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Hockey stick graphs present the global or hemispherical mean temperature record of the past 500 to 2000 years as shown by quantitative climate reconstructions based on climate proxy records. These reconstructions have consistently shown a slow long term cooling trend changing into relatively rapid warming in the 20th century, with the instrumental temperature record by 2000 exceeding earlier temperatures.

The term "hockey stick graph" was coined by the climatologist Jerry Mahlman, to describe the pattern shown by the {{#invoke:Footnotes|harvard_citation_no_bracket}} (MBH99) reconstruction, envisaging a graph that is relatively flat with a downward trend to 1900 as forming an ice hockey stick's "shaft" followed by a sharp, steady increase corresponding to the "blade" portion.<ref name="Monastersky" /><ref name="Weart HS" /> The reconstructions have featured in Intergovernmental Panel on Climate Change (IPCC) reports as evidence of global warming. Arguments over the reconstructions have been taken up by fossil fuel industry funded lobbying groups attempting to cast doubt on climate science.<ref>{{#invoke:Footnotes|harvard_citation_no_bracket}}, {{#invoke:Footnotes|harvard_citation_no_bracket}}</ref>

Paleoclimatology dates back to the 19th century, and the concept of examining varves in lake beds and tree rings to track local climatic changes was suggested in the 1930s.<ref name="Weart CO2">{{#invoke:Footnotes|harvard_citation_no_bracket}}</ref> In the 1960s, Hubert Lamb generalised from historical documents and temperature records of central England to propose a Medieval Warm Period from around 900 to 1300, followed by Little Ice Age. This was the basis of a "schematic diagram" featured in the IPCC First Assessment Report of 1990 beside cautions that the medieval warming might not have been global. The use of indicators to get quantitative estimates of the temperature record of past centuries was developed, and by the late 1990s a number of competing teams of climatologists found indications that recent warming was exceptional. {{#invoke:Footnotes|harvard_citation_no_bracket}} introduced the "Composite Plus Scaling" (CPS) method used by most later large scale reconstructions.<ref name="Fingerprints (1990s-2000s)">{{#invoke:Footnotes|harvard_citation_no_bracket}}</ref><ref>{{#invoke:Footnotes|harvard_citation_no_bracket}}.</ref> Their study was featured in the IPCC Second Assessment Report of 1995.

In 1998 Michael E. Mann, Raymond S. Bradley and Malcolm K. Hughes developed new statistical techniques to produce {{#invoke:Footnotes|harvard_citation_no_bracket}} (MBH98), the first eigenvector-based climate field reconstruction (CFR). This showed global patterns of annual surface temperature, and included a graph of average hemispheric temperatures back to 1400 with shading emphasising that uncertainties (to two standard error limits) were much greater in earlier centuries.<ref name="Wahl 2007">{{#invoke:Footnotes|harvard_citation_no_bracket}}</ref> {{#invoke:Footnotes|harvard_citation_no_bracket}} independently produced a CPS reconstruction extending back for a thousand years, and {{#invoke:Footnotes|harvard_citation_no_bracket}} (MBH99) used the MBH98 methodology to extend their study back to 1000.<ref name="Weart HS" /><ref name="TAR 2.3.2.2" />

A version of the MBH99 graph was featured prominently in the 2001 IPCC Third Assessment Report (TAR), which also drew on Jones et al. 1998 and three other reconstructions to support the conclusion that, in the Northern Hemisphere, the 1990s was likely to have been the warmest decade and 1998 the warmest year during the past 1,000 years.<ref name="TAR 2.3.2.2" /> The graph became a focus of dispute for those opposed to the strengthening scientific consensus that late 20th century warmth was exceptional.<ref name="Part three guardian">{{#invoke:Footnotes|harvard_citation_no_bracket}} "Part three: Hockey stick graph took pride of place in IPCC report, despite doubts".</ref> In 2003, as lobbying over the 1997 Kyoto Protocol intensified, a paper claiming greater medieval warmth was quickly dismissed by scientists in the Soon and Baliunas controversy.<ref name="revkin August 5, 2003" /> Later in 2003, Stephen McIntyre and Ross McKitrick published {{#invoke:Footnotes|harvard_citation_no_bracket}} disputing the data used in MBH98 paper. In 2004 Hans von Storch published criticism of the statistical techniques as tending to underplay variations in earlier parts of the graph, though this was disputed and he later accepted that the effect was very small.<ref>The Decay of the Hockey Stick, Nature "Climate Feedback" blog post by von Storch. "... we do not think that McIntyre has substantially contributed in the published peer-reviewed literature to the debate about the statistical merits of the MBH and related method." (comment by von Storch & Zorita, May 7, 2007 07:35 PM, in response to multiple comments on their failure to acknowledge McIntyre and McKitrick's contributions)</ref> In 2005 McIntyre and McKitrick published criticisms of the principal components analysis methodology as used in MBH98 and MBH99. Their analysis in was subsequently disputed by published papers including {{#invoke:Footnotes|harvard_citation_no_bracket}} and {{#invoke:Footnotes|harvard_citation_no_bracket}} which pointed to errors in the McIntyre and McKitrick methodology. Political disputes led to the formation of a panel of scientists convened by the National Research Council, their North Report in 2006 supported Mann's findings with some qualifications, including agreeing that there were some statistical failings but these had little effect on the result.<ref name="Part four guardian">{{#invoke:Footnotes|harvard_citation_no_bracket}}, "Part four: Climate change debate overheated after sceptics grasped 'hockey stick'".</ref>

More than two dozen reconstructions, using various statistical methods and combinations of proxy records, support the broad consensus shown in the original 1998 hockey-stick graph, with variations in how flat the pre-20th century "shaft" appears.<ref name="Part four guardian" /><ref name="Frank 2010">{{#invoke:Footnotes|harvard_citation_no_bracket}}.</ref> The 2007 IPCC Fourth Assessment Report cited 14 reconstructions, 10 of which covered 1,000 years or longer, to support its strengthened conclusion that it was likely that Northern Hemisphere temperatures during the 20th century were the highest in at least the past 1,300 years.<ref name="AR4 2k years" /> Further reconstructions, including Mann et al. 2008 and {{#invoke:Footnotes|harvard_citation_no_bracket}}, have supported these general conclusions.


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