The ‘hockey stick’ graph of surface temperature change over the past millennium and implications for climate change assessments was the subject of two hearings held by the U.S. House of Representatives Energy and Commerce Subcommittee on Oversight and Investigations, on 19 and 27 July. These hearings marked only the second time that the committee has discussed climate issues since George W. Bush became president.
The hearings were called to investigate papers published in 1998 and 1999 by Michael Mann (Pennsylvania State University) et al. that showed that surface temperatures for most of the past millennium were flat and then rose significantly in the late twentieth century like the blade of a hockey stick. Their graph of temperature in the Northern Hemisphere during the past 1000 years was reconstructed from proxy data, such as tree rings, ice cores, and other indirect data sources.