A Rapid Reversal

…of the magnetic field, that it, has been proposed in a new paper by Sagnotti et al. in Geophysical Journal International.  An associated press release from Berkeley (home of the sixth author) breathlessly reports that this shows that magnetic field reversals happen far more quickly than models predict–within 100 years. The evidence is from paleomagnetic directions in rapidly accumulating lake beds accumulating during the last reversal of the magnetic field (the Bruhnes-Matayama reversal). The press release, after suggesting that our electrical infrastructure would be at risk of a rapid reversal, warns

And since Earth’s magnetic field protects life from energetic particles from the sun and cosmic rays, both of which can cause genetic mutations, a weakening or temporary loss of the field before a permanent reversal could increase cancer rates. The danger to life would be even greater if flips were preceded by long periods of unstable magnetic behavior.

“We should be thinking more about what the biologic effects would be,” Renne said.

Now this is an interesting paper, but a few points are in order.  First, this is a record from one spot; there is considerable evidence that during a reversal, while the dominant dipole field decays towards zero, higher order fields (like an octopole field–one with 8 poles instead of the 2 of a dipole field) remain.  What this means is that a reversal in one place might be at a different time or rate than another owing to the dominance of the higher order field terms; the reversal process overall could still be taking a longer time (1000 years is a pretty typical result from models and observations). This locale could just be sitting at a spot where you don’t see the directions wander about as much as other places. Second, as the press release somewhat grudgingly notes, there is no evidence in the fossil record for extinctions associated with reversals. It has been a popular idea widely tested and so far, nothing has stood up.  Since there are the higher order fields present during a reversal, it isn’t as though there is no protection at all on the surface of Earth during a reversal. And while energetic particles from the Sun might be more common, it is worth noting that the ozone layer would still be there protecting from UV light.

The press release more than the paper reminds GG of some issues with trying to isolate rapid changes in the earth system in the geologic past.  First, most geologic records are pretty slow and often spotty; for a long time, geologists, having nothing else to offer, would say that things happened gradually (dinosaurs gradually died off, glaciers slowly melted away, etc.). This was an artifact of the available records, but with greater detail in some places, some processes suddenly looked quite fast.  Dinosaurs might have gone extinct in a few hours or days.  The beginning of the Younger Dryas just after the end of the last ice age looks pretty fast when you look at some oceanic records. However, such rapid change found in one place may not mean the whole globe changed at once; for instance, an ocean current might shift to move a location from under warm water to under cold water while the global impact might be more subtle.  The impact hypothesis for the extinction of dinosaurs passed that test: records from around the world found the same thing as at Gubbio Italy.  The Younger Dryas onset is a bit more uncertain;  whether the Younger Dryas is even a global event or just a northern hemisphere event is still being argued, let alone the rapidity of the onset in the southern hemisphere. And so it is with this observation: is it really a global reversal in under 100 years or a peculiarity of this site’s position with respect to the higher order fields that might dominate within a reversal?

How is this paper groundbreaking? It is probably the highest resolution record we have of the last reversal and one of the highest recorded by sediments across any reversal.  However, it is far from the first (as the authors note in the paper’s introduction) to indicate that very rapid field reversals have occurred: some lava flows have variations in paleomagnetic direction between the rim and the interior of the flow suggesting rates of change of the paleomagnetic direction up to 6° per day (you could do a reversal in a month at that rate).  Because these individual lava flows don’t record the entire reversal, it can be hard to determine their overall place within a reversal, but the evidence of rapid magnetic field changes has been out there awhile.

Where do we go from here?  You would want to see equivalently high resolution records from other parts of the world, especially if they can be tied closely with absolute age dates like those reported here. (That is a tall order–they probably do exist but can be hard to find). If indeed the entire process takes under 100 years, that would be a valuable piece of information.  We’ve only really recently been able to model magnetic reversals in the earth, so violating the predictions of the current set of models isn’t necessarily saying that we have missed something big; it may just be that slightly different parameters need to be tried. So examining the sensitivity of these models to the duration of a reversal would be worthwhile.

Anyways, it is an interesting a useful article and, well, a typically overblown press release.

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