July 24, 2007 > TechKnow Talk
When North Becomes South
Imagine hiking in the wilderness and checking a compass to get your bearings. The needle spins around lazily, offering no clues as to direction. Or perhaps the needle fixes on south, instead of north. As amazing as it seems, both of these scenarios will likely occur someday. How far in the future this will happen is the topic of scientific inquiry and debate.
The earth is a giant magnet. Surrounding its solid core and below the less fluid mantle is a massive quantity of liquid metal, primarily iron and nickel. Due to the high temperatures and pressures in this region, atoms are ionized, or stripped of electrons. These electrically charged particles, spinning with the earth's rotation, create a huge magnetic field extending out thousands of miles around the earth.
Just as with any magnet, the magnetic field or "flux" exits near the south pole, wraps around through space and enters near the north pole. This creates an invisible, protective blanket--called a magnetosphere--around the earth that intercepts much of the solar wind, composed of charged particles and high-energy radiation from the sun. It is the interaction of the magnetosphere with the solar wind that creates the brightly colored lights known as the Aurora Borealis at high latitudes. The direction of this magnetic flux along the earth's surface also aligns a compass with "true" or magnetic north.
In fact, the points of entry and exit of the magnetic field do not align exactly with the earth's rotation. The magnetic poles are inclined about 11 degrees from the axis of rotation. This is probably due to non-uniformly distributed deposits of magnetic material in the earth's mantle as well as the dynamic nature of the liquid core material itself. Further evidence that our liquid magnet is ever-changing is the drift of the north magnetic pole. Just a few years ago, it was over extreme northern Canada, but has moved into international waters, apparently headed for Siberia, at a rate of about 25 miles per year.
But the most dramatic indication that we live on a very dynamic magnet is the repeated reversal of the earth's magnetic field throughout the planet's history. The flux lines completely reverse course, exiting from the north pole, and entering through the south, then vice versa. This happens at extremely variable intervals, ranging from less than 10,000 years to many millions of years between reversals, with an average in the neighborhood of 250,000 years. The most recent reversal, which resulted in the magnetic orientation we experience today, occurred 780,000 years ago.
How do scientists know this? Ancient lava flows and sedimentary rocks contain particles of iron and other materials that aligned themselves with the magnetic field extant at the time the rock solidified. By examining the orientation of these particles in rocks from various geologic eras, a long history of magnetic flip-flops has been documented.
No one knows for sure why magnetic reversals have occurred. Many scientists believe that convection currents in the molten core may develop in such a way as to destabilize the field and cause it to become disordered, or even to collapse completely, before being re-established in a reversed polarity. Computer models seem to confirm that this is possible. Others have proposed that reversal of the field results from dramatic celestial events such as impact with a comet or asteroid.
Science is also unclear on how long the reversal process takes. It may happen suddenly, within a few hundred years, or it may take thousands of years, preceded by a gradual weakening of the field. There is evidence that at least some past reversals were signaled by many millennia of weakening field strength.
Another unknown is whether the field strength actually goes to zero for some period of time and the planet loses its magnetism entirely. Alternatively, the field may enter a period of chaotic and rapidly changing magnetism, perhaps characterized by several short-lived, weak magnetic poles, from which it eventually emerges with a reversed polarity. In this scenario, there would be some magnetic field throughout the reversal process, offering a level of protection against the solar wind.
Interestingly, we do have a handy example to study right here in our own solar system. The sun reverses its magnetic polarity with regularity every eleven years. Unfortunately, the physics associated with the sun's magnetism are very different than those on earth, so comparisons are difficult. For example, the sun flips its magnetic poles at times of maximum field strength, whereas earth's reversals apparently occur when the field is weakest.
Are we approaching another magnetic reversal? The magnetic field has been getting weaker for at least 2000 years, losing about 35% of its strength in that period. Even more alarming, the rate of decrease seems to be accelerating. Currently, it is weakening at about 7% per century. At this rate, the magnetic field could become weak enough to collapse entirely in as little as a thousand years, or if the rate continues to accelerate, perhaps much sooner.
But these data need to be weighed in an historical context. The field strength has varied greatly over the eons. It has been dramatically stronger, at least five times stronger, than it is today. It has also weakened to its current level before, and then began strengthening again, without a reversal. No one knows if this current cycle will lead to a reversal, though many scientists believe it is probable.
How would life on earth be affected by a magnetic reversal? As the magnetosphere protects us against the damaging solar wind, the answer would depend on how weak the field became before flipping and strengthening again and for how long its weakened state endured. The heightened exposure to radiation from space would likely increase cancer rates and mutations, and it if lasted long enough, could possibly result in extinctions, as plants and animals could be bombarded for decades or centuries by strong radiation, affecting their ability to successfully reproduce.
Some scientists have also postulated that a very weak magnetic field would have devastating effects on climate, which would dramatically alter the entire earth ecosystem. It seems likely that animals such as geese and whales, which rely on magnetism for annual migrations, would also be at risk of failing to locate their breeding and feeding sites in a weak field and, following a pole shift, could even migrate in the wrong direction.
The good news for our descendants is that fossil records show no mass extinctions, excessive mutations, or other impacts associated with past reversals. In fact, there is no evidence that any of the past reversals have negatively affected life in any widespread manner whatsoever. Our ancestors have survived several reversals, with no apparent deleterious effects.
So the dire predictions of mankind's demise put forward by some alarmists may be dismissed. However, our way of life may change rather dramatically. A useless compass would be the least of our concerns. For example, in an attempt to minimize the exposure to cancer-causing radiation, people may choose to live and work in bunker-like buildings, perhaps even underground.
The weakened magnetosphere would subject our satellites to a level of radiation their on-board electronics are not currently designed to withstand. Even in today's relatively strong field, satellites have been damaged and even rendered useless by intense bursts of solar radiation, and by passing through the South Atlantic Anomaly, an area where the magnetosphere dips low over the earth, providing orbiting satellites a reduced level of protection from cosmic radiation.
Here on earth, though the atmosphere itself would offer some meager protection, all our electronic systems would be at risk of failure, a problem not encountered by our ancestors of 780,000 years ago. The potential impact would be tremendous, affecting all facets of modern life. If we are still relying on satellites and earth-based electronic systems for communications, navigation, transportation, and power generation when we enter the next reversal period, we will need to devise a way to make them impervious to high levels of radiation exposure.