It is now known, but not well publicized, that the earth's main magnetic field is decaying relatively rapidly. This paper considers a physical basis for this decay and experimental determinations that support it. The conclusion is reached that it is an exponential decay and that the half-life is 1400 years. This rate of decay is assumed to have been constant since the origin of the magnetic field. It is then shown that the life of the earth’s magnetic field should be reckoned in thousands, not millions or billions, of years. It is also shown that the stronger magnetic field in the past and its shielding effect would alter radio carbon dates, reducing the previously held dates.