http://en.wikipedia.org/wiki/Earth's_magnetic_field#Magnetic_field_decay Quote:
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The earth's magnetic field strength was measured by Carl Friedrich Gauss in 1835 and has been repeatedly measured since then, showing an exponential decay with a half-life of about 1400 years. This could also be stated as a relative decay of about 10% to 15% over the last 150 years.
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Earth appears to be heading toward another pole reversal. Once the reversal takes place, to the best of my understanding, the magnetic field's power is replenished from all the excitement of the change.