Going back millions of years into Earth’s history, our planet’s magnetic field has frequently gone its own way. The magnetic north pole has not only wandered through the north, but it has changed places with the south magnetic pole, taking up residence in the Antarctic. Going back millions of years, there’s a regular pattern of pole exchange, with flips sometimes occurring in relatively rapid succession.
In those terms, our current period of pole positioning is unusually long, with the last flip occurring nearly 800,000 years ago. But the magnetic field has grown noticeably weaker since we started measuring it over a hundred years ago, it has grown noticeably weaker. The poles have wandered a bit, and there’s an area of even more dramatic weakening over the south Atlantic. Could these be signs that we’re due for another flip?
Probably not, according to new research published with the refreshingly clear title, “Earth’s magnetic field is probably not reversing.” In it, an international team of researchers reconstructs the history of some past flips, and argues that what’s going on now doesn’t much look like previous events.
The work relies on reconstructing the global magnetic field tens of millions of years ago. Whenever rock is formed—from sediment deposits or volcanic eruptions, for example—the Earth’s magnetic field influences how small particles of magnetic materials line up within the newly formed rock. That influence gets locked into place as a magnetic signature in the rock, one we can read today. Combine that with rocks we can get dates on, and it’s possible to tell what Earth’s magnetic field was doing in the distant past.
But, as we mentioned above, the Earth’s magnetic field isn’t entirely uniform; it changes location and can develop weak points. To get a more complete picture, the researchers pulled out data on the magnetic field’s strength and orientation from around the globe. This was then plugged into a global model, which took locational information into account to build an estimate of the entire magnetic field’s strength during the period of two reversals.
The period covered was between 30,000 and 50,000 years ago. We mentioned earlier that the magnetic field has had the same orientation for nearly 800,000 years, so this would seem like a rather dull period. But the reality is that the Earth’s magnetic field has only had that orientation for of the time. About 41,000 years ago, there was something called the Laschamp event, where the field reversed for only about 500 years. It also covered something called the Mono Lake excursion, where the poles shifted positions dramatically before returning to closer to the geographic poles.
The good news? In neither of the events did the magnetic field look much like today’s. Rather than still having strong poles (as we currently do), the magnetic field as a whole was relatively weak. And, rather than having a single area of weakening as we currently see, there were multiple areas around the globe, including near the equator, where the local magnetic field reversed shortly before the entire planet’s field flipped. During this time, the elevated presence of some specific atomic isotopes indicate that the weakened magnetic field allowed more cosmic rays to reach the surface.
Flips that were flops
But the changes during these times were often rapid, so we shouldn’t be entirely confident that what we’re seeing might not suddenly resolve into something new and more similar to earlier events. But researchers, as their paper’s title implies, don’t think that will be the case. And that’s because their model has identified a couple of times when the magnetic field looked a lot like the current one, and there wasn’t a flip.
These occurred 48,500 and 46,300 years ago. In both cases, there was an area of low magnetic field intensity in the southern hemisphere; in one case, centered over the south Atlantic, and on the other just to the west, on the far side of South America. At least one of these corresponds to a period of elevated production of some specific isotopes, again suggesting that the field had weakened.
So the authors argue that these are the real analogs of our current situation, and they resolved without a big flip. “We infer that for excursions to occur,” they write, “a weakening of the field across much of the globe spreading from multiple sources is required, and not just localized weakening expanding from an South Atlantic Anomaly-like feature.”
And that’s relatively good news. In addition to the possibility that a pole flip would make countless compasses obsolete, the increased cosmic rays allowed into the atmosphere by a weakened magnetic field could also cause problems. That’s because, in addition to creating some new isotopes, this sort of radiation can damage DNA. There’s been a semi-regular worry about the prospect that a magnetic pole reversal would burden life on Earth with a greatly elevated rate of mutations, so this study is reassuring.
Less reassuring, however, is the fact that the researchers identified a few additional periods where production of these isotopes spiked without anything obvious going on in the magnetic field. There are clearly some things we still need to get a handle on.