"Reversals have been documented as far back as 330 million years. During that time more than 400 reversals have taken place, one roughly every 700,000 years on average. However, the time between reversals is not constant, varying from less than 100,000 years, to tens of millions of years. In recent geological times reversals have been occurring on average once every 200,000 years, but the last reversal occurred 780,000 years ago. At that time the magnetic field underwent a transition from a "reversed" state to its present "normal state".
geolab.nrcan.gc.ca
Does the magnetic field drop to zero gauss? Dire predictions follow upon the heels of this theory. Electronic devices would all be at risk: there may be damage to, or complete loss of, all near-earth-orbiting satellites and possibly the space station itself. Effects on life forms could range from migrating birds losing their sense of direction to immune system decline and even widespread die-off from radiation-induced cancers.
Losing its protective magnetic envelope, the atmosphere would expand and become thinner, possibly leading to altitude sickness near sea level. No longer filtered out, deadly cosmic rays would kill most if, not all, living creatures on the surface. Only those living in deep caves would be safe. This scenario has prompted some to build underground bunkers in hopes of surviving.
Countering this frightening vision, NASA predicts that, rather than declining to zero gauss, the magnetic field would become disordered. Thus we might for short time have more than one north and south pole on the planet. This official scientific stance says that the magnetosphere which shields us from cosmic radiation would not entirely disappear either. Thus, while communications would be erratic and perhaps at times completely inactivated, humans would find ways to survive. However, there are dissenters in the ranks, pointing to the vast South Atlantic magnetic anomaly and radiation damage to satellites over that region attributed to weakening of the protective magnetosphere.
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