The Earth’s magnetic field — which deflects harmful space radiation from the surface — has been weakening, losing about 10 percent of its strength over the last two centuries, and the decay may have been accelerating in recent years.

That weakening led to speculation that the magnetic field may be on the cusp of disappearing during a reversal, when the planet’s north and south poles flip, which could have consequences for civilization and life. But geoscientists have little knowledge of what has been occurring with the magnetic field during recent millenniums. The current decline could be just a squiggle of typical fluctuations.

So they turned to pottery and the record keeping of ancient bureaucrats.

In a paper published on Monday in The Proceedings of the National Academy of Sciences, an international team of scientists examined 67 jar handles collected from excavations of Judah, an Iron Age kingdom encompassing the region around Jerusalem between the eighth and second centuries B.C. Embedded in these handles, scientists say, are records of how the magnetic field rose and fell during those centuries.