14 DAY TRIAL // Oxygen isotope analysis from tree rings could help settle the question of whether hurricanes are getting stronger and more frequent, say researchers from University of Tennessee.
This could be the newest method in studying 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.
This 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.
Shallow-rooted trees, such as pines, take their water from the surface, and the sources vary depending on rainfall. In hurricanes, the oxygen contained the water is slightly different structurally.
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.
"Well-organized tropical cyclones, such as major hurricanes, produce large amounts of precipitation with distinctly lower (by as much as 10 percent) oxygen isotope compositions than typical low-latitude thunderstorms," the researchers said.
The Tennessee team reconstructed the last 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. Consulting older trees could expand the record to 500 or 600 years, she points out.
Their findings confirmed the recent hurricane record and suggested a way to track the weather record going farther back in time - they wrote in Proceedings of the National Academy of Sciences.
"Recent studies suggest a sharp increase in hurricane activity and intensity since the mid-1990s," they wrote in their report.
However, weather cycles may last for many decades. "Before about 1900, systematic records of hurricane occurrence are fragmentary in many localities and rely predominantly on documentary records such as ship logs and news media," they wrote.
"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.