990s as a weak spot in the Earth's magnetic field, and could indicate a large area of the liquid core changing its polarity. It seems to be the only large area of weakness, though, so who knows if it is simply a temporary phenomenon, or a sign of things to come.
The European Space Agency is launching its 'Swarm' magnetic field mission next month. With two satellites in polar orbit to examine the magnetic field directly, and one satellite in far orbit to examine the effects of solar activity on the field, the mission could bring us much closer to understanding the Earth's magnetic field and gain more insight on how it is changing.