One way to understand hurricanes is by looking at historical records and working out different factors at play during the time. But that approach is limited since the National Hurricane Center did not start keeping official records of storms until the mid-1800s.
So how to study weather that predates record-keeping? One way is to look at weather's casualties - shipwrecks.
A University of Arizona-led team combined tree-ring records with data gleaned from Spanish shipwrecks and found there was a dramatic 75 percent drop in the number of Caribbean hurricanes from 1645-1715. That period corresponds with a time when there was a known decrease in sunspot activity and cooler temperatures in the Northern Hemisphere.
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"By combining shipwreck data and tree-ring data, we are extending the Caribbean hurricane record back in time and that improves our understanding of hurricane variability," said lead author Valerie Trouet, an associate professor in the UA Laboratory of Tree-Ring Research in a press release.
Finding data on shipwrecks was no problem for the researchers since ship traffic between Spain and the Caribbean in the 17th and 18th centuries was bustling. And because the ships carried valuable goods such as gold, Spain was diligent in its record-keeping of their comings and goings. When a ship didn't reach its destination, records showed it.
Those records were also supported by a book commonly used by treasure hunters today - Robert F. Marx's "Shipwrecks in the Americas: a complete guide to every major shipwreck in the Western Hemisphere."
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Another clue to hurricane activity in this period is revealed in tree rings since tree growth slows during years with hurricanes. The University of Arizona team combined the two sets of data, shipwrecks and tree rings, and discovered the hurricane patterns from both overlapped from 1707-1825. The slowdown in storms revealed by both corresponded to a period known as the Maunder Minimum.
"We didn't go looking for the Maunder Minimum," Trouet said. "It just popped out of the data."
The Maunder Minimum refers to a period of low sunspot activity. Because Earth receives less solar radiation during lulls in sunspot activity, the Northern Hemisphere was cooler. So the scientists were able to infer that the lull in Caribbean hurricanes corresponded to a time when Earth received less solar energy.
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This may all seem like a lot of legwork, but the researchers point out that finding new ways to study hurricanes is critical as climate models suggest big storms will become more frequent as the planet warms.
The research is published in Proceedings of the National Academy of Sciences.