Researchers confirm that surface temperatures in the Northern Hemisphere were warmer over the last 10 years than any time during the last 1300 years, and, if the climate scientists include the somewhat controversial data derived from tree-ring records, the warming is anomalous for at least 1700 years.

"Some have argued that tree-ring data is unacceptable for this type of study," says Michael Mann, associate professor of meteorology and geosciences and director of Penn State's Earth System Science Center. "Now we can eliminate tree rings and still have enough data from other so-called 'proxies' to derive a long-term Northern Hemisphere temperature record."

The proxies used by the researchers included information from marine and lake sediment cores, ice cores, coral cores and tree rings.


"We looked at a much expanded database and our methods are more sophisticated than those used previously," says Mann. In today's (Sept. 2) online edition of the Proceedings of the National Academy of Sciences, the researchers note, "Conclusions are less definitive for the Southern Hemisphere and globe, which we attribute to larger uncertainties arising from the sparser available proxy data in the Southern Hemisphere."