Ships' Logs and Archeomagnetism

摘要

Since the mid-19th century, geophysical measurements have shown that the strength of Earth's magnetic field has steadily decreased by about 5% per century. At this rate, the field should disappear entirely in about 2000 years. But what about variations in the field before the start of this drop in strength? On page 900 of this issue, Gubbins and colleagues report important contributions to the study of such geomagnetic secular variations (1). By analyzing the record back to 1590, the authors find some surprising variations that offer clues to what is happening with Earth's field.