Is the Earth’s magnetic field about to flip?

[Note: this was originally intended for the latest edition of The Accretionary Wedge, now up at Clastic Detritus, which asked the geoblogosphere to look to the geological future. Sadly, it took much longer than I thought it would, and is therefore a bit late - but what's a few days to a geologist?]
ResearchBlogging.orgIt’s fairly common knowledge that the Earth’s magnetic field periodically reverses its polarity. At the moment, magnetic field lines run from the south pole to the north pole, and point up in the southern hemisphere and down in the northern hemisphere, as in the figure on the left below. But at many points in the past, the field lines (and compasses, if they’d been invented) pointed south, and, as the figure on the right below shows, were directed upwards in the northern hemisphere and downwards in the southern hemisphere.

normal_reversed_field.png

Rocks record the direction of the ambient magnetic field as they form, allowing us to reconstruct the history of these reversals. In the next figure, periods when the field is “normal” (the same as the present day) are in black, and periods when it is in the opposite, “reversed” polarity are in white. The field last flipped over about 780,000 years ago (0.78 million years); previous reversals occurred about 0.99, 1.07, 1.19, 1.2, 1.77 and 1.95 miilion years ago.

Reversal record for the last 2 million years
The last couple of million years worth’ of reversals. Each polarity interval, or ‘chron’, is named after either a famous palaeomagician (Brunhes, Matayama) or the location where it was first identified (Olduvai).

You can’t help but notice that the typical period between the reversals in the last couple of million years is a lot less than 780,000 years, which is why you might hear talk about us being ‘overdue’ a reversal. Is this true? When can we next expect the field to reverse? And should we care if it does?


The question of whether we should be expecting a reversal is most easily addressed by taking a longer view. The symmetric pattern of magnetic anomalies either side of mid-ocean ridges, give us a continuous record of magnetic reversals stretching back almost 200 million years. This shows that the time between reversals is not constant, varying from a few hundred thousand years to many millions of years, as it did in the Late Cretaceous normal “superchron” (the big black block between about 85 and 125 million years ago) where the field retained the same polarity for around 40 million years.

Geomagnetic polarity timescale

This longer view suggests that trying to predict the geomagnetic future from the ‘periodicity’ of past reversals is a risky enterprise, to say the least. But that’s not the end of the story; by looking at the recorded behaviour of the magnetic field prior to previous flips, it might be possible to identify precursors that may herald the next one. We palaeomagicians are particularly interested in how the strength of the Earth’s magnetic field varies during reversals, because geodynamo theory suggests that reversals are probably more likely to happen when fluctuations in outer core convection weaken and destabilise the dipole. This prediction is particularly relevant in light of the fact that the strength of the Earth’s magnetic field has been steadily decreasing, at a rate of about 5% a century, since we first started directly measuring it in the mid 1800s. Is this a prelude to a reversal? Without a better idea of how the geodynamo behaves over geological time, there’s no way of knowing.

modern_mag_field_intensity.jpg
Source: BGS

Fortunately, rocks can preserve information about the palaeointensity (literally, ancient intensity) of the Earth’s magnetic field. Effectively, the stronger the magnetizing field, the stronger the rock’s magnetisation will be. If you measure the magnetisation acquired by your rock samples in artificial magnetic fields of known value, you can use this experimentally determined relationship to infer the strength of the geomagnetic field required to impart the original remanent magnetization.

how to determine palaeointensity

In practice, reliable measurements of absolute palaeointensity can be quite tricky*. However, although absolute values would be nice, we’re actually most interested in relative changes in the strength of the field over time – and if you can find some nice continuous sedimentary sections with constant mineralogy, you can be reasonably sure that changes in the intensity of the magnetisation as you move up- or down-section are mainly due to changes in magnetic field strength at the time of deposition (in other words, if the characteristics of the recording medium stay constant, then variations in the record must be due to changes in the input signal). Increasing numbers of relative paleointensity records, mainly from measurements of marine sediment cores, have been published in recent years. If you’re clever, like Jean-Pierre Valet et al. were in their 2005 Nature paper, you can stitch and stack** the most reliable of these records together, to produce a composite curve of the last 2 million years of relative variations in geomagnetic field strength.

Sint-2000 composite relative palaeointensity curve

There’s no other word for it – these data are beautiful. Every switch in polarity occurs when the field is at its weakest – the palaeointensities are lower during reversals than at any other point in this record. Rather intriguingly, slightly less extreme dips in the field intensity appear to be associated with what are called excursions – periods where changes in the direction of the recorded magnetization show that the magnetic poles have wandered quite a long way from the geographic poles, but without the field reversing. So, just as geodynamo theory predicts, a weakening of the magnetic field seems to be associated with less dipole-like behaviour, and reversals. It’s interesting to note that the field seems to have been stronger, on average, since the last reversal, which might explain why this current polarity interval has lasted longer than the preceding ones.
The other interesting thing about this record is that the field has behaved in quite a consistent way during every reversal. In the figure below, Valet et al. plot the changes in field intensity across each individual change in polarity atop one another, with time decreasing from right to left. In each case, the reversal is preceded by at least 20,000-40,000 years of fairly continuous decay in field strength to about the same (very low) value, with a much more rapid recovery in field strength following the transition. In this context, a couple of centuries’ worth of field decay is not particularly significant, especially since the present field strength (about 8 on the scale in this figure) is still a lot higher than the value reached during all of these reversals (<2).

Intensity variations through the last few magnetic reversals

To really place recent field behaviour in context, we really need a better idea of what was going on geomagnetically prior to the instrumental record. As it turns out, archaeological sites are a very good source of absolute palaeointensity data: – human artefacts like hearths, forges, and kiln-baked pottery are pretty good magnetic recorders, with easy to understand behaviour. In a recent paper, Knudsen et al. use the wealth of palaeointensity measurements from the last 12,000 years or so to produce a best-fit model of dipole strength for the Holocene (the grey shaded region is the estimated error – and note that the time axis here runs the other way to all the other figures…).

Intensity of the geomagnetic field in the Holocene

This shows that for most of the last 12,000 years, the field strength was actually lower than it is today; it increased between about 4,500 and 2,500 years ago, and after this peak- which is high even when you look over the last 2 million years – it has been decreasing again for most of the last 2,000 years. If you compare the duration and magnitude of the recent decrease (the red box in the figure below) to the field behaviour for past reversals, it doesn’t look like we have to worry about the field reversing for a while – it looks like it needs to continue weakening for a few thousand more years, and reach a much lower strength, before a reversal is going to happen.

Past reversals compared to recent geomagnetic variation

Of course, another thing you should get from the Valet et al. paper is that a full reversal sequence is not an instantaneous event; our compasses will not point north today and south tomorrow. Instead, the geomagnetic field will weaken, and the magnetic poles will start to wander to lower latitudes, and possibly multiply, over a period of hundreds and thousands of years. It looks dramatic from the perspective of Deep Time, but during a reversal the changes over a human lifetime will probably be little different from the secular variation we see today. No extinction event has ever been linked to a magnetic field reversal, and I think that we might just cope with the next one too – whenever it might occur.

*The magnetic behaviour of a sample in response to fields applied over timescales of maybe a few minutes in the lab might be very different to its response over the thousands or tens of thousand of years that it might take to be magnetised in the wild. Problems with complex magnetic mineralogy, and overprints, and possible thermal alteration during the experiments, are also rife.
**Stacking is just an averaging technique, and helps to remove noise and variations in the field local to the region each core was taken from, producing a smoother global signal.
Jean-Pierre Valet, Laure Meynadier, Yohan Guyodo (2005). Geomagnetic dipole strength and reversal rate over the past two million years Nature, 435 (7043), 802-805 DOI: 10.1038/nature03674
M KNUDSEN, P RIISAGER, F DONADINI, I SNOWBALL, R MUSCHELER, K KORHONEN, L PESONEN (2008). Variations in the geomagnetic dipole moment during the Holocene and the past 50 kyr Earth and Planetary Science Letters, 272 (1-2), 319-329 DOI: 10.1016/j.epsl.2008.04.048

Categories: basics, geology, geophysics, palaeomagic, paper reviews
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Comments (42)

  1. Sean C says:

    Umm. Yes we should care about when the poles flip. We store most of our digital world on magnetic media. I can’t imagine an event like this would be gentle on them. Then there’s the considerations surrounding the magnetosphere and how it protects us from solar rays. If there was even a slight ripple in our protective covering, I think we’d be in for a world of hurt.
    At the same time, i’m glad that it shifts, at least there’s something beyond our understanding out there.

  2. Greg Laden says:

    If the poles reverse, all the 0’s will become 1’s and the 1’s will become 0’s!!!!!11!!!
    But seriously, this is a nice post and should be distributed to Earth Science teachers everywhere.
    One interesting question is what happens to oganisms, especially birds, that use the magnetic signal to orient for long distance migrations or other navigation. It is probably true that some birds rely heavily on the magnetic field, and literally would get lost. This might include birds like loons, where the new brood of a given year find their way to their wintering grounds without the benefit of adults (the adults head out weeks earlier). In most cases, migrating birds probably use a set of different inputs possibly including sun position, prior learning (map building) etc.
    Presumably adjustments are made during periods of excursion, but one would think that rapid shifts would cause a certain amount of disruption and confusion, and maybe a bit of unnatural selection ….

  3. spyra says:

    Darn. I would have looked forward to that pole change during my lifetime. :\

  4. rpenner says:

    “I can’t imagine” — argument from personal ignorance and personal incredulity is the hallmark of all truly memorable anti-scientific thinking.
    Sean C, we aren’t talking about the Earth’s magnetic field going away but of it pushing energy from the dipole mode to the higher multipole moments. The magnetic field does little to protect us from “solar rays” except in the antique sense where rays includes charged particles. Our atmosphere does a great job of protecting us from these charged particles, by itself.
    Charged particles from the solar wind (~ 1 keV) would slowly erode Earth’s atmosphere if the magnetic field went away. Cosmic rays ( ~ 1 GeV and up) get through today would be unchanged. Intermediate charged particles ( ~ 1 keV – 1 MeV) would cause aurora effects but would cause little ground effects at sea level. And as the magnetic field is not expected to go away even if a reversal was imminent, I think the main effect would dubiously-phrased articles about the “northern lights” “moving south” during the generations prior to the actual reversal.

  5. AnonyMoose says:

    “magnetic media. I can’t imagine an event like this would be gentle on them.”
    Don’t worry about it, magnetic media are exposed to much stronger magnetic fields. Walk around your car and work area with a compass. Don’t forget to check a CRT-based computer monitor, the kind which many people put on a computer, and the steel shelves and desk.

  6. BrianR says:

    Great post Chris … I’ve updated the Wedge post. Better late than never!
    One question — what’s the deal w/ the Cretaceous superchron? Did robust Atlantic spreading keep the dipole from destabilizing somehow?

  7. Silver Fox says:

    Chris, this is a really great post, and you’ve explained everything very clearly. I didn’t really understand the field mimimums before or that we should be able to see a reversal coming a at least a couple centuries in advance. And glad this made it into the Wedge.

  8. natural cynic says:

    Greg

    Presumably adjustments are made during periods of excursion, but one would think that rapid shifts would cause a certain amount of disruption and confusion, and maybe a bit of unnatural selection…

    Why would it be “unnatural selection” when it is the environment that effects migrating behavior. Would you consider [ancient] climate warming that isolates populations on islands as unnatural?
    As a study in evolution, this shifting of magnetic poles could be a nice example of rate of variation and selection on how the magnetic sense changes IOW, can it keep up> And also how evolution might emphasize other clues such as celestial and/or terrestrial navigation.

  9. Chris Rowan says:

    I’ve always figured that migrating species don’t rely solely on their magnetic sense – its used in conjunction with other cues from the sun and other landmarks. This would provide some redundancy. Either way, they seem to be able to cope.
    As for the Cretaceous Normal Superchron, palaeointensity measurements from that era suggest that the field was very strong, and therefore stable. As for why, it must be something to do with changes in the heat flux across the core/mantle boundary (higher flux=stronger field); some think that this might be related to deep subduction of plate fragments, or the more dramatic ‘slab avalanches’.

  10. Bob O'H says:

    the magnetic poles will start to wander to lower latitudes, and possibly multiply, over a period of hundreds and thousands of years.

    Multiply? Eek!
    Can you explain?

  11. agm says:

    Re: dipole and earth’s field changing
    It’s somewhat inartful to say that the earth’s magnetic field is a dipole. What’s more useful (at least for those not used to this) is to say that the dipole is the dominant term when we want to represent our observations of the earth’s magnetic field using a multipole expansion, such as the spherical harmonics scheme in the IGRF/DGRF models (which is a reasonable description given that the earth’s geometry, composition, the math of multipole expansions, etc). And within about 2-2.5 earth radii distance of the earth’s center, it’s pretty reasonable to consider the earth’s field as mostly a dipole, with modifications locally based on space weather, secular variation/evolution of the geodynamo, mineral deposits, and other circumstances dependent on altitude and situation.
    When the OP says multiply, wander, and so on, this means that the relative strengths of various terms needed to make this a useful description of the earth’s field (the dipole, quadrupole, octopole, and so on) will change. The total field is described in this sort of model by adding up all the terms with coefficients with large magnitudes, so the net field looks different as the magnitudes of these coefficients vary and the signs flip.

  12. fulf says:

    Lovely post, positively thrilling. Thank you.
    I second Bob’s request for more explanation of multiple poles; agm made some sense, but I’m largely still wondering ‘why’ and a little bit of ‘how’.

  13. Ed Yong says:

    Great post Chris. Clearly wrong of course. Years of reading X-Men comics have taught me that reversing the Earth’s magnetic field can happen very quickly and causes massive global catastrophe. Fortunately it’s usually averted through superheroic intervention. So there. Take that with your fancy “facts” and “science”. ;-)

  14. Chris Rowan says:

    Convection in the outer core normally seems to have a fair degree of large scale organisation, which produces a simple, dominantly dipolar, magnetic field. What seems to happen during reversals is that convection becomes more disorganised; this means that the magnetic field produced is also more complicated, with shorter wavelength quadropole and octopole components becoming stronger at the expense of the longer wavelength dipole field. If they grow large enough, they will start to dominate the overall field geometry; hence you could get two or more “north” and “south” poles wandering over the surface.

  15. David Marjanović says:

    Umm. Yes we should care about when the poles flip. We store most of our digital world on magnetic media. I can’t imagine an event like this would be gentle on them.

    Why would a weakening of the magnetic field be a problem for magnetic media? A (very, very, very) drastic strengthening would be.

    Then there’s the considerations surrounding the magnetosphere and how it protects us from solar rays. If there was even a slight ripple in our protective covering, I think we’d be in for a world of hurt.

    Then why has this never happened before? Look at the third illustration of this post, the one that looks like a barcode. None of the changes, not one, is associated with even a small mass extinction event.
    There was a paper a few years ago that calculated that the stronger multipole field during inversions would induce a magnetic field in the ionosphere, which would take over the protection function of the magnetosphere. Unfortunately I forgot where it was published.

  16. fulf says:

    Thanks Chris, I think I can visualize it now.

  17. BAllanJ says:

    And I thought that these outer core convection cells are influenced by the position of the continents (thicker crust). So do these periods of long term stability in field direction correlate to particular continent positions? (Non geo type commenting here, so if it’s an obvious q, that’s why)

  18. Austin says:

    Thanks Chris,
    I just emailed this article to my granddaughter. She, at her young age, is having occasional difficulty explaining to her friends (of the young earth Christian persuasion) her belief that scientific evidence is the best model for understanding the physical world. My continuing advice to her is to use actual scientific findings to educate her friends. This article (with some summarizing from her Granddad) will provide yet another educational tool for her. Again, thanks.

  19. GAC says:

    “Why would a weakening of the magnetic field be a problem for magnetic media? A (very, very, very) drastic strengthening would be.”
    Don’t pay so much attention to the troll. From the data it looks like by the time a reversal occurs, we may have no need for magnetic storage anyway. And if there were some sort of crucial devices that needed protected, we’d have plenty of time to find ways to shield them.

  20. Neil B says:

    I saw a Nova show about this flipping of the Earth’s B field a few years ago. They said the field was already getting more irregular and we should expect more “holes” (weak spots) away from the main poles and increased radiation and Auroras even in the short term (century not millennium order.)

  21. PS: The term “paleomagician” is evocative and cute, but is it a proper title for someone who studies magnetism and not magic?

  22. Porzitski says:

    Hello,
    I surfed in while looking at other things and got interested in the subject. The main thing that strikes me is that I see no mention of mechanisms, processes, forces or energy that might account for a flipping of the Earth’s magnetic field in the future (or for that matter, in the past). I see a reasonable argument for a decline in the strength of our magnetic field, but nothing that indicates (even remotely) impending reversal. Did I miss something? (theories on apparent historical events don’t count)
    PS – thanks for posting your work, I appreciate the willingness to share info and ideas on the web. Best wishes, PP

  23. David Marjanović says:

    The main thing that strikes me is that I see no mention of mechanisms, processes, forces or energy that might account for a flipping of the Earth’s magnetic field in the future

    The chaotic behavior of convection in the outer core, which is mentioned in the post.

  24. Superman says:

    Don’t worry folks. If the poles start to reverse I will make a large demagnetizer in my Superfortress and fly it around the world to stabilize the situation. I can’t do much about overpopulation however, or political stupidity. You Earthlings are on your own there.

  25. saxifraga says:

    Excellent post. I’ve never really known the background data for the magnetic excursions.

  26. Fascinating post, dude!

  27. Brendan C says:

    What would be the climatological effects, if any, of a reverse of the magnetic fields?

  28. DaveK says:

    Ok, here’s something to get paranoid about.
    One of the side effects of magnetic reversal is that for a period of time the earth’s overall magnetic field will be very irregular. During this time of an irregular magnetic field, the solar wind can do a very scary thing… our once-smooth magnetic shield becomes rough, and causes “eddies” in the solar wind. This, in turn, will result in big chunks of atmosphere being ripped off into interplanetary space. Over the time of past reversals, this probably doesn’t practically amount to much. But what if the reversal got “stuck” so to speak? The earth could relatively quickly (geologic terms) get stripped of air.
    There are some who posit that this is what happened to Mars some time in the past.

  29. Chris Rowan says:

    DaveK -this hasn’t happened in probably thousands of reversals in the last couple of billion years, so I don’t think this is really something to start worrying about. I’m not sure that a ‘stuck reversal’ is even possible: the Earth’s rotation tends to make a strong dipole a much more stable state.
    In Mars’s case, it’s more that it is smaller, and lost all its internal heat much faster than Earth, which weakened convection in the outer core (and mantle) pretty early in its history, so its magnetic field just faded away.

  30. Lab Lemming says:

    It looks like the Greeks picked a perfect time to discover lodestones…

  31. Ryan says:

    Chris: Be careful about Mars, it isn’t as simple as talking about it losing all its heat. There are two really good counterexamples in the terrestrial planets: Mercury is much smaller than Mars and almost definitely has an active dynamo, Venus is much bigger than Mars and does not have an active dynamo.
    There could be a number of reasons for Mars’s dead dynamo, it still has a molten core, so if it could suddenly start losing alot of heat, presumably the dynamo could start up tomorrow (well, geologically tomorrow).
    My point is, that the causes for the death of the dynamo aren’t well enough understood to be saying things without noting the extreme uncertainties in our knowledge of a dynamo that died billions of years ago.

  32. Ryan says:

    I forgot to mention, good post!

  33. Lab Lemming says:

    >There could be a number of reasons for Mars’s dead dynamo, it still has a molten core
    Says the modelling- until we get a seismometer up there we won’t be able to confirm that hypothesis.

  34. Ryan says:

    Using radio tracking of Mars Global Surveyor some people confirmed that there is a part of the outer core that remains liquid to this day. The reference is Science 300, 299 (2003).
    Admittedly they don’t know how big the inner core is, and probably won’t until we get seismometers up there.

  35. Tom Moore says:

    On the subject of extinctions, you might find this older paper interesting:
    http://www.nature.com/nature/journal/v259/n5540/abs/259177a0.html
    Nature 259, 177 – 179 (22 January 1976); doi:10.1038/259177a0
    Influence of ancient solar-proton events on the evolution of life
    G. C. Reid†, I. S. A. Isaksen*†, T. E. Holzer‡ & P. J. Crutzen§

  36. TuMmBLleWeEd says:

    well we know that the field itself reverses as found in iron deposits in the Atlantic pointing in different directions. It’s been doing this since the formation of earth. But, as far as a polar shift, where the crust itself moves, putting the north pole somewhere in saudi, I don’t know. That would destroy darn near everything. Not to mention shifting seas and a magnetic storm with winds speeds around 600 mph. Lets just hope its a magnetic reversal and not a polar shift.

  37. TuMmBLleWeEd says:

    A question ? If the poles did shift and the crust did move, how would this affect the gravitational pull the moon has on the earth and the earth has on the moon ?

  38. Chris Rowan says:

    Your ‘if’ presumes an event for which there is absolutely zero evidence. Just in case anyone was wondering.

  39. Brazos says:

    What do honey-bees use for navigation?

  40. Steven Smith says:

    Great info. I think we are in the starting process of a pole reversal. This past fall i noticed the geese were flying North. Very odd indeed I thought! Then this last week I read an article on whales being beached by the 100’s in Australia. This event was not limited to the one time but has happened several times over the last few months.
    I do believe a new age is almost upon us and that we will be enlightened when the pole shift occurs. When you see people doing mad acts as if during a full moon but 100x stronger, we will know the time is upon us. My prediction is major changes starting somewhere between Dec 2011 and February 2012 which is a precurser for the shift which will occur on the equinox of Dec 21, 2012.
    Prepare yourselves for 3.5 days of darkness when the earth stops rotating and the 6 hrs it will take for pole changes!

  41. Chris Rowan says:

    I’d thank you for the compliment, apart from the fact that you clearly haven’t actually read the actual post – if you had, you would find out that my ‘great info’ basically boils down to (i) field unlikely to reverse for several thousand years and (ii) when it finally does, it will take a few thousand years more to do so. Oh, and the Earth will not stop spinning, the stars will not fall from the sky, and the Great Old Ones will not return to eat us all (probably).
    The risk of further idiot comments means that this thread is now closed.

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