Centuries of hurricane records have been discovered in the rings ofsoutheastern US pine trees. This arboreal archive may contain criticalinformation about how the Atlantic hurricane factory responds over thelong term to natural and human-induced climate changes, say researchersat the University of Tennessee, Knoxville.
In a "proof of concept" study of the oxygen isotopes found in thecellulose of late-season growth in annual growth rings from pine treesnear Valdosta, Georgia, a team led by Claudia Mora found they couldidentify all known hurricanes that hit the area over the past fiftyyears.
But that's just the beginning, says Mora, who is scheduled topresent some of her team's findings on Thursday, 11 August, at EarthSystem Processes 2, a meeting co-convened by the Geological Society ofAmerica and Geological Association of Canada this week in Calgary,Alberta, Canada. "We've taken it back 100 years and didn't miss astorm," said Mora.
Since a century is a very short time when it comes to climatechange, she and her team applied their new technique to old trees fromother parts of the Southeastern US and found a tropical cyclone recordspanning 227 years. They've even found additional climate informationgoing back as far as 1450 AD.
"What we're trying to do is understand frequency of hurricanesand how variable their occurrence is over the long-term," said Mora."We're trying to come up with a reliable way to establish this."
Mora's group divided each individual annual tree ring in thetrees into early-year and late-year growth. That way they could isolatethe late-year hurricane season. Then they searched all the woodytissues for any sudden drops in a particular oxygen isotope: oxygen-18.That is the hurricane signal, Mora said.
What makes drops in oxygen-18 so telling is that it matches up with alittle known talent of all hurricanes: they are very good at depletingthe air of oxygen-18, Mora says. Consequently, there are unusually lowconcentrations of oxygen-18 in the water that rains out of hurricanes.So when shallow roots of Southeastern trees like the longleaf pine andslash pine suck up that low-O-18 hurricane rain water, the same unusualisotopic signal is preserved in the woody tree cells that start growingas soon as the sun breaks through the storm clouds.
The trees pick up the storm water in the dozen or so daysimmediately after the storm, according to what other researchers havelearned about how pines exploit rainwater, says Mora.
Of course, not every hurricane drops rain on Valdosta,Georgia, says Mora. So to get a fuller picture of hurricane frequenciesher team has already begun looking at and searching for more locationsand old living trees or well-preserved dead trees in the SoutheasternUS , she said.
The matter of hurricane frequency has taken on greaterimportance recently as the Eastern US is seeing more hurricanes andclimate researchers have begun asserting that there's reason to believeglobal warming - at least partially human-influenced - may be causingthe increase.
The best way to differentiate natural from anthropogenicincreases in hurricane occurrence is to have a long history ofhurricanes and other tropical cyclones to compare with, Mora explains.
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