Long before geophysicists suspected that a flip of the magnetic poles might be in progress, they began delving into a concept that startled them. It was the early 1960s. The theory of reversals was just beginning to creep into respectability. Its implications were breathtaking. Among them: Did reversals kill off or mutate species and therefore affect patterns of evolution? This suggestion went far beyond the idea that the magnetic field provides a refuge from cosmic radiation and shelters our atmosphere from solar winds that would rip it away. It was metaphysical: Did the inner machinations of the molten core help determine what lives and dies on the crust?
The first salvo, in 1963, stemmed directly from the discovery of the Van Allen belts. What would happen to all that radiation trapped in the belts when the poles reversed? Would solar wind be able to bathe the Earth in radiation, causing rampant genetic mutations? And had it
done so during previous pole flips?
The author of the page-and‑a‑half paper, Robert Uffen of the University of Western Ontario, hypothesized: yes.
“It is becoming increasingly apparent that the Earth is a heat engine the internal workings of which have controlled not only geological phenomena such as mountain building, volcanoes, and earthquakes, but also geochemical phenomena such as the development of the atmosphere and the oceans; geophysical phenomena such as the magnetic field and radiation belts; and even biological phenomena like the origin and evolution of life,” Uffen concluded.
The next step was to examine the rock record. This time, it was to look not only for the magnetic memory locked in rock, but also for its archive of fossils. It was obvious that reversals did not kill off all life, because life had persisted continuously on the planet for at least 3.6 billion years. But had previous reversals led to mass die-offs? At a first pass, there was little evidence. For one thing, the Earth had experienced just five mass extinctions. But there had been hundreds of reversals and near-reversals. Therefore, reversals didn’t cause mass extinctions, or at least not always. The logic of that line of reasoning broke down under scrutiny, though. Reversals last for perhaps a few thousand years — or less — and the paleontological record is rarely precise to that time scale. It’s hard even to find a global rock record for such a short period, much less a record of species gone missing forever within it. We have evidence of the five mass extinctions because they spanned millions of years.
As researchers dug further into the data, some peculiarities began to spring up. Two of the mass extinctions coincided with abrupt changes in the tempo of reversals. The first was the one 252 million years ago at the end of the Permian period. It is known as the Great Dying because 95 per cent of species on the planet vanished. The second was the one that killed off the dinosaurs and many other species 65 million years ago at the end of the Cretaceous period. A superchron, when the Earth’s magnetic field did not change for tens of millions of years, came before each. By contrast, during those two mass extinctions, the field reversed many times. One theory was that during the superchrons, species evolved without the need to adjust to the rigours of reversals, and so when reversals came, so did pulses of extinction. That may offer a bit of comfort about the vulnerability of species to a reversal today. Ever since the dinosaurs vanished, we have been in a relatively fast-paced pulse of reversals, which may have built some level of protection into the genetic code of species now on Earth.
By 1971, the scientific exploration had turned to comparing an index of the change in the number of taxonomic animal families over the past 600 million years — a measure of rates of extinction but not mass extinction — against the timing of reversals. There was an astonishingly high correlation, the author, Ian Crain of the Australian National University in Canberra, found. But why? Did reversals foster extinction and, therefore, the emergence of new species to replace them? Pointing to lab experiments, Crain proposed that the low magnetic field itself was the killer, causing difficulty in movement and reproduction.
But perhaps there was another killing mechanism. New findings in the 1970s and 1980s were showing that the magnetic poles are important — in both large and surprisingly small ways — for navigation in almost every species studied. Many use the field to find food, mates, breeding spots, and wintering areas. But, for example, radishes also align their roots according to the field, and dogs prefer to urinate facing north-south rather than east-west, as long as they are off leash and not in the midst of a geomagnetic storm. What happens when the poles are reversing? Can species that rely on the poles to navigate still get where they need to go? If not, do they die en masse?
What of the perils of radiation? The long-standing belief was that the Earth’s thick atmosphere provides a physical barrier against a full blast of solar and cosmic radiation whether the magnetic shield holds or not. Exposure to radiation while you are in an airplane, for example, increases along with altitude and latitude, suggesting that the atmosphere is a filter except near the poles, where field lines converge. But what if the field were decimated? A clue came from ocean sediment records. They showed an increase in radioactive beryllium, a marker of the collision of cosmic particles with the atmosphere during the last reversal. That meant more cosmic particles were getting into the upper atmosphere before colliding and scattering damaging secondary radiation. But it was not a sign that the destructive energetic particles themselves were reaching the surface, just that secondary radiation was.
And then there was the investigation into damage not from ionizing particles but from a lack of ozone. The Dutch chemist Paul Crutzen, who won a Nobel Prize in 1995 for his work on the ozone hole, showed in 1975 that when solar protons produced ions in the stratosphere, that led, through other chemical reactions, to the widespread destruction of the ozone layer. In turn, that allowed damaging ultraviolet radiation to reach the surface of the Earth. Other investigators found that during a reversal, vast swaths of ozone would vanish, allowing greater amounts of ultraviolet B radiation to strike the surface of the Earth, especially near whatever poles there were at that time. Ultraviolet B radiation is not ionizing, but it can affect living tissue in myriad destructive ways. Skin cancer and long-term damage to the eyes and to the immune system are all linked to the rays. More recently, the French geophysicist Jean-Pierre Valet proposed that the disintegration of the ozone hole could be one factor in the final die-off of the world’s Neanderthal population. The last small populations vanished at the time of the Laschamp excursion 40,000 years ago, when the field was at one-tenth of its normal strength. Neanderthals’ tendency to be fair-skinned and redheaded suggests they were especially susceptible to ultraviolet B damage, just as modern humans with that coloration are.
In the end, the evidence of how past reversals had affected past life — and therefore how they would affect life during a future reversal — was slender, largely theoretical, and inconclusive. The German physicists Karl-Heinz Glassmeier and Joachim Vogt, who did an extensive review of the relevant studies in 2010, concluded, “It is yet too early to decide in which way magnetic field driven bio-chemical effects influence evolution on Earth.” The implication is that they do.
Excerpted fromThe Spinning Magnet by Alanna Mitchell. Copyright © 2018 by Alanna Mitchell. Published by Viking, an imprint of Penguin Canada, a division of Penguin Random House Canada Limited. Reproduced by arrangement with the Publisher. All rights reserved.
View video transcript