STATE COLLEGE, Pa., Sept. 2 (UPI) -- U.S. climate scientists say the Northern Hemisphere's surface temperatures were higher during the past decade than at any time during the last 1,300 years.
The researchers said if they include somewhat controversial data derived from tree-ring records, the warming is anomalous for at least 1,700 years.
"Some have argued tree-ring data is unacceptable for this type of study," said Penn State Associate Professor Michael Mann. "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 marine sediment cores, ice cores and coral cores.
The scientists said their findings show that, with caveats, tree-ring data can be used, but even without that data, it's clear the anomalous nature of recent warmth, which most scientists believe to be a result of human activity, is a reality.
The study that included University of Massachusetts Professor Ray Bradley, Professor Malcolm Hughes and researcher Fenbiao Ni of the University of Arizona; researchers Zhihua Zhang and Sonya Miller of Penn State; and Assistant Professor Scott Rutherford of Roger Williams University appears in the online edition of the Proceedings of the National Academy of Science.