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Magnetic Storm (2003)
PBS Nova
Date Added: 13 years ago.
Documentary Description
Like the plot of a sci-fi B movie, something weird is happening deep underground where the constant spin of Earth's liquid metallic core generates an invisible magnetic force field that shields our planet from harmful radiation in space. Gradually, the field is growing weaker. Could we be heading for a demagnetized doomsday that will leave us defenseless against the lethal effects of solar wind and cosmic rays? "Magnetic Storm" looks into our potentially unsettling magnetic future.
Scientists studying the problem are looking everywhere from Mars, which suffered a magnetic crisis four billion years ago and has been devoid of a magnetic field, an appreciable atmosphere, and possibly life ever since, to a laboratory at the University of Maryland, where a team headed by physicist Dan Lathrop has re-created the molten iron dynamo at Earth's core by using 240 pounds of highly explosive molten sodium. The most visible signs of Earth's magnetic field are auroras, which are caused by charged particles from space interacting with the atmosphere as they flow into the north and south magnetic poles.
But the warning signs of a declining field are subtler—though they are evident in every clay dish that was ever fired. During high-temperature baking, iron minerals in clay record the exact state of Earth's magnetic field at that precise moment. By examining pots from prehistory to modern times, geologist John Shaw of the University of Liverpool in England has discovered just how dramatically the field has changed. "When we plot the results from the ceramics," he notes, "we see a rapid fall as we come toward the present day. The rate of change is higher over the last 300 years than it has been for any time in the past 5,000 years. It's going from a strong field down to a weak field, and it's doing so very quickly."
At the present rate, Earth's magnetic field could be gone within a few centuries, exposing the planet to the relentless blast of charged particles from space with unpredictable consequences for the atmosphere and life. Other possibilities: the field could stop weakening and begin to strengthen, or it could weaken to the point that it suddenly flips polarity—that is, compasses begin to point to the South Magnetic Pole.
An even older record of Earth's fluctuating field than Shaw refers to shows a more complicated picture. Ancient lava flows from the Hawaiian Islands reveal both the strength of the field when the lava cooled and its orientation—the direction of magnetic north and south. "When we go back about 700,000 years," says geologist Mike Fuller of the University of Hawaii, "we find an incredible phenomenon. Suddenly the rocks are magnetized backwards. Instead of them being magnetized to the north like today's field, they are magnetized to the south."
Such a reversal of polarity seems to happen every 250,000 years on average, making us long overdue for another swap between the north and south magnetic poles. Scientist Gary Glatzmaier of the University of California at Santa Cruz has actually observed such reversals, as they occur in computer simulations (view one in See a Reversal). These virtual events show striking similarities to the current behavior of Earth's magnetic field and suggest we are about to experience another reversal, though it will take centuries to unfold.
Some researchers believe we are already in the transition phase, with growing areas of magnetic anomaly—where field lines are moving the wrong way—signaling an ever weaker and chaotic state for our protective shield.
Geophysicist Rob Coe, also of the University of California at Santa Cruz, may have even found a lava record in Oregon that charts the magnetic mayhem that ensues during a period of reversal. The picture that emerges may not be up to Hollywood disaster standards, but considering that human civilization has never had to cope with such a situation before, it could be an interesting and challenging time.
For a sequence of images of this documentary with many images of the aurora borealis and the magnetic fields, please click here.
Impact on Animals
by Peter Tyson
Would a dramatic change in the Earth's magnetic field affect creatures that rely on it during migration? Late on a January night in 1993 I found myself on a beach on the Pacific coast of Costa Rica, kneeling in the sand beside a leatherback sea turtle. Like a giant mango with wings, the huge black turtle had hauled herself up the beach in great stentorian gasps of air and was laying her eggs in a pit she had laboriously scooped out with her hind flippers.
Knowing basic facts about her ecology and physiology, I was in awe. How her kind, the largest living reptiles, had been around for 120 million years. How she lived solely on jellyfish, a thing more water balloon than animal. How she could collapse her lungs and dive to depths that would cause you or me to implode. How she had traveled thousands of miles around the Pacific Ocean, only to return there to the very beach she was born on years before.
That navigational and homing ability astonished me more than any other. How did she navigate around a trackless wilderness larger than the world's total land area and find her way back to that same short ribbon of sand? One hypothesis was just starting to be floated in those days: that to aid their long-distance migrations leatherbacks and other sea turtles appear to use the Earth's magnetic field (see Figure 1).
When I learned recently that our planet's magnetic shield is rapidly weakening and may be ready to reverse its polarity, causing compasses to point south, I immediately wondered what that would mean for leatherbacks and the many other species that use the magnetic field to orient themselves and find their way around. Could they withstand a significant dwindling of the field's strength or even a reversal? Or might extinctions, perhaps mass extinctions, be in the offing?
Animal magnetism
One of the first concrete signs that animals can tap into the magnetic field was observed, as in many a great discovery in science, by chance. It was the fall of 1957, and Hans Fromme, a researcher at the Frankfurt Zoological Institute in Germany, noticed that several European robins he kept in a cage were becoming restless and were fluttering up into the southwestern part of the cage. Nothing unusual there: it was known that migrating birds in cages become edgy at that time of year, and European robins in Germany migrate southwestwards to Spain to overwinter.
What made it striking was that the birds were in a shuttered room. They could see neither visual landmarks, nor their fellow, non-captive robins, nor the sun or stars, which were known to serve them as navigational aids. Clearly they were acting on something invisible, and Fromme deduced it must be the Earth's magnetic field.
Numerous experiments undertaken by him and others since then have shown that many living things avail themselves of the magnetic field. Organisms as diverse as hamsters, salamanders, sparrows, rainbow trout, spiny lobsters, and bacteria all do it. "I would go so far as to say that it's nearly ubiquitous," says John Phillips, a behavioral biologist at Virginia Polytechnic Institute and State University who himself has detected this ability in everything from fruit flies to frogs. (There's no scientific evidence that humans have this "sixth sense," though curiously, our brains do contain magnetite, the mineral thought to aid other animals' brains in detecting the field.)
How do we know organisms have this ability? A standard method to test for it is to throw a magnetic curve ball, as it were, at experimental subjects. In an effort, for example, to determine if the blind mole rat, a subterranean rodent that builds a home of branching tunnels with no exits to the surface, can sense the magnetic field, Tali Kimchi and Joseph Terkel of Tel Aviv University built an eight-armed maze within a device in which they could alter the magnetic field. They then tested two groups of rats—one in the Earth's magnetic field and the other in a field shifted by 180°—to see whether they had directional druthers for siting their sleeping nests and food chambers. The first group showed a significant preference to build their beds and pantries in the southern part of the maze, while the second group opted for the northern sector.
So they can sense it, but can they use it like we do a compass, to orient themselves? In another experiment, Kimchi and Terkel trained 24 blind mole rats to reach a goal box at the end of a complex labyrinth. Then, when all had mastered the task, they had half the rats do it again under the natural field and half under a reversed field. Lo and behold, the latter rats' performance fell far short of that achieved by their magnetically unmanipulated fellows.
Undersea superhighways
Other animals take things a step further than the blind mole rat, using the magnetic field like we do the Global Positioning System, to determine their location on the surface of the Earth and using that to negotiate unseen pathways during migration.
Kenneth and Catherine Lohmann of the University of North Carolina at Chapel Hill and their team have shown through many experiments that during their 8,000-mile migration around the Atlantic Ocean, young loggerhead sea turtles can detect not only the field's intensity but its inclination, the angle at which magnetic field lines intersect the Earth. The turtles use these two pieces of information, which vary at every point on the planet's surface, as navigational markers that help them advance along their migratory route (see Figure 2).
Sometimes this navigational ability can serve its practitioners only too well. A mystery long bedeviling marine biologists is why otherwise healthy whales beach themselves, often in large groups. In the early 1980s, a British biologist named Margaret Klinowska first noticed a correlation between where whale strandings tended to occur along the coasts of England and where magnetic lineations written into the seafloor intersect those coasts. (These lineations, or anomalies, are different from those produced by the main magnetic field.) Joe Kirschvink of the California Institute of Technology and his colleagues later showed a similar association on the east coast of the U.S.
Whales, it seems, follow these magnetic lineations during migration (see Figure 3). "If that's your game plan, and you get off track, and you follow a sharp magnetic anomaly that curves and runs into the coast, bang, you end up on the beach," says Kirschvink. Because whales are very social, if the leader makes this mistake, so does its entire pod, hence the mass strandings.
Rising to the occasion
If whales can run into trouble when the field is reasonably strong, what might happen to them and other creatures that rely on it if the field becomes feeble or even flips? Hans Fromme had found in Frankfurt that when he placed his European robins into a steel chamber and reduced the strength of the ambient magnetic field by a third, the birds' flutterings were no longer directional. This suggested that the birds needed the magnetic field to be a certain intensity to be of use. But Fromme's colleague F. W. Merkel later showed that the birds were able to acclimatize to the new magnetic field within a number of days.
Indeed, the researchers I spoke with all thought that organisms would be able to adjust to an acute weakening or even complete reversal of the magnetic field. "My gut reaction is it's not going to have an impact," says Frank Paladino, the Indiana-Purdue University leatherback researcher whose project I was visiting that night in 1993.
History seems to back this up. There is no firm evidence that the many magnetic field reversals that have taken place throughout our planet's history (see When Compasses Point South) have coincided with or triggered extinctions. Reversals take hundreds if not thousands of years to complete, and because for any one type of animal that represents hundreds or thousands of generations, species have time to accommodate to the change. Moreover, Kirschvink notes that even if the main dipole field were to collapse—an event that can last for up to 10,000 years during a reversal—residual fields 5 or 10 percent as strong as the main field would remain on the surface, and animals would be able to use those quite well for migration.
So as I watched that leatherback in Costa Rica use her oar-like front flippers to expertly disguise her newly laid nest with sand and then begin dragging her massive bulk back to the surf, I needn't have worried, it seems, that she and others like her might lose their way and thus rupture the cycle leatherbacks have maintained since the Age of Dinosaurs. That's a relief considering how many threats she and other wild animals already face today.
Source: PBS Nova
When Compasses Point South
If all the compasses in the world started pointing south rather than north, many people might think something very strange, very unusual, and possibly very dangerous was going on. Doomsayers would have a field day proclaiming the end is nigh, while more rational persons might head straight to scientists for an explanation.
Fortunately, those scientists in the know—paleomagnetists, to be exact—would have a ready answer. Such reversals in the Earth's magnetic field, they'd tell you, are, roughly speaking, as common as ice ages. That is, they're terrifically infrequent by human standards, but in geologic terms they happen all the time. As the time line at right shows, hundreds of times in our planet's history the polarity of the magnetic shield ensheathing the globe has gone from "normal," our current orientation to the north, to "reversed," and back again.
The Earth is not alone in this fickleness: The sun's magnetic shield appears to reverse its polarity approximately every 11 years. Even our Milky Way galaxy is magnetized, and experts say it probably reverses its polarity as well. Moreover, while a severe weakening or disappearance of the magnetic field would lay us open to harmful radiation from the sun, there's little evidence to date that "flips" per se inflict any lasting damage (see Impact on Animals).
It might sound as if scientists have all the answers regarding magnetic reversals. But actually they know very little about them. Basic questions haunt researchers: What physical processes within the Earth trigger reversals? Why do the durations and frequencies of both normal and reversed states seem random? Why is there such a disproportionately long normal period between about 121 and 83 million years ago? Why does the reversal rate, at least during the past 160 million years, appear to peak around 12 million years ago?
All these questions remain unanswered, though experts like Dennis Kent, the Rutgers University geologist who supplied NOVA with updated figures for the time line, are hard at work trying to answer them. In the meantime, not to worry. Reversals happen on average only about once every 250,000 years, and they take hundreds if not thousands of years to complete.
Even the weakening currently under way may be a false alarm. The field often gets very weak, then bounces back, never having flipped. As Ron Merrill, a magnetic-field specialist at the University of Washington remarked when asked whether we're in for a reversal: "Ask me in 10,000 years, I'll give you a better answer." So hang on to your compass. For the foreseeable future, it should work as advertised.—Peter Tyson
Source: PBS Nova
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