Earth’s magnetic field

The BBC has an article by Nic Fleming about Earth and its magnetic field:

The Earth's magnetic field, which protects us from potentially dangerous solar radiation, is gradually losing its stability. No need to move underground or build space colonies just yet, though: the changes are taking place over millions of years.
You might assume that compasses will always point north, but in fact the magnetic poles have swapped places many times in the Earth's history. Earth scientists have long suspected that these flips are becoming more frequent, and that the magnetic field was less prone to pole reversals in the distant past. Now the most detailed analysis of the geological evidence to date suggests that the field really is slowly destabilizing. Whereas in the distant past it reversed direction every five million years, it now does so every two hundred thousand years.
The Earth's magnetic field is powered by the heart of the planet. At its center is a solid inner core surrounded by a fluid outer core, which is hotter at the bottom. Hot iron rises within the outer core, then cools and sinks. These convection currents, combined with the rotation of the Earth, are thought to generate a "geodynamo" that powers the magnetic field. The last major reversal was 781,000 years ago
Because of changing temperatures and fluid flows, the strength of the magnetic field varies, and the positions of the north and south magnetic poles shift. These shifts leave traces in rocks. When lava cools, metal oxide particles within the rock become frozen in the direction of the prevailing magnetic field. So scientists can work out the historic positions of the magnetic poles by examining and dating lava samples.
As a result we know there have been nearly two hundred magnetic pole reversals during the last hundred million years, and that the last major reversal was 781,000 years ago.
So are these reversals becoming more or less common? In theory, it depends on what is happening to the Earth's core.
Researchers believe the inner core is slowly growing, as the outer core cools and solidifies. That should mean more frequent flips. Simulations, by Gary Glatzmaier of the University of California at Santa Cruz and his colleagues, suggest that a bigger inner core would be more of an obstruction to currents in the outer core, making for a more unstable magnetic field. But it is hard to verify this, because in older rocks the evidence of magnetic field direction is less well preserved. So Toni Veikkolainen of the University of Helsinki in Finland, assembled a swathe of existing data from rock samples between five hundred million and three billion years old. First, Veikkolainen weeded out all the less-reliable data. For example, he rejected all samples containing hematite, because it can form a long time after the rest of the rock, leading to muddled data. He also left out slow-cooling rocks like granite, and threw out samples that were known to have tilted, unless they could be accurately corrected based on other evidence. Having whittled down around three hundred data sets to fifty, Veikkolainen estimated how frequently magnetic pole reversals occurred at different points in Earth's past.
The evidence points to a more stable field in the very far past and fewer reversals
He found that reversals happened about once every four million years between five hundred million and one-and-a-half billion years ago. But in an earlier time, between one-and-a-half billion and three billion years ago, the magnetic field only flipped once every five million years.
That is much less often than in the last hundred and fifty million years, when the field has flipped every six hundred thousand years. In the last ten to twenty million years, it has sped up even more, to once every two hundred thousand years.
"The evidence points to a more stable field in the very far past and fewer reversals," says Veikkolainen. "This looks to me to be the most thorough study that has been done so far, and it does reinforce many of the conclusions of earlier work, so I think it's pretty good evidence," says Robert Coe of the University of California at Santa Cruz.
Are we due for another flip? It's hard to say. Data collected by the European Space Agency's Swarm satellite array reveals the Earth's magnetic field has recently been weakening, at a rate of around five per cent per decade. The field does change all the time, but a rate of five per cent a century is more normal, leading some to speculate a pole reversal may be imminent.
It is unclear precisely what would happen if the field weakens greatly or disappears for a time during a reversal. However, scientists believe power grids and communications systems are potentially at risk.

Rico says he'll be happy if it waits until he's gone to do the next one...