Here is a PBS NOVA Special program on the earth's magnetic field.
Watch a simulated reversal of Earth's magnetic field, from the first signs of instability to the final, inevitable flip.
Per NOVA website on the Magnetic Storm
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 simulationser.