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Monday, June 19, 2006
The Disaster You Didn’t Expect
You wake up one day, and notice the compass on your Swiss Army watch points south.
Tap tap. Must be broken, right?
Thwak! A disoriented pigeon slams into your picture window. Strange. You turn on the Today Show, and they’re speaking about the orbiting space shuttle. Its navigation system has just gone out, NASA is scrambling, and…the power goes out.
Ah well, might as well go out for your morning run. But when you approach the beach, you see crowds gathering. Along the sand, exhausted sea turtles are washing ashore, and people are trying to rescue them. You run your hand over your face. You’ve only been outside a few minutes, and yet your face feels burned, as if you’d been outside for hours. Such a strange day.
There’s a cry for help. One of the turtle rescuers has collapsed. “His pacemaker has stopped,” the man’s wife screams, as she lifts her head from his chest.
You reach for your cell phone, to call for an ambulance. But the phone won’t work. The phone company’s satellites are down.
It turns out, your south-pointing compass was right. The Earth’s magnetic polarity has flipped.
Cool scenario for a disaster movie, huh? (It’s been done, sort-of, in “The Core.”)
But the Earth’s magnetic poles couldn’t really flip. Could they?
Credits: M. A. Shea, Geophysics Directorate, Philips Laboratory
Scientists say the Earth’s magnetic polarity has shifted many times in geological history, and we are, in fact, overdue for another flip. Some think that evidence of magnetic holes in the South Atlantic indicates that it has already started.
Given the importance of magnetism to navigation, animal migrations and atmospheric conditions, the European Space Agency has plans to launch a cluster of satellites four years from now, called SWARM, which will begin monitoring changes in Earth’s magnetic field.
Swarm satellite illustration credit: European Space Agency
What is magnetism?
Magnetism is a force that’s created by the movement of charged electrons. It can be an attractive force or a repellent force. The Earth’s magnetism is created by the turbulent movement of molten iron within its outer core.
Writing in Science on June 16, 2000, Chicago geophysicist Bruce Bennett describes how it works:
“Earth evolved into a layered body early in its history. Molten metal (mainly iron) descended to form the present-day core, while silicates and oxides were confined to a thick shell called the mantle. The innermost part of the core is now solid, whereas the outer portion is liquid.”
Benett says the inner core is hot — about 5,000 degrees Celsius — but cooling. The outer core, under enormous pressure from the weight atop it, is likewise incredibly hot, churning like liquid water. The constant movement of this iron, propelled by convection, is what creates the magnetic field. Scientists call it a geodynamo.
Earth cutaway credit: Created by Jeremy Kemp. Based on elements of an illustration by USGS.
Add to this picture plate tectonics and planetary spin, and you have a system complex enough to inspire development of models and predictive maps, like some sort of geological weather forecasts.
If you think about macaroni in a rapidly boiling pot of water, you can imagine how this outer core churns. But given its size and its rate of movement, the good news is, most scientists think an outright flip could take thousands of years to mature, meaning the earth’s creatures would have time to adapt to a reversal of magnetic polarity.
There’s interesting evidence to support the theory that the magnetic field is changing. Writing in Scientific American in April 2005, Gary Glatzman and Peter Olson pointed to mineral evidence which suggest a polarity switch has not happened in 780,000 years. Yet that same evidence suggests that the average span between shifts has been about 250,000 years.
Add to that evidence from satellite observations.
“When measured at the surface, the dominant structure of this field is called the dipole, which most of the time is roughly aligned with the earth’s axis of rotation. Like a simple bar magnet, this field’s primary magnetic flux is directed out from the core in the Southern Hemisphere and down toward the core in the Northern Hemisphere. (Compass needles point to the earth’s north geographic pole because the dipole’s south magnetic pole lies near it.)
But the satellite missions revealed that the flux is not distributed evenly across the globe. Instead most of the dipole field’s overall intensity originates beneath North America, Siberia and the coast of Antarctica.”
“Evidence from the geologic record shows that past reversals occurred over relatively short periods, approximately 4,000 to 10,000 years. It would take the dipole nearly 100,000 years to disappear on its own if the geodynamo were to shut down. Such a quick transition implies that some kind of instability destroys the original polarity while generating the new polarity.
In the case of individual reversals, this mysterious instability is probably some kind of chaotic change in the structure of the flow that only occasionally succeeds in reversing the global dipole.”
The addition of the SWARM satellite data should provide better information. The satelites will be launched in 2010.
If the magnetic poles really did switch, compasses would indeed point south. Migratory animals like birds, bees, sea turtles, whales and some fish would be forced to adapt. The magnetic shield that prevents some radiation from reaching the Earth would be weakened in places, affecting satellites, power grids and communications. Cancer rates would rise. Crop yields would fall.
But if indeed it happens, it won’t happen overnight. Would it still qualify as a disaster?
