At the Nantes meeting, the question of whether the poles are in the throes of a reversal was like Banquo’s ghost: unwelcome and mainly invisible. There was no session on it. Several posters touched on it but only at a slant, as if in code. When, just once, a scientist asked a direct question about it after a talk, the presenter neatly deflected. Yet everyone there knew that the responsibility to find out—if that were possible—rested uniquely with this community of scientific minds and with no one else. And not just the responsibility to find out but also to warn society if the switch were to show signs of arriving, just as climate scientists have had to shoulder the task of explaining the effects of high carbon dioxide concentrations in the atmosphere. Instead, it felt as if the geophysicists were chastened. Having seriously considered the possibility that a reversal was looming in the heady decades after satellite data began streaming in, they had pulled back.
I sought out Peter Olson, an emeritus professor at Johns Hopkins University, in Baltimore, Maryland. One of the world’s most eminent geophysicists, he wrote a famous commentary for Nature in 2002 titled “The Disappearing Dipole.” In it, he referred to the “amazingly rapid decline” in the dipole’s strength in the past 150 years. While carefully noting that it was “premature” to suppose that the dipole would continue to decline until it was gone, he pointed to the growing reversed-flux patches and wrote that they suggest an attempt at a reversal “may be underway.” That article accompanied another famous one in the same issue of the journal, also suggesting that the satellite data show the dynamo might be gearing up for a reversal. In Nature in 2008, Gubbins added that our current situation “could be the start of a reversal, but we have not yet reached the point of no return.” None of this was declarative—or too much different from what Finlay was able to conclude when I saw him—but it was a little bolder than the dominant thread today. And the scientific idea that a reversal was imminent was sexy enough to have penetrated the public discourse. There was a spate of articles in the popular press in the 1990s and even a 2003 movie, The Core, starring Aaron Eckhart and Hilary Swank. Its premise was that the core stops rotating, the Earth’s magnetic shield fails, and deadly microwave radiation immediately starts killing people by halting their pacemakers. Civilization is also threatened. It’s been panned as the worst physics movie ever.
Olson looked slightly resigned when I asked him about a reversal. We were assembling in the expansive main floor of the conference center in Nantes with the other participants, to have a group photo taken. It was hot and some people wore shorts, sandals, and short-sleeved shirts. Sure, this could be the foreshadowing of a reversal, Olson said. But it’s more likely just a phase. If the South Atlantic Anomaly were to expand to 30 percent of the Earth’s surface, then that’s a reversal. (Today, depending on how you measure it, it is just over 20 percent, up from 13 percent in 1955.) Will that happen? It’s simply not predictable, he said. It’s like predicting when and where a hurricane will hit a decade from now. One of his areas of research has been on the effects of a reversal on mass extinction. He hasn’t been able to find a link. “It’s not as if people haven’t thought deeply about this,” he said, adding that it would make the whole conference—here he gestured to the throngs of scientists—more relevant. As I thought about it, I realized that the meeting did not have the air of urgency one finds at, say, a conference on climate change, where the effects of the scientific phenomenon are playing out in every backyard in the world, sometimes with deadly consequences.
Next, I tracked down Cathy Constable, a geophysicist at Scripps Institution of Oceanography at the University of California, San Diego. An international leader in the art of using statistical techniques in geophysics, she and her co-investigators have been tireless in constructing models of the magnetic field that go back in time millions of years. She’s used them to examine what the field looked like during previous reversals and has compared those scenarios to today’s. During a break between sessions, I asked her if we are in the throes of a reversal. “Hell no!” she said. “Not in my lifetime!”
In 2006, Constable and Monika Korte from GFZ German Research Center for Geosciences in Potsdam published a paper meticulously outlining the case for an imminent reversal and doing the math to assess the likelihood. It was like reading a crisp legal brief. The assumption was that the more we know about past reversals, the better we can predict whether one is happening now. So, is a reversal overdue? The last one was 780,000 years ago and reversals have been happening about three times every million years for the past 90 million years. Therefore, some say, it’s time. Constable and Korte crunched the numbers and found that contention statistically suspect. Maybe. Maybe not. Over time, an interval of more than 780,000 years between reversals is not wholly unusual.
What about the argument that the dipole is decaying fast? Constable and Korte pointed to the fact that while, yes, it is waning, its pace while doing so is in line with what’s been going on for the past 7,000 years. Nothing unusual there. At other times during the past 7,000 years, it has decayed just as fast or even faster without prompting the poles to flip. Not only that, but it is still strong compared to the dipole’s intensity at the moments of other reversals. It’s even strong compared to the long-term average over the past 160 million years; it’s nearly twice that figure. The role of the South Atlantic Anomaly in triggering a reversal was a little tougher to analyze. Their best conclusion: There is an absence of evidence to support a looming reversal.
But the fact that so far there is no certainty on the timing of a reversal doesn’t stop geophysicists from trying to figure it out. For example, the north magnetic pole, which has always wandered, has begun moving at a full gallop to the north-northwest at the pace of about 55 kilometers a year. (The south magnetic pole, by contrast, continues to meander sedately.) An animated video of its track just since 1999 shows a breathtaking race across the High Arctic. It’s a clear indication that the field is changing rapidly, that something is afoot in the outer core.
Not only that, but a new batch of papers has used novel methods to come to different conclusions about a reversal. A French study looked at the past 75,000 years of sedimentary and volcanic data as well as ice cores from Greenland, where radioactive isotopes of beryllium and chlorine had been deposited over time. The concentration of those isotopes, created when cosmic rays battered the Earth’s upper atmosphere, is a good measure of the intensity of the Earth’s dipole. The more there are, the lower the dipole’s strength. The authors, one of whom, Carlo Laj, traveled to Pont Farin in 2002 to redo Brunhes’s measurements, found excellent matches in the records for the Laschamp near-reversal, a nod to the precision of the method. Their conclusion: The field is decaying so fast that a reversal may be irreversibly under way, but the poles themselves won’t reverse for at least five hundred years. But since the risks of a reversal lie not in the actual shift of the poles but in the strength of the magnetic shield to protect the Earth from dangerous radiation, this finding implies that the next five hundred years or more could be the ones to watch. Not much comfort.
A study by two Italian researchers took an altogether different, highly controversial approach. They used a theoretical systemics approach to examine the geomagnetic field, looking at how it interacts with other systems governing the planet. The behavior of the South Atlantic Anomaly in particular led them to conclude that today’s field is “rather special” and is approaching a critical transition. They even put a date on the point of no return: 2034, give or take three years. That’s not when the reversal will happen, but when it becomes inevitable.
At the University of Rochester geophysicist John Tarduno and others did an ingenious study examining the burned clay huts of Iron Age Bantu-speakers who lived in villages along Africa’s Limpopo River beginning in about 1000 CE. These are some of the few ancient readings from the southern hemisphere. When the rains failed, these early farmers torched their storage huts in a cleansing ceremony, heating up the magnetite-rich clays well past their Curie point. As they cooled, they took on the magnetic memory of the day. Tarduno, working with archeologists, has discovered that this part of Africa displayed a low field seven hundred years ago. The field then regrew in strength before weakening again to form part of the South Atlantic Anomaly.
The findings are significant, Tarduno argued, because the weak patch in the magnetic field, then and now, lies over the edge of an unusual formation in the mantle at the boundary of the core. It’s a place millions of years old with steep sides, where seismic waves move with unusually low velocity. Tarduno posited that this piece of mantle affects the movement of molten iron in the outer core, changing its magnetic flow. In turn, this shifts the field’s direction, producing the reversed-flux patches now visible, and saps the field of strength on the surface. Tarduno’s proposal was that rather than being triggered by random phenomena in the core—or related to Finlay’s gyre—reversals might be triggered by this oddity in the mantle, particularly if several reversed-flux patches link up. While Tarduno stopped short of saying a reversal is nigh, he stressed the dramatic decay of the dipole over the past 160 years, calling it “alarming.”
And new findings keep emerging, questioning the basic understanding of how the Earth’s field works. A fascinating paper published in 2017 examined the handles of clay jars made in the Levant near Jerusalem between 750 BCE and 150 BCE. The handles were stamped with royal Judean seals when they were still wet, which means the date of their firing can be pinpointed with a high degree of precision and therefore so can the date of the magnetic field they record. This degree of exactitude in the rock record is rare. Just before 700 BCE, the field’s intensity spiked up by about 50 percent. At that time, the field was already strong compared to today, so with the spike it became nearly twice as strong as it is now. The odd thing is that it decayed abruptly too, waning by more than 25 percent in just thirty years. That’s far more rapid change than geophysicists have believed the outer core is capable of. If it’s real, it suggests a degree of volatility previously unimagined.
For their part, French geophysicists Jean-Pierre Valet and Alexandre Fournier adjured their colleagues to keep heart. In an exhaustive review paper, they argued that the answer to understanding what happens during a reversal lies in closer examination of sedimentary rocks. They argued for better techniques in studying rocks’ magnetic memory, especially to track the field in the throes of a transition. Perhaps greater use of new magnetometers that test rock samples so small as to be nearly microscopic. Perhaps the beryllium isotope readings. “Despite many unresolved questions we are far from pessimistic and consider the quest for a proper description of polarity transitions to not be hopeless,” they wrote.
Back and forth. Back and forth. Like so much of the lengthy investigation of the Earth’s fickle magnet, the questions outstrip the techniques that could provide definitive answers.
So where to go from here? Finlay had his eyes trained on what might emerge from a whole new angle of investigation. Not squeezing more information from more ancient rocks. Not building ever more detailed analysis of what rocks are saying. Not calculating ever more realistic numerical models of the field. Instead, trying to physically reproduce the self-sustaining dynamo in the heart of the Earth. That means rather than being stuck on the surface, or at the boundary between the mantle and the core (which is as deep as the math will allow maps to be constructed), geophysicists could go even deeper, right into the mysteries of the outer core itself. One of the promising experiments is in a lab in Maryland run by Daniel Lathrop. The whole geophysical world, Finlay confided, is holding its breath waiting to see if Lathrop’s experiment will “dynamo”—that is, make a dynamo on its own. Perhaps a plot for The Core II, starring Hilary Swank?