Oxygen taken from tree rings could help settle the question of whether hurricanes are getting stronger and more frequent, US researchers said on Monday.
Dana Miller and colleagues at the University of Tennessee at Knoxville said they had used the method to reconstruct a 220-year history of cyclone activity for southern Georgia.
Their findings confirmed the recent hurricane record and suggest a way to track the weather record going farther back in time, they write in this week’s issue of the 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. But 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.
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.
“Well-organised tropical cyclones, such as major hurricanes, produce large amounts of precipitation with distinctly lower (by as much as 10 per cent) oxygen isotope compositions than typical low-latitude thunderstorms,” the researchers wrote.
These isotopes — chemical variants of oxygen — are taken up by the tree and stored in the cellulose. There it remains until the tree is cut down. The researchers could easily find the oxygen isotopes from different years by counting the tree rings, just as a child would.
The researchers found evidence of three big storms in the 1870s, for instance. “Other decades of apparent activity include the 1840 and 1850 decades, 1800-1820 decades, and 1770s decade,” they wrote.