Is the Earth’s Magnetic Field Going to Reverse: Part I
Since it’s Wednesday, here’s something research-related. Here’s the number one question that most people ask me when I tell people my field of study: Is the Earth’s magnetic field going to reverse? And if it is, are we doomed? I figure, since a lot of people ask me this, they must be interested. And since they think it’s related to my field of research, it must be. So I’ll do my best to answer the question. I’m probably going to have to break my answer into a couple of parts – at least two.
By the way, I’m using “research-related” loosely here: much of this isn’t really part of my own research. When I was a real honest-to-godness paleomagnetist, I used to tell people I studied the early history of the Earth’s magnetic field. This basically means I looked at very old rocks (mostly 2.7 billion years old, to be precise) to determine how strong the magnetic field of the Earth used to be. I didn’t look at magnetic reversals very much. I only have a pretty cursory understanding of dynamo theory – the physics of the deep-earth processes that generate the magnetic field – and only a passable working knowledge of the historical magnetic field. Plus, I’m moving away from this area of research. I’m starting to look at more environment-related questions – basically, using the magnetic properties of sediments, rocks, and manmade materials to track pollution in the Puget Sound area. So some of you readers may be more qualified to answer this question than I am. But I figure, since people ask me, I should have an answer…
…and my answer is, “Yes.” Yes, the magnetic field is going to reverse itself. The north magnetic pole will almost certainly change places with the south at some point in the future, as it has many times in the past. But we cannot say with any certainty when this will happen. Saying that the magnetic field will reverse itself is kind of like saying that the Dow Jones Industrial Average will go up. We can forecast that it will do so in the future – however unlikely it may seem right now – but we can’t say whether it will do so tomorrow.
The evidence for past magnetic reversals is pretty solid. If you want the details, read The Road to Jaramillo by William Glen. We’ve seen plenty of magnetic reversals in the past (first documented by Bernard Brunhès). There is evidence from different kinds of rocks (lava flows and sedimentary rocks) and from rocks in diferent places (lots of locations on land and on the seafloor) that Earth’s north magnetic pole has previously switched positions with the south pole. The most recent reversal of positions was only about 780 thousand years ago. There are well-documented reversals going back a few hundred million years, and a good indication of reversals 2.7 billion years ago.
However, we can’t predict the future behavior of Earth’s magnetic field very well for a few reasons: (1) the physics of what generates it are complicated and poorly understood, and (2) we have very limited observations. Before we go any further, a little background: the main part of Earth’s magnetic field is produced by the movement of the molten iron and nickel outer core. It is not produced by the inner core, despite what you might hear in the movies. Because iron and nickel are conductive, swirling these molten metals around causes electrical charge to move. This produces a magnetic field. This is basically Ampère’s law, something you may have heard if you took my intro physics class. At the same time, magnetic fields also change the motion of electrical charges (the Lorentz force, also from intro physics…). The interaction between these two effects alone makes it nearly impossible to predict what will happen to the magnetic field in the future. To further complicate things, the outer core is a turbulent mess. It is crystallizing as it cools, forming the inner core. It is convecting as it gives up energy to the mantle above it. The mantle itself is changing with time, changing the temperature at the top of the core. We don’t even know the chemistry of the core all that well. No wonder, then, that we can’t forecast magnetic reversals. Consider weather forecasts. How much do you trust your TV weather-person (no offense, meteorologists)? And yet we know some basic things about how the atmosphere works (interaction with surface topography, moisture, general wind patterns, etc.) that allow us to predict weather at least a few days out. We don’t know a lot of these things about the core.
What’s more, we can only observe the outer-core system indirectly, and we have only been recording the Earth’s magnetic field for the past few hundred years. That’s just a moment when compared to the time since the last reversal. It’s even a short amount of time compared to how long we think it takes the magnetic field to reverse (a few thousand years). Think about the analogy with weather forecasts: how well would you be able to forecast the weather if you were only able to see Earth from a satellite? And what if you had only observed it for a few hours? I guess you could compare geomagnetism to trying to forecast the weather on, say, another planet.
We paleo- and geomagnetists find it incredibly exciting that the magnetic field is still so unpredictable. We are fascinated by the complexity of the geodynamo. A lot of us are continually trying to come up with more ways to squeeze more knowledge out of the precious little data we have about the changing magnetic field of the Earth. Some geophysicists are looking at ships’ navigation records to get a longer detailed record of changes in the magnetic field. Some are looking at old rocks, even lunar rocks and meteorites, to get an idea of what’s happened to Earth’s magnetic field over a very long time scale. Some are looking at what Earth’s magnetic field is like during a reversal. Are there signs (so-called “precursors”) that precede a magnetic reversal? Are there conditions that make reversals more or less likely to happen? These are all big questions that we’re still working on. As an aside, in what is perhaps the coolest and most dangerous kind of experiment associated with geomagnetism, you can simulate the physics of the outer core by making a scale model of the Earth’s core using liquid sodium (highly flammable! kids, don’t try this at home!).
OK – enough about why we don’t know when Earth’s magnetic field will reverse next. I’m tired and it’s late. Next Wednesday, I’ll explain why some geomagnetists think that there’s a good chance that there will be a reversal sooner than later, and why others think that the chance is slim to nonexistent (I’m in the latter camp). Then the following week I’ll explain what the consequences of a magnetic reversal might be (spoiler alert: the consequences are not very dire). Later on I’ll get to blogging about how old rocks keep a record of magnetic fields, and why I get to use equipment that my wife thinks I made up (how much nerdier can you get than “cryogenic magnetometer?” Much.).