Inside the Hockey Stick

This curve, known as the hockey stick because of its shape, represents an estimate of how the temperature of the earth's Northern Hemisphere has changed over the past millennium.

To reach back centuries before the invention of the thermometer, Michael E. Mann, an associate professor of meteorology at Pennsylvania State University, and his colleagues used so-called proxy records, derived from tree rings, corals, and ice cores. In a 1999 study, the scientists overlapped that evidence

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