Earth?s magnetic field has been monitored carefully since the 1830s, when the German polymath Karl Friedrich Gauss invented a way to measure its intensity. Since then, the field has decayed at the ?startling rate of about 5 percent per century …. British geophysicist David Gubbins and his colleagues have [examined] data hidden in the logbooks of ships that navigated the planet?s oceans in the 16th, 17th, and 18th centuries. The results have allowed Gubbins to build a remarkable picture of the behavior of Earth?s magnetic field in the centuries before detailed measurements were possible. …
It turns out that measurements of the direction of the field relative to the position of the sun were common between 1590 and 1840. ?Mariners made extremely accurate measurements, because their lives depended on it,? says Gubbins. Over the past 20 years or so, he and others have been mining this data from the many thousands of ships? logs that have survived in museums and archives, an endeavor that has occupied a steady flow of graduate students. Gubbins says there are 50 000 measurements alone in the records of the British East India Company, which had a monopoly on sea trade between Britain and India for much of the period that interests him.
Gubbins has now combined these data with the paleointensity measurements to calculate that Earth?s field was probably stable prior to 1840, or at least decaying at a much slower rate than it is now.
So what caused the sudden decline after 1840? Gubbins says it is due to regions of reversed magnetic field flux appearing in the Southern Hemisphere in the late 18th century, probably as a result of small thermal changes in Earth?s core. The field?s abrupt drop is consistent with other studies, he says. Data from older rock analyses suggest that the intensity of Earth?s field has declined by as much as 40?percent over the past 2500?years, at an average rate of 1.6?percent per century. That?s much slower than the current rate… ?It?s just coincidence,? he says, ?that today?s period of rapid change began at about the time we became able to measure it.? – spectrum