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.