Finding North
As I wondered what to write about this morning, my partner and I joked about a North Pole-themed journal. It is Christmastime for my family; in books, movies, and holiday décor we see images of a candy-cane striped pole emerging from the snowy drifts. Some of the questions I received from students about the MOSAiC Expedition wondered if I might encounter Santa. So perhaps a journal about the north pole would be well-received. But what about the scientific content?
At first it was just a joke, but as we talked more about it Nathan and I realized that we have many questions about the other north poles – namely the magnetic north pole and geographic or “true” north. There is also the town of North Pole, Alaska, but that’s not the focus of my journal today. Instead, I want to dive in to why we have both a magnetic north pole and a geographic north pole, and then spend some time learning about how the north pole (and south pole) are shifting and why this matters.
This diagram shows the geographic north and south poles compared to the magnetic north and south poles. Magnetic field lines are also shown. Image courtesy the National High Magnetic Field Laboratory.
Geographic North Pole
The geographic north and south pole are determined by the Earth’s rotation. They represent the two end points of the axis the Earth spins around. This axis is not completely stationary because Earth is not a perfect sphere. The axis drifts and wobbles, which means that geographic the north and south poles shift and move. This is called polar motion. The axis moves on different time scales, from short day to day variation to decades-long differences. These changes are driven by a combination of tides, winds, and other oceanographic and atmospheric processes; solar forcing; the intrinsic year to year wobble of the axis; and global cycles of liquid water and ice. But there are other factors that operate on a longer time scale than these subtle changes and wobbles. During the 20th Century, the axis shifted an average of about 4 inches (10 centimeters) per year, or approximately 33 feet (10 meters) over the course of those 100 years. The direction of movement is towards Labrador, Canada.
Recently, NASA and other scientists used models to help explain the driving factors in this shift. Their study showed that the path of the north pole was determined by changes in the mass of the earth. The Earth isn’t a perfect sphere and the surface is dynamic. As an area of the surface gains or loses mass, this affects the balance of weight around the sphere and basically makes it a little lopsided. In particular, three factors seem to explain both the direction and speed of the century-long shift of the poles.
One of the factors is the movement of material in the mantle. This movement is driven by heat from the Earth’s core. Mantle convection is responsible for the movement of tectonic plates at the Earth’s surface and slowly re-distributes material. As the plates and molten mantle move, so does the balance of mass on Earth.
The second driving force is caused by glacial rebound. During the last ice age (which ended about 12,000 years ago) massive glaciers pressed down on land forms. As they melted, glacial rebound occurred. Where glaciers and ice sheets were present, the weight of them pushed the Earth’s crust deeper into the mantle. When the ice melted away, the land slowly began to rebound. Released from the weight of the ice, the land is expanding upward. Imagine a couch cushion. When you sit, your weight creates an indent on the cushion. When you stand up, you might notice the sunken area stays for a while. But then it slowly starts to fill back in and rebound to it’s original shape. That is, more or less, what is happening with glacial rebound. The “butt-print” of the now-melted glaciers is filling back in. And as that happens, it redistributes mass on the Earth and affects the axis on which the Earth spins.
That is not the only reason why glacial rebound is important. It significantly alters the way sea-level rise affects different places on Earth. In my hometown, for example, glacial rebound (and tectonic uplift) are causing the land to rise faster than the sea. In Kachemak Bay, Alaska we are actually gaining land even as global seas rise. There are some really interesting collaborative studies happening in Kachemak Bay to understand the impacts of this more.
But back to the poles! Scientists identified a third factor that is likely shifting the location of the geographic north and south poles: recent melting of ice on Greenland. As the Greenland ice sheet melts, water flows off this one part of the Earth’s surface and is distributed around the globe. Increases in global air temperatures drove a lot of melting in the 20th Century. About 7,500 gigatons of ice was lost from Greenland. That weight was then distributed more evenly around the Earth’s ocean. But how much is a gigaton? A gigaton is equivalent to the weight of approximately 6 million blue whales! The loss of 7,500 gigatons of ice is like if 45 billion blue whales were hanging out on the land of Greenland for thousands of years and then, in the course of about 100 years, all decided to swim to different parts of the ocean. That would be a big change in mass. And the melting of the ice sheet on Greenland was a big change in mass. Big enough to affect the balance of mass on Earth and contribute to the changes in the Earth’s axis. Loss of ice sheets and glaciers elsewhere, like Antarctica, play a role too but Greenland’s location near but offset from the pole makes the Earth’s axis especially sensitive to changes in mass there.
Other researchers suggest that climate change impacts the wobble of the axis in a different way. Their study suggests that movement of the geographic north pole maps very closely with changes the distribution of water on land. Prolonged patterns of drought in some regions and flooding in other regions can change the balance of mass on the earth and may also be pulling or pushing the axis.
Okay, so that’s the geographic north pole. It’s location changes day to day, year to year, decade to decade, and century to century. The longer-term changes in location seem to be driven mostly by mantle convection, glacial rebound, and ice-sheet loss. Altogether, the pole has shifted about 33 feet in the last 100 years. What about the magnetic north pole?
Magnetic North Pole
The magnetic north pole (and south pole) are determined by the Earth’s magnetic field. And they are moving even faster!
Driven by processes in the Earth’s core, our planet is surrounded by a magnetic field (region around a magnetic material within which the forces of magnetism act). This is really important to life on Earth, since the magnetic field helps to protects us and our atmosphere from solar radiation and cosmic rays. This energy is deflected along the magnetic field towards the magnetic poles. This also plays a key role in the creation of Aurora Borealis (Northern Lights) and Aurora Australis (Southern Lights).
The magnetic field – and thus the magnetic poles – are known to move over time. Interestingly, the magnetic north pole and magnetic south pole move independent of each other. They can move at different speeds or in various directions. When it was first measured by western scientists in 1831, the magnetic north pole was located on the Boothia Peninsula very far north in Canada. Since then it has moved about an average of 6 miles (9 kilometers) per year towards the Central Arctic Ocean. Over about 170 years, it moved over 1,200 miles (2,000 kilometers). But in the past two decades, the magnetic north pole has started to move a little faster. Current estimates are that it is moving approximately 30 miles (50 kilometers) each year, and headed towards Siberia, Russia.
The magnetic north pole has shifted hundreds of miles since 1831. Image from Nature and the World Data Center for Geomagnetism/Kyoto University.
Some researchers even think that the instability in the magnetic poles may be a clue that we are headed toward a magnetic reversal. In this slow process, the magnetic field weakens and eventually the north pole and south pole switch positions. This happens from time to time. Evidence shows that the last reversal was nearly 800,000 years ago. Some research indicates that these reversals happen about every 400,000 years, but with lots of variability. So sometimes it is millions of years between reversals and sometimes it is a lot less.
Why does the movement of the magnetic poles – and the potential for a magnetic reversal – matter? In the short-term, movement of the magnetic poles can make navigation difficult. From a basic compass to complex GPS systems used to navigate planes, navigation technology relies on predictions of where the magnetic north pole is. Earlier in 2019, scientists had to update the model they use to predict the location of the magnetic north pole earlier than they had planned. It was just moving faster than they had predicted.
More significant changes in the magnetic north pole (such as those associated with a magnetic reversal or temporary or incomplete reversal) could have impacts for migratory animals. Some animals, such as migratory birds, sea turtles, and salmon, may use cues from the magnetic north pole to navigate. It makes sense that they can use this information to navigate north-south. In the northern hemisphere, they could orient towards north simply by following the strongest magnetic cue. But it also seems that some species of birds can use declination. Declination indicates the difference between geographic north-south and magnetic north-south. Birds likely use patterns of star location or sun location to help them navigate in relation to geographic north-south. Research with one species of warblers now indicates that they can somehow use the difference between geographic north-south and magnetic north-south to know if they also need to fly east or west. So if the magnetic north pole changes location significantly or the magnetic field weakens, it may make it difficult for migrating animals to orient themselves and head in the right direction.
This chart shows predictions of magnetic declination for the year 2020. Image from NOAA National Center for Environmental Information, 2019.
If the magnetic field weakens a lot, it can also cause problems for humans. Remember that the magnetic field helps to shield the earth from radiation? Well, if it weakens, more solar radiation and cosmic rays can penetrate the Earth’s atmosphere. These charged particles can cause problems for satellites, aviation, and electrical systems. The impacts of recent solar storms might give us an idea of what it would be like to have a weaker magnetic field, since the solar storms bombard the Earth with extra charged particles. Power outages are sometimes caused by increases in radiation from solar storms; a solar storm around Halloween 2003 required planes to be re-routed, interrupted communications systems, and damaged satellites. The good news about a weakening of the magnetic field is that we likely have plenty of time to prepare ourselves, and our technology, for it. New research indicates that these reversals take place over thousands of years.
Who knew the north poles could be so complicated? It has been fascinating to learn a little more about something I tend to take for granted. I’ll be paying closer attention now to news about the magnetic north pole and the geographic north pole.
For those that want more, there is actually a third north pole – the geomagnetic north pole! Read more about it here. Oh, and that article also mentions something called the north pole of inaccessibility. That is the point in the Arctic Ocean farthest from any shoreline. Which of these 4 north poles do you think the MOSAiC Expedition will come closest to?
Education Extension
The storymap How Many North Poles Does the Earth Have? is an excellent resource for exploring geomagnetism, the different types of north pole, magnetic reversal, and the ways that animals use magnetic fields to migrate. There is a lot of information here! It is probably best for older students, as well as interested adults. A great source of background information for elementary teachers as well.
This unit on navigation is intended for middle school students and includes a multitude of science and engineering activities that build on each other.
And there is a fun lesson about magnetic north, declination, and orienteering that uses lodestones (magnetic rocks) for hands-on exploration.
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