Dramatic variations in the Earth's main magnetic field during the 20th century

Analysis of the International Geomagnetic Reference Field (IGRF) 1900-2000 shows that the Earth's main magnetic field has changed dramatically during the 20th century: its dipole moment has decreased by 6.5% since 1900, the s trengths of its quadrupole and octupole have increased by 95% and 74%, resp ectively, four major planetary-scale magnetic anomalies on the Earth's surf ace have enhanced by 21%-56%, and the magnetic center has shifted 200 km to wards the Pacific Ocean. These time-variation features are similar to the b ehavior before a geomagnetic polarity reversal.