Tree rings may be the newest aid in piecing together the frequency of hurricanes over the centuries. Researchers have matched individual rings to documented stormy seasons going back to the 1860s and to presumed active seasons occurring up to a century earlier. If validated, this proxy record would help climatologists understand natural variations in storm frequency, although it would probably not shed light on the controversial issue of changes in hurricane strength.
To reconstruct the climate and weather events of the past, before written records, researchers must plumb every indirect source of evidence they can find. In the case of hurricanes, sources such as windblown sand layers lying amid pond muck offer spotty resolution at best, says Claudia Mora of the University of Tennessee. Trying to build a better record, she and her colleagues scraped flecks of wood from the rings of longleaf pines, long-lived softwood trees common along the U.S. coasts of the Gulf of Mexico and the south Atlantic. During big storms, water vapor containing the heavy isotope oxygen 18 condenses and falls first, rendering local precipitation deficient in that isotope for up to several weeks. Trees incorporate the depleted oxygen into their cellulose, and the longleaf pine in particular relies heavily on precipitated water, so isotope variations should show up strongly between rings. Counting rings would then reveal whether a given year had any hurricanes, although it could not pick out individual storms or their intensities.
The team reconstructed 220 years of hurricane activity in the area around southern Georgia, representing storms that struck anywhere from the adjacent gulf to South Carolina. "Ours is the first long, coherent record," says Mora, whose group publishes its findings online September 18 in the Proceedings of the National Academy of Sciences USA. Consulting older trees could expand the record to 500 or 600 years, she points out.
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"It's a provocative paper," says atmospheric scientist Judith Curry of the Georgia Institute of Technology. "Now people need to take a careful look at it. How you interpret tree rings in terms of one climate variable isn't real simple." She notes that a reliable proxy record for hurricanes would be very useful as a way of evaluating models of storm activity.
