The hockey stick: a retrospective - Climate Change: The Facts - page 147

Figure 2: Northern Hemisphere temperature chart as it appeared in the 2001 IPCC
report (the 'Hockey Stick' graph)
Source: IPCC, “Summary for Policymakers: Report of Working Group I of the Intergovernmental Panel on Climate
Change,” in Climate Change 2001: The Scientific Basis: Contribution of Working Group I to the Third Assessment
Report of the Intergovernmental Panel on Climate Change, ed. J. T. Houghton, Y. Ding, D. H. Griggs, M. Noguer, P. J.
van der Linden, X. Dai, K. Maskell and C. A. Johnson (Cambridge: Cambridge University Press, 2001), accessed July
15, 2014, http://www.grida.no/publications/other/ipcc_tar/?src=/climate/ipcc_tar/wg1/005.htm figure 2.20
A very brief summary of the problems of the hockey stick would go like this. Mann's
algorithm, applied to a large proxy data set, extracted the shape associated with one small
and controversial subset of the tree rings records, namely the bristlecone pine cores from
high and arid mountains in the US southwest. The trees are extremely long-lived, but grow
in highly contorted shapes as bark dies back to a single twisted strip. The scientists who
published the data had specifically warned that the ring widths should not be used for
temperature reconstruction, and in particular their twentieth century portion is unlike the
climatic history of the region, and is probably biased by other factors. 8 Mann's method
exaggerated the significance of the bristlecones so as to make their chronology out to be
the dominant global climatic pattern rather than a minor (and likely inaccurate) regional
one; Mann then understated the uncertainties of the final climate reconstruction, leading to
the claim that 1998 was the warmest year of the last millennium, a claim that was not, in
reality, supportable in the data. Furthermore, Mann put obstacles in place for subsequent