Background
Perhaps no single item related to the debate over CO2-induced global warming has been as controversial as the Northern Hemispheric "hockeystick" temperature reconstruction of Mann et al. (1998, 1999) and the extended global version of Mann and Jones (2003), all of which temperature histories depict a level of late 20th-century warmth that is, as their creators claim, unprecedented over the past one to two millennia.

What was done
McIntyre and McKitrick, the authors of the important new paper that is the object of this review, critically analyzed the procedures that resulted in the "hockeystick" nature of the above-mentioned temperature reconstructions from proxy climate data, i.e., their abrupt and dramatic late 20th-century increases in temperature that appear alien to all that preceded them.

What was learned
First, in an analysis of an unusual data transformation employed by Mann et al., which strongly affects the resulting principal components (PCs) of their tree-ring based temperature reconstructions, McIntyre and McKitrick discovered that when tested on persistent red noise, the unusual method nearly always produces a hockeystick-shaped first principal component (PC1).  "In effect," as they say, "the Mann et al. (1998) data transformation results in the PC algorithm mining [our italics] the data for hockey stick patterns."

Second, the insightful Canadians demonstrated that the unusual data transformation used by Mann et al. "effectively selects only one species (bristlecone pine) into the critical North American PC1, making it implausible to describe it as the 'dominant pattern of variance'."  Interestingly, the selected tree-ring records were ones that had been developed and analyzed by Graybill and Idso (1993), who had suggested they were particularly good candidates for exhibiting CO2-induced increases in growth (as opposed to temperature-induced increases in growth, as assumed by Mann et al.).  In fact, Graybill and Idso described in detail how they had investigated "the possibility that changes in climate during the past century might be responsible for the unusual increases in ringwidth growth" exhibited by the trees," considering temperature, precipitation and computed drought values; and it was their stated conclusion, which was clearly available for Mann et al. to read before using Graybill's data, that "it is notable that trends of the magnitude observed in 20th century ringwidth growth are conspicuously lacking in all of the time series of instrumented climatic variables that might reasonably be considered growth-forcing in nature."  These facts are particularly "disquieting," as McIntyre and McKitrick put it, "given that the NOAMER PC1 has been reported to be essential to the shape of the Mann et al. (1998) Northern Hemisphere temperature reconstruction."

Third, McIntyre and McKitrick show that the Mann et al. (1998) benchmarks for significance of their Reduction of Error statistic are "substantially under-stated," and, using a range of cross-validation statistics, they show that their 15th-century reconstruction "lacks statistical significance."

What it means
With these three solid strikes against the hockeystick, one would hope it would be laid to rest, never again to raise its blade, in the continuing quest to determine the true nature of earth's temperature history over the past two millennia.

References
Graybill, D.A. and Idso, S.B.  1993.  Detecting the aerial fertilization effect of atmospheric CO2 enrichment in tree-ring chronologies.  Global Biogeochemical Cycles 7: 81-95.

Mann, M.E., Bradley, R.S. and Hughes, M.K.  1998.  Global-scale temperature patterns and climate forcing over the past six centuries.  Nature 392: 779-787.

Mann, M.E., Bradley, R.S. and Hughes, M.K.  1999.  Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations.  Geophysical Research Letters 26: 759-762.

Mann, M.E. and Jones, P.D.  2003.  Global surface temperatures over the past two millennia.  Geophysical Research Letters 30: 10.1029/2003GL017814.