There is the major 'bar magnet' field called the dipole component, and then there is the crustal field. The crustal field was created when molten rock solidified and remembered how the local field was oriented. Over time, and millions of years of evolution, the crust has become a patchwork of local fields that add together to equal about 10% the strength of Earth's main field. Geologists use this crustal field to identify oil and ore-bearing regions that register as slight magnetic changes as instruments scan across them. This field more or less stays put, and moves around very slowly as the continents move, or as new crust is produced. The main ' bar magnet' field, which helps us navigate by compass, is created by currents in the liquid outer core of Earth. Its strength increases and decreases, and the locations of its north and south poles move around on the surface, depending on how these currents flow. As the orientation of these currents change within Earth, the main field also moves around on the surface. This is what causes cartographers to re-make their harbor maps every few decades to keep up with the changing magnetic bearings for 'true north'.
During the last few hundred years scientists have had equipment sensitive enough to track Earth's field accurately. The intensity of the main field seems to be falling by about 5% per century. This means that if the rate stays constant, Earth's main field will be no stronger than the irregular crustal field in about 1,000 or 2,000 years. But is it really going to be a steady decline? We don't know because we have never been able to study other reversals with year-by-year resolution. The latest Canadian Arctic Survey in 2001 reported an accelerating change in the motion of the pole. After staying near the area of Northwest Canada for nearly a century, it will be moving rapidly to stand nearly directly over the North Pole by about 2030, and then by the 22nd century it will be in Northern Siberia! Because the magnetic field is a critical buffer, shielding the atmosphere from cosmic rays and charged particles from the Sun, you might think that the health hazard posed by Earth loosing its magnetic field is rather fearful to think about. The geologic record, however, shows that Earth's polarity flips every 250,000 years or so. Presumably, half way through each reversal, the field dips to a strength near the level of the permanent, but highly irregular crustal field. There are no identifiable fossil effects from previous reversals, so perhaps they are not as much of a problem for the biosphere as we might suppose. On the other hand, perhaps other species are less susceptible to genetic damage than we humans. We know that many species of bacteria, birds and fish can sense the direction of the magnetic field and use it for migration and finding food in murky waters. These animals, or the less adaptable members of a species, may perish 'suddenly' when the field changes and they can no longer rely on it to orient themselves. We have never experienced loosing our main field. The fossil record is pretty mute about the biological effects. No one really knows if the condition of zero-field will last 5 years, 10 years or 500 years. By the way, in March 2003, Paramount Pictures released an awful movie called 'The Core' which showed hell breaking loose as the field began to fall to zero. The biggest reason not to expect anything to happen this time is that, 780,000 years ago when the last one happened, it was a complete washout. No grand extinction events. No evidence for atmospheric chaos. Nothing.
 |
This answer was updated in 2011.
See my books:
The Astronomy Cafe (1998) and
Back to the Astronomy Cafe (2003) for more FAQs in printed form. Author: Dr. Sten Odenwald, Copyright 2011
Return to Ask the Astronomer |