Scientists connected with a 2001 report by the Intergovernmental Panel on Climate Change (IPC) found in an analysis of Northern Hemisphere temperatures over the past millennium, a sharp rise starting around 1900. Their “hockey stick” graph and conclusion that human activity caused this sudden warming drew fire from politicians and critics. The National Research Council now lends support to the hockey stick. It finds that tree rings, ice cores and other evidence suggest with a high level of confidence that the last decades of the 20th century were warmer than any comparable span in the past four centuries. And like the IPC work, its report, released June 22, expressed less certainty in temperature reconstructions going back a millennium because of the scarcity of precisely dated evidence before the 17th century. The council noted that the available data did suggest that many locations were hotter in the past 25 years than during any other quar ter-century period since the 10th century.