Scientific American magazine described him as the "Man behind the Hockey Stick", referring to his reconstruction of temperatures, originally published in 1998. The hockey stick was a term coined for a chart of temperature variation over the last 1,000 years, which suggested a recent sharp rise in temperature caused by human activities. The high-profile publication of the data led to the controversial "hockey stick" being used as a key piece of supporting evidence in the third assessment report by the United Nations' Intergovernmental Panel on Climate Change (IPCC) in 2001.
Dr. Mann has been personally involved in the debate over climate change. In testimony before the US Senate in 2003, he stated:
It is the consensus of the climate research community that the anomalous warmth of the late 20th century cannot be explained by natural factors, but instead indicates significant anthropogenic, that is human influences... More than a dozen independent research groups have now reconstructed the average temperature of the northern hemisphere in past centuries... The proxy reconstructions, taking into account these uncertainties, indicate that the warming of the northern hemisphere during the late 20th century... is unprecedented over at least the past millennium and it now appears based on peer-reviewed research, probably the past two millennia.