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Tree Rings Show History Of Hurricanes

GAINESVILLE, Fla. (AP) _ Scientists at the University of Tennessee have discovered a new database containing records of dozens of previously undocumented hurricanes.

That database is tree trunks.

By comparing two types of oxygen in the growth rings of longleaf pines, researchers have been able to determine when and where hurricanes made landfall, including information as far back as 1420 in some places.

Researchers wanted to know whether the increase in hurricanes in recent years is due to a natural variation in the global climate, global warming or perhaps a combination of factors. While the research has not definitively answered the question, it will likely be a variety of factors, said Claudia Mora, an isotope geochemist and one of the study’s principal investigators. Mora is a University of Tennessee professor and head of the Department of Earth and Planetary Science.

Modern hurricane record keeping involving weather instruments began around the time of World War II, although some spotty or partial written records date back to the 16th and 17th centuries. Although important, the records are not sufficient to determine a pattern of hurricane frequency.

"To statistically make an argument, you must have longer records than 10 to 50 years," Mora said.

The tree ring records are as old as the tree, sometimes 200 years or more. The tree rings have been accurate, so far showing only one year in the past 50 in which it appeared there was a hurricane when none was recorded by the National Weather Service.

The first tree rings examined came from 220-year-old trees being cut down and removed from the Valdosta State University campus, where geography professor Henri Grissino-Mayer was then teaching.

The old pines were used as a basis for the study, which was completed after Grissino-Mayer was hired at the University of Tennessee.

Mora said the research focused on the longleaf pine trees because they have a shallow root system which tends to take up a lot of moisture from the soil instead of the groundwater used by other species like oak trees.

"Water drawn from the soil would be more closely tied to a hurricane and maximize our chances of seeing this variation," Mora said.

Researchers were able to avoid cutting down trees to see their rings by boring a half-inch hole into a tree’s trunk to extract a sample of wood containing growth rings.

"That was another advantage to working with the pines - they are so resinous that the resins would quickly fill those boring and the tree could survive," Mora said.

Working with about 70 samples, including some dating back to 1420, researchers looked at the dark portion of a ring produced during each year’s growth. The dark areas grew from about July to October, the same time period when 90 percent of hurricanes that hit the U.S. would strike the Southeast.

Using analytical instruments that became available about five years ago, scientists were able to use about 100 micrograms - a sample smaller than a particle of dust - to find oxygen isotopes that are unique to hurricanes. Isotopes share an atomic number and have nearly identical chemical behaviors, but have a different atomic mass. In the tree ring study, the isotopes being examined are oxygen 16 and oxygen 18.

"An isotope is a chemical fingerprint and they have become major tools for things like tracking elephant feeding patterns and even for police departments," Mora said.

She and the other researchers determined that the ratio between oxygen isotopes in a tree ring changes when the tree has been in a hurricane, when more oxygen 16 and less oxygen 18 is present.

The next step in the process of studying the trees will be to look for even older specimens to sample, as well as to take a closer look at the isotopes’ position in the tree rings to try to determine whether hurricanes tended to occur earlier or later in the season.

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