Drifting poles, the planet’s churning core and the threat to life on Earth
Can we avoid the fate that befell Mars? The Red Planet’s magnetic shield failed, and its atmosphere was blown away by the sun, leaving it a barren, sterile wasteland. Is Earth heading the same way?
Earth’s magnetic field, known to scientists as the magnetosphere, has been an integral part of the biosphere since life on the planet began. Bacteria, plants and animals are known to be affected by its orientation. Many species of birds would quite literally be lost without it—it is the cornerstone of migration strategies that allow them to escape harsh northern winters, for example.
Humans cannot consciously sense magnetism in the same way as many animals, but we still gain enormous benefit from the Earth’s field. Not only does it appear to keep our atmosphere in place, it also protects us from intense solar radiation and electrical storms that would otherwise play havoc with our electrical grids, satellites and aircraft communications. If Earth’s magnetic shield is failing, we need to know sooner rather than later.
We may never know which human civilization was the first to make explicit use of Earth’s magnetic field. Until relatively recently, it was thought to be the Chinese, who used magnetic minerals known as “south-pointing fish” to align their buildings in accordance with the principles of feng shui. However, reliable evidence for this practice dates back no further than 400 BC which means the oldest magnetic artifact is most probably a piece of the mineral magnetite found in Veracruz, Mexico, home of the Olmec.
The Olmec were, it is thought, the first civilization of the New World, existing between 1000 and 1400 BC. The piece of magnetite, unearthed in the early 1970s, had been fashioned into a bar that would offer little friction when set on the ground, and scored with a groove in the middle of one end. It looks, to all intents and purposes, like a compass needle.
When physicist John Carlson reported the discovery of the Olmec magnetite, he pointed out that the Olmec people constructed their buildings on alignments 8 degrees west of north. This, he said, was “a curiosity.” But taken with other evidence gathered over the subsequent centuries, it is more than a curiosity—it is evidence that the Earth’s magnetic field is far from constant. And that is why we think it may currently be failing.
Modern measurements of the Earth’s magnetic field only began two centuries ago, but we do have older evidence of shifting fields. Examine the orientation of over a hundred Danish churches built during the 12th century, for example, and you’ll see that they sit around 10 degrees off from today’s magnetic east-west line. As with the Olmec buildings, it is highly likely that when these churches were built, compasses pointed in a different direction than today.
A more reliable account of Earth’s magnetic field began at the start of the 19th century, when Alexander von Humboldt took field measurements while traveling in the South Atlantic. He found that the intensity of the field decreased in this region. In 1804, von Humboldt reported his finding to the Paris Institute, but counterclaims soon emerged, throwing the issue into confusion. Von Humboldt eventually took the matter to the German mathematician Carl Friedrich Gauss, and asked for help in constructing an atlas of magnetic observations. Gauss, a polymath who had made important discoveries in disparate scientific fields, was already investigating terrestrial magnetism, and was only too keen to help. By 1840 he had written three significant papers on magnetism—including a way to define the Earth’s field—and constructed a mobile magnetic observatory that would exclude all fields but the Earth’s.
Gauss’s geomagnetic atlas was published in 1836. Measurements of Earth’s field have continued since Gauss’s first efforts, and we now have a 150-year record. One of the key findings is that the magnetic North Pole moves. It was first pinpointed by explorers in 1831, then again in 1904. During that interval it had moved 50 kilometers (31 miles). During the 20th century, the pole has moved north at around 10 kilometers (6.2 miles) per year, though that movement appears to be speeding up. It is currently moving at around 40 kilometers (25 miles) per year.
That’s not the only change: records show that at midlatitudes, compass needles are drifting about 1 degree per decade. There really is a blip in the South Atlantic too: satellite measurements tell us that under the Atlantic Ocean, west of South Africa, field lines appear to converge, forming a magnetic pole. This “South Atlantic anomaly” exhibits its own reversed magnetic field lines, which now cover much of South America, and confuse our overall view of the Earth’s field. Then there’s the issue of the general weakening of the field. Taken as a whole, Earth’s magnetic field has lost 10 percent of its strength since Gauss’s measurements began. To understand what that means for the future, scientists have tried to uncover the roots of the field.
The fact that the Earth has a North and a South Pole might tempt us to think that its field arises from something like a bar magnet buried deep in the Earth. Unfortunately, things are nowhere near as simple as that. The Earth’s magnetic field arises from a churning sphere of molten iron and nickel deep in the heart of the planet. This, the Earth’s inner core, is a solid ball of iron 1,250 kilometers (775 miles) across. It is fiercely hot, at thousands of degrees, and only the pressure bearing down on it from the weight of the rest of the planet keeps it from melting.
Surrounding the inner core is the molten metal that creates the magnetic field. Heat from the inner core courses through this liquid, creating convection currents that move hot liquid metal up toward the mantle, the layer beneath the crust. This hot liquid cools as it rises, and so falls back down. The motion of this metallic conductor creates electricity, which is always accompanied by a magnetic field. The combination creates a self-sustaining “geodynamo” that maintains Earth’s field.
This geodynamo creates an extremely complex magnetic field. As the Earth spins on its axis, the field lines become twisted, creating new currents within the liquid outer core. This creates new magnetic field lines, and a new magnetic field can sometimes grow within the core. Typically, this adds to the current field, but if its orientation is shifted relative to the dominant field, it can sometimes detract from the overall magnetization of the Earth.
This may be what is happening with the South Atlantic anomaly, and it may be what is causing the apparent weakening of the Earth’s field. However, researchers can’t be sure, because the dynamics of the field created by such an enormous geodynamo are too complex to yield up their secrets to mathematical models. Frustrated geodynamo researchers are supplementing their mathematical models by creating their own real-world geodynamos. Typically, this involves some highly perilous equipment. If you want molten spinning metal in your lab, you can’t use a metal that melts at thousands of degrees. The best candidate is something like sodium, which melts at temperatures of just under 100 Celsius.
That said, sodium has its own dangers. It can burn in a fierce explosion on contact with water or air, for example. Nevertheless researchers have succeeded in spinning balls of molten sodium to simulate what is going on beneath our feet. The results have been impressive: self-sustaining magnetic fields do indeed form, and they do exhibit the kind of complex behavior seen in the Earth’s geodynamo. They even exhibit occasional “reversals,” when the North and South Poles swap places. During that process, the magnetic field fades and becomes much more complex, then grows again, but with a reversed polarity.
For a period of time, during a reversal there is no clearly defined field. So, could this happen with the Earth’s magnetic field, with potentially disastrous results? Unfortunately, even these simulations have not yet proved accurate enough for us to make forecasts for the Earth’s field. The best we can do, it seems, is to look at the evidence frozen into the planet’s rocky crust, and try to extrapolate our findings.
In the molten rock that pours from volcanoes and the gaps between tectonic plates in mid-ocean ridges, magnetic crystals—tiny grains of magnetite, for example—are free to move, and will orient themselves to the direction of the Earth’s magnetic field. When that rock cools, that orientation is frozen in, creating a rock whose magnetic field points toward its era’s magnetic north. By dating rocks and noting their magnetic orientation, researchers can build up a picture of how the direction of “north” has changed over millennia. This is how we gained the first evidence for a shield failure. In 1904, geomagnetic studies of the Massif Central mountains of southern France revealed that the orientation of the magnetic crystals in the rocks was significantly shifted from what it would be today. In the 1920s, similar observations were made across the world, and the field of paleomagnetism was born.
We now have evidence that, during the past 20 million years, the Earth’s field has collapsed and reversed more than 60 times. These reversals have occurred every half-million years or so, and can take thousands of years to complete. However, it is by no means a clockwork phenomenon. Sometimes, as happened during the age of the dinosaurs, no flips happen for tens of millions of years. We haven’t seen a reversal for 780,000 years now. So, does that mean we are due one? Is that why the Earth’s field is currently fading at what seems like an alarmingly fast rate?
We know, thanks to the logbooks kept during Captain Cook’s journeys in the South Seas, that the current failure only started relatively recently. We have mariners’ logs that date back to 1590, which record, amongst many other things, the direction of the Earth’s field and the angle at which the field lines enter the Earth. It was a useful navigational trick—in many ways the sailors’ lives depended on it. We have recorded a decline in field strength since Gauss began measuring it in 1840, but the ship’s logs show no change between the 1590 value and Gauss’s field strength.
Of course, it may be that we haven’t enough data to draw any firm conclusions; the “South Atlantic anomaly” may be leading us astray, for example. So should these strange measurements and discoveries give us cause for concern? Given the crucial role Earth’s magnetic field has played—and continues to play—in the development of life on Earth, the answer has to be yes.
The blue-green planet we call home sits roughly 93 million miles from the sun. We are in the “Goldilocks Zone,” where life can thrive in certain regions because the climate is not too hot and not too cold. But the sun produces more than heat. The surface of the sun is a turbulent mass of plasma, a gas composed of charged, high-energy particles. The sun is constantly losing these particles, which travel through space as the “solar wind.” Our magnetic field directs most of these around Earth. Crucially, only a small proportion of the particles reach the Earth’s surface.
When particles from the solar wind hit the atmosphere at higher latitudes, some create a cascade of energized particles. That energy is released as the fluorescent, shimmering glow of the aurora borealis, the Northern Lights. Some of the particles, though, reach the surface of Earth as radiation. This radiation from the solar wind has been, in some ways, a positive force. It may, for instance, be responsible for directing some of life’s evolution on Earth. This radiation can damage DNA, which imposes mutations in the genetics of terrestrial life, facilitating the process of evolution.
However, the radiation is also a danger. If it is too intense, mutations in DNA can lead to sterility, cancer—even, possibly, extinction for some species. The fact that this radiation hasn’t wiped out life on Earth is largely due to the fact that our magnetic field deflects most of the solar wind. So what would happen if it failed?
We know that our magnetic field came into existence at least 3.2 billion years ago. The earliest known life forms were in existence 3.5 billion years ago. The implication seems obvious: life has evolved in a magnetic field, and may require it. The moon and Mars both had magnetic fields around 4 billion years ago, but neither body has one now—nor do they harbor life, as far as we know.
Physicists’ best guess about the reason for that is that their small size meant they cooled quickly, losing the heat necessary to keep a liquid core churning. Earth’s larger size maintains the heat in its core, while its tectonic plates cool the mantle relative to the core. That temperature difference keeps the convection currents strong, stirring the iron-rich molten rock and maintaining our field.
And here is another connection to life: Earth’s field maintains our atmosphere. The magnetic field’s deflection of the solar wind means that the atmosphere is not buffeted by the solar wind’s particles. Maps of Mars’s little remaining ionosphere show that it is thickest where Martian rocks have maintained their magnetism. It seems that, if you lose your magnetic field, the atmosphere goes with it. So, Earth’s magnetosphere does not just protect us from radiation. It also allows our atmosphere to form and develop, giving us oxygen to breathe. Are we about to lose the very air we breathe?
The answer is almost certainly not. The reversal is most likely happening, but all our experiments and observations seem to indicate that any magnetic reversal will take a few thousand years, at the very least. During this process the Earth’s field will weaken, and become massively more complex, but it will remain strong enough to hold on to our atmosphere. It may not be a disaster in other ways, either.
The humans living on Earth at that time will almost certainly be at risk of receiving much more solar radiation. But no one yet knows whether that will really prove a problem. It is possible there could be mass extinction because of DNA damage, but there are so many other factors in play over those kinds of timescales that anything is possible. The last reversal didn’t wipe out our ancestors, and by then we might have developed the technology to create our own artificial radiation shield. Earth’s natural shield might well be failing, but this time we are ready, willing and able to face the consequences.
