Number 154 (Story #3), December 3, 1993 by Phillip F. Schewe and Ben Stein EARTH'S MAGNETIC FIELD fluctuates in strength over time. For example, 2000 years ago the field was 40% stronger than it is today and it continues to decrease at a rate of 7% per century. A plot of geomagnetic intensity over the past four million years, made using measurements of sedimentary samples brought up from the ocean floor, exhibits an asymmetrical sawtooth pattern with a time constant of about a half million years. Scientists at the Institute of Earth Physics in Paris found that during a period of stable polarity the intensity fell slowly but that after a polarity change the intensity regenerated rapidly. (Jean-Pierre Valet and Laure Meynadier, Nature, 18 Nov. 1993.)