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WIREs Clim Change
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The climate of the past millennium and online public engagement in a scientific debate

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Abstract After the publication in 1999 of a reconstruction of the Northern Hemisphere temperature popularly known as “the hockey‐stick,” climate scientists and Internet bloggers engaged in a heated and often stalemated public debate on the validity of paleoclimate reconstructions and on their implications for the wider question of anthropogenic climate change. The Internet emerged as an important medium for channeling the direct participation of the public almost at the same level as the professional climate scientist. It also allowed dissemination of largely unfiltered information. I argue that, although paleoclimate research did benefit in some technical aspects, the public debate around the hockey‐stick focused on issues that were not scientifically central, like the existence of the Medieval Warm Period or the characterization of 1998 as the warmest year of the millennium. In contrast, much more relevant points, such as constraining the value of climate sensitivity using reconstructions of past climate, remained restricted within purely academic circles. The public resonance of the hockey‐stick debate was also clearly framed by the politicization of climate science and the impacts of the series of IPCC reports, in particular the Third Assessment Report, published in 2001. This resonance was amplified by the expanding use of the Internet. The Internet represented a new bidirectional channel through which the public and academia could interact to achieve a transparent, democratic, and participative evaluation of science. I argue that, although we could have hoped for a positive outcome of the hockey‐stick controversy, this opportunity for a new constructive public engagement in scientific debates was missed. This article is categorized under: Paleoclimates and Current Trends > Paleoclimate Social Status of Climate Change Knowledge > Sociology/Anthropology of Climate Knowledge
Original figure of the hockey‐stick graph as a reconstruction of the Northern Hemisphere temperature anomalies over the past millennium, as published in 1999 in Geophysical Research Letters (Mann et al., ). The year 1998 is highlighted as the warmest year in the past millennium. The shading represents the uncertainty in the estimated temperature. This figure was almost exactly reproduced in 2001 in the summary for policy makers of the IPCC Third Assessment Report (IPCC, ), Working Group 1, but without highlighting the year 1998. The figure caption there, however, ended with the sentence: “..Nevertheless the rate and duration of warming of the 20th century has been much greater than in any of the previous nine centuries. Similarly, it is likely that the 1990s have been the warmest decade and 1998 the warmest year of the millennium
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Reconstructions of the Northern Hemisphere temperature deviations from the mean over the past millennium as published by the IPCC Fourth Assessment Report (Jansen et al., ) in year 2007, thus summarizing the state‐of‐the art during the controversy. The reconstructions are derived mainly from dendroclimatological records and have been smoothed to highlight timescale variability of 30 years and longer. The record labeled PS2004 is exceptionally derived from borehole temperature data). The hockey‐stick graph (labeled MBH99) displays smaller temperature variations than most other reconstructions
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Time series of the frequency of occurrence in the internet of the expressions “medieval warm period” and “Climate sensitivity,” obtained with Google trends. Climategate e‐mails (Maibach et al., ; Grundmann, ) were released in December 2009
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Social Status of Climate Change Knowledge > Sociology/Anthropology of Climate Knowledge
Paleoclimates and Current Trends > Paleoclimate

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