I’ve talked before about solar storms, specifically about how they are caused by the internal workings of the Sun causing its magnetic field to weaken and reverse. I had the idea at the time to follow that post up with one about geomagnetic reversal, as the mechanism behind it is (in theory) similar, as is its depictions in pop culture. I was reminded of this idea from some (unrelated) recent news headlines, so I figured better late than never. If you’ve ever heard about geomagnetic reversal, it’s probably through films like 2012 or through “History” Channel “documentaries” about the Mayan apocalypse. Fortunately, the truth appears to be far less terrifying.
Earth’s Magnetic Field
I would strongly recommend re-reading my previous post on solar storms, but I will repeat one of the important bits here. When picturing a magnetic field, it’s useful to imagine a series of lines running from the magnet’s north pole to its south pole. Anything affected by magnetic fields, from charged particles in space to the needle of a compass, will follow these paths until they run into something that stops them. If you stood at Earth’s magnetic south pole* with a compass and turned your compass sideways, the needle would point directly at the ground. Pull the same trick at the magnetic north pole, and the needle would point straight up away from the ground. It’s important to remember that these lines don’t start/stop at the magnet’s poles but instead pass through the magnet’s interior to create continuous loops. It is inside the magnet that the activity which generates the magnetic field occurs, which is always some form of electrically charged material moving around.
Beneath Earth’s crust (the part you’re standing on), the next layer down is the mantle. This is a layer about 2,900 kilometers deep (1,800 miles) made up of rock under high heat and pressure. While it’s still solid, this heat and pressure causes it to behave like a very slow-moving viscous liquid. Beneath the mantle is the core, consisting of the outer and inner core. The inner core, located at the center of the Earth, is a sphere 2,440 kilometers wide (1,520 miles) made of solid iron and nickel. The pressure in the core is 3.6 million times that of the atmosphere at sea level, and that pressure crushes the core into a solid despite it being 6,000°C (10,800°F). The outer core is under less pressure, but is similarly hot due to radioactive elements in the mantle, meaning the iron and nickel here behaves like a liquid. Being a liquid means this metal can flow, so it’s pushed around by convection currents and coriolis forces. And an electrically conductive fluid moving around in a rotating, convecting container generates a magnetic field. Currents of electromagnetic force are pulled into the planet through its magnetic south pole, travel through the complex of moving liquid metal inside the core, are expelled from the planet at its magnetic north pole, and then flow right back to the starting point at the south pole to complete the loop. This field deflects high energy radioactive particles away from the Earth’s surface, making complex life possible. Because Earth’s magnetic field is generated by such a dynamic process, the field itself is dynamic. The north and south magnetic poles move by between 5 to 60 kilometers a year and the field does wax and wane in strength.
Reversal
When lava is ejected onto the Earth’s surface, small metallic crystals in the lava align themselves north-to-south with the planet’s magnetic field. When the lava hardens into volcanic rock, these crystals become stuck in place, forever pointing toward where the poles were when that rock first formed. If this lava was spilled close to one of the poles and the rock contains radioactive elements that can be used for radioscopic dating, these rocks can be used to determine where the poles were at the time they formed. But in 1905, scientists found rocks where these tiny compasses were frozen pointing in the wrong direction. What was supposed to be the north pole was where the south pole is and vice versa. Since the magnetic poles can’t get too far from the geographic poles (the currents of molten metal generating the magnetic field are closely aligned with the Earth’s axis of rotation), the only explanation was that some internal process reversed the direction of Earth’s magnetic field. This led to the development of the theory of geomagnetic reversal, along with questions about what physically happens to the planet during this process.
While the exact mechanism of geomagnetic reversal is still contested, the leading theory is that the process is similar to that of the Sun. Since Earth’s outer core is a liquid, it doesn’t necessarily have to spin at the same speed as the Earth’s crust. In fact, there’s evidence that the Earth’s inner core (the solid part) rotates slightly faster than the crust. This uneven rotation means that over time, the magnetic currents moving through the core get coiled around the core like a spiral. The tighter this spiral becomes, the more stress is put on the magnetic field until it starts to kink. These kinks would cause changes to the Earth’s overall magnetic field and, if they were large enough to reach Earth’s surface, would create a smaller north and south pole elsewhere on Earth’s surface. As more of these kinks are created, Earth’s normal north and south pole would grow weaker as the overall magnetic field stops self-reenforcing. Over time though, these new mini-poles will be pulled toward the geographic north and south pole by thermal currents within the core. These mini-poles would coalesce to form a new definite north and south pole, once again reforming a normal magnetic field. The Earth does not have a preference for which side of the field is which, so there’s no reason these new poles couldn’t be on the opposite side of where they once were.
This is the leading theory for how geomagnetic reversals occur, but our understanding is still incomplete. While the mechanism of magnetic reversal appears to be the same as that of the Sun (and other bodies in our solar system with magnetic fields) Earth’s field varies far slower and not on nearly as predictable a cycle. Looking at the paleomagnetic record, geomagnetic reversals happen on average every 450,000 years, though they have happened as frequently as every 10,000 years or as infrequently as every 50 million years. The cycle is almost certainly longer because the Sun is significantly more fluid than Earth, meaning different parts can spin at vastly different speeds, thus twisting the magnetic field far harder. While Earth’s outer core is a liquid, the overall solidity of the planet prevents its spin becoming too out of sync with itself. But statistical modeling of historic magnetic reversals have found no pattern to their frequency. This has led some to suggest that reversals might be triggered by random major events, such as large asteroid impacts or the sudden melting of the polar ice caps, but most experts consider this very unlikely. The most recent reversal was 780,000 years ago, and we can’t predict when the next one will be.
Cause for Concern?
How long a reversal would take is also contested. Different estimates put the duration of a reversal as quick as a single human lifetime or as long as 10,000 years. During such a time period, the Earth would have multiple magnetic poles scattered across the planet, moving far faster than the poles move today (fast in the context meaning tens of kilometers per year). During these transitions, the strength of Earth’s magnetic field would also weaken to roughly a tenth of its normal strength. This is what led to many scientists at the time believing geomagnetic reversals might trigger extinction events. It was theorized that the temporary reduction of the planet’s magnetic field would allow particles of solar wind and cosmic rays to reach Earth’s surface, radically increasing the amount of radiation every living thing receives. It was also theorized that the increased churning of the planet’s core could trigger increased volcanic activity and that the cycle of geomagnetic reversal might line up with the cycle of extinction events. Most of the research supporting this theory was from the middle of the 20th century and has almost completely fallen out of favor in recent decades.
As paleontologists have gotten better at studying prehistoric biodiversity, it has become very clear that there is no connection between magnetic reversals and extinction events. Reversals appear to happen far more often than significant extinction events and the majority of mass extinction events have probable causes not linked to reversals. It is true that our understanding of both the frequency of these reversals and drops in global biodiversity is still fairly rudimentary; it is very difficult to look at the distant past and make assertions with any real certainty. But there is no statistically significant connection between the frequency of reversals and extinctions. Additionally, ice cores data from the most recent magnetic reversal (42,000 years ago) show no significant changes to Earth’s climate (that weren’t better explained by other sources) and while there were measurably more radioactive isotopes in the ice from this period, the isotope levels weren’t high enough to suggest significant global irradiation. Given how frequently these reversals occur, it’s questionable if complex terrestrial life could have developed at all if they caused significant extinction events.
So, what would it be like to live through a geomagnetic reversal? While Earth’s biosphere has survived and will survive such events, our complex civilization has its own strengths and vulnerabilities. But the change would be incredibly slow, giving us plenty of time to prepare and adapt. First off, compass navigation would be rendered nearly impossible as there would now be multiple, far weaker magnetic poles scattered across the planet. Many modern electronics, including cell phones, do have inbuilt magnetometers to orient themselves against Earth’s magnetic field, and planes and ships still carry compasses as a backup in case their satellite navigation systems fail. But given how long we would have to adapt our technology and how GPS doesn’t rely on the magnetic field, this wouldn’t be the disaster it would’ve been a few decades ago. There are animals who can detect Earth’s magnetic field and use it to navigate, particularly migratory species, who might fare more poorly during a magnetic reversal. That said, these species did survive the last reversal. What health effects the excess radiation exposure could cause is not certain. Earth’s atmosphere is also very effective at blocking radiation (hence why auroras are high in the sky), but a weaker magnetic field would mean more high energy particles make it to the surface. The fact that the biosphere has survived magnetic reversals before does show that the adverse effects wouldn’t be extreme, though slightly elevated cancer risks wouldn’t be completely out of the question.
And of course, there is the concern of how our electrical infrastructure would fare in such a world. I’ve talked before about how solar storms interact with Earth’s magnetic field to potentially disrupt electrical grids. Charged particles from coronal mass ejections hit the magnetic field and distort its shape. Moving magnetic fields generate electrical current, so bending the Earth’s magnetic field generates current in the Earth’s crust, which then generates their own magnetic fields that can produce electrical current in transmission lines. Since these lines already have electrical current running through them, this additional current can be enough to overload the lines and damage critical equipment. If Earth’s magnetic field weakens, it will become easier to distort, making these electrical storms more common and more severe. Additionally, worse solar storms could cause more frequent disruptions to radio communications and could pose more risks to satellites and astronauts outside Earth’s atmosphere. Now, as I said in that last post, there are ways that scientists and engineers can mitigate damage to electrical infrastructure, and it stands to reason that we will continue to get better at doing so as time goes on. And again, the magnetic field won’t go away all at once; we would see decades, possibly centuries, of steadily worsening solar storms against which to prepare. Losing our planet’s breakwater against these storms will certainly make weathering them more of a challenge, but it’s absolutely within the realm of doability.
As it stands, no one knows when the next geomagnetic reversal will be. If reversals follow a definite cycle, it is not a particularly regular one. There are scientists (and others) who worry that the next reversal might be sooner rather than later due to ‘recent’ developments. Over the past century, Earth’s magnetic field has dropped in strength by 10% and the poles have gone from moving roughly 15 kilometers per year to roughly 55 kilometers per year. Some have proposed that this could indicate we are at the very beginning of the next reversal. That said, most geologists are skeptical of this claim because the paleomagnetic record shows that the magnetic field has varied by this much in the past without reversing. We still know astoundingly little about this topic, so making assertions about events that occur so infrequently on the geologic timescale isn’t particularly wise. Should a geomagnetic reversal start in our lifetimes, it will likely be one of the biggest problems our civilization has had to deal with. But it would be something we have the tools to deal with, and we will only get better at dealing with it over time. As it stands, geomagnetic reversal should not be the global disaster we are most concerned about.
For More Details
*Counterintuitively, the Earth’s geographic north pole is actually closest to its magnetic south pole. The names of the magnetic poles come from how compasses work, with the north pole of the compass pointing northward. But north is attracted to south, so this means that the north of the compass is attracted to the planet’s south pole, which is near the planet’s north. Such things happen when you name things before you understand them.
All information presented on this and other posts, to the best of my ability, represents the most up-to-date scientific research in the relevant fields. All opinions presented here are my own.
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