SAN FRANCISCO -- Changes in tree-ring density in the Arctic may be evidence of changes in light intensity during the trees' growth, according to a new study by San Francisco State University researcher Alexander Stine.
The finding has direct implications for the tree-ring "divergence problem," a phenomenon that has received considerable media attention but has been widely misinterpreted, said Stine, an assistant professor of Earth & climate sciences.
Tree rings consist of a low density ring, which forms early in the growing season, and a high density ring that forms late in the growing season. In colder parts of the world, the dense latewood rings tend to be denser during warm years. Temperature records inferred from Arctic tree rings do a good job of tracking temperature up until the 1960s, but subsequent Arctic tree-ring densities did not keep pace with increases in temperature, a discrepancy that is called the divergence problem.
Climate scientists have been aware of the divergence problem for some time, and it was mentioned in the emails from the Climatic Research Unit at the University of East Anglia, where it became the focus of attention during the 2009 "Climategate" controversy.
The divergence is not a problem for understanding modern climate change in the Arctic, Stine explained, "because we have thermometers and those thermometers tell us it's warming. But it's a problem because if we want to use these tree rings as a proxy for temperatures of the past, we need to make sure that we understand what's happening now."
Stine sees the new findings as the "bright side" of the divergence problem, one which he hopes will lead to a more informed discussion about climate change. "We could learn more about past variations in light intensity at the Earth's surface, and we may be able to deepen our understanding of both trees and climate."
With his colleague Peter Huybers at Harvard University, Stine set out t
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San Francisco State University