UT Researchers’ Work Reveals 220-year Hurricane History

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KNOXVILLE — New research by two University of Tennessee professors could help us better understand hurricanes by looking to an unusual source: tree rings.

By analyzing the rings of trees in areas that are hit by hurricanes, UT professors Claudia Mora and Henri Grissino-Mayer have found that the oxygen isotope content in a ring will vary if the tree was hit by a hurricane during that year.

Their research is being published in this week’s early online version of the Proceedings of the National Academy of Sciences, one of the world’s most cited multidisciplinary scientific journals.

There has been a significant increase in the number of hurricanes hitting the Southeast since the mid-1990s, and scientists have sought to determine the cause for the upswing. Some question exists about whether the increase is part of a regularly occurring cycle of activity, or whether it is being brought about by a cause such as global climate change.

The problem facing this analysis is that the current documented history of hurricane activity in the Southeast dates back only about 100 years — not enough time to establish a cycle that might last many decades at a time.

By looking at older trees, Mora and Grissino-Mayer have been able to create a record of hurricane activity dating back 220 years, more than double the current record.

“We think this can shed light on whether we’re looking at a long-term pattern, or something that could be caused by human activity,” said Mora, professor and head of UT’s Department of Earth and Planetary Sciences.

One notable aspect of their research is the accuracy with which the tree-ring oxygen analysis is able to show when a hurricane hit an area. Comparing the tree-ring data to the National Weather Service data over a 50-year period, the tree-ring data showed only one year in which their data reported a hurricane that was not in the list of recorded storms.

The initial data were collected from trees on the campus of Valdosta State University, where Grissino-Mayer was previously a faculty member. The two professors then expanded their research are to swampy nearby Lake Louise, where they were able to find even older trees preserved beneath the waterline.

Different isotopes of oxygen present in the tree rings are the key to knowing whether hurricanes hit the tree. The moisture carried by hurricanes carries a different ratio of oxygen-18 to oxygen-16 than the normal rain that trees absorb. When that moisture falls near a tree, it is absorbed, and that ratio of oxygen is reflected in that year’s ring.

“The level of resolution with this measure is key,” said Grissino-Mayer, a UT associate professor of geography. “Other proxy measures of hurricanes are not able to look at a year-by-year basis.”

Mora and Grissino-Mayer also noted that this opens the door for research to go back even further than 220 years, as older trees are discovered in hurricane-prone areas, perhaps as old as 500 years.

The next steps of their research are already underway. Research teams recently traveled to areas near Pensacola, Fla., and Charleston, S.C., to collect tree samples to analyze, with the hopes of building a broader geographic sample.

Mora and Grissino-Mayer are also working to improve the resolution at which they can examine when the oxygen isotope ratios are different within a tree ring, specifically looking at determining whether storms hit early or late in the hurricane season of the year in which a tree ring grew.

The lead author on the article was UT earth and planetary sciences doctoral student Dana Miller, now a researcher at Oak Ridge National Laboratory, whose dissertation was written about the new findings.

The research was funded by grants from the National Science Foundation, along with the UT President’s Initiative in Teaching, Research and Service.

An abstract of the article, as well as a full-text PDF, are available online at http://www.pnas.org/cgi/content/abstract/0606549103v1. The article was also published online by the National Science Foundation.


Contacts:

Jay Mayfield (865-974-9409, jay.mayfield@tennessee.edu)
Claudia Mora (865-974-2366, cmora@utk.edu)
Henri Grissino-Mayer (865-974-6029, grissino@utk.edu)

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