The Hockey Stick is Bad Science

The four examples I chose were randomly chosen as disparate techniques which share almost nothing in common but are all published reconstructions showing hockey stick shape. Statistics Critiques Pro raises an interesting point about the statistical basis used for the analyses of MBH. Pro incorrectly refers to it as “component analysis”. It is actually PCA (Principal Component Analysis) and yes it is a somewhat advanced technique but Pro’s assessment that climatologists are not statistics experts and failed for this reason is undercut almost immediate when his first reference is Ross McKitrick, an economist. In point of fact many scientists (myself included) have to have some experience with statistical analysis of data as part of our tool box. But no one argues that more statistical expertise (on both sides) would be of value The critiques cited by Pro attributed to Ross McKitrick in the book “Shattered Concensus” (not a peer reviewed scientific publication) are not available to me, but I am very much aware of McKitrick and analyses of Mann et al’s data as described in the article he co-authored with Stephen McIntyre [1], and, in many ways, it appears to be flawed statistically. First of all the centering convention chosen does not significantly impact important factors of the PCA analysis in this case. The MBH98 analysis is very insensitive to the centering convention whether it is that baseline chosen by McKitrick and McIntyre (M&M) or MBH. Apparently the argument against the centering convention by M&M is predicated on their failure to use the proper number of PC’s base on “selection rules” (Priesendorfers Rule N). With the M&M centering they should have retained 5 principal components whereas with MBH 2 should be retained. With MBH the hockey stick shape shows up in the first PC whereas in the M&M’s constructions it shows up in the 4th PC. It’s still there. BUT McKitrick and McIntyre erroneously truncated THEIR PC’s down to 2. So yes, when you misapply the procedure you can, if needed, eliminate the hockeystick shape. In addition M&M incorrectly eliminated several proxy data sets resulting in spurious warming in the 15th century [2] Pro then also notes the large reliance on a small set of tree data from Graybill and Idso. MBH’s reliance on a technique of PCA called an “eigenvector-based CFR Technique” takes into account the regional climatic effects that Graybill and Idso were originally focusing on and, according to a study by Wahl (2007) [3] this data carries “important climate information at the level of eigenvector patterns in global surface temperatures. These results are valid notwithstanding issues concerning these proxies’ empirical relationship to local/regional surface temperatures after 1850…” (so the implication that this data is somehow improper is ameliorated to some extent). Further “…these proxies do not generate spurious increases in calibration fit that thereby downweight the value of other (presumably more locally climate-correlated) proxy series.” Wahl even finds that eliminating the Bristlecone/Foxtail pine data set still results in a single bladed hockey stick shape! Finally Wahl’s analysis do NOT support the upward “correction” of McIntyre and McKitrick in the 15th century and “leave unaltered the single-bladed hockey stick conclusion of strongly anomalous late 20th century temperatures. “ Further they conclude that M&M’s reconstruction showing 15th century temperatures higher than those of the mid 20th century “…do not have climatological meaning.”[3] As for the point about “infilling” errors on the data, this is presumably in reference to the St. Anne (“Gaspe”) data. Removal of this data set may result in a slight upward adjustment of only about 0.05deg) but does not alter the finding of anomalous warming in the late 20th century. Wahl finally notes: “Overall, the primary outcome from our results is that the work reported in MM03, MM05a, and MM05b does not provide substantive reason to invalidate the general conclusion of anomalous warmth in the later 20th century derived from the MBH reconstruction method and proxy data framework. We find that this result is neither an artifact of selection of the proxy series nor the result of formation or application of PC summaries in the reconstruction procedure” The Wegman hearing It should be noted that during this hearing Wegman was only asked to provide information about the “centering” of the PCA analysis, and strangely not whether it altered the final hockeystick result (!) His finding was that centering made a difference in the first principal component. Which was already acquiesced to. The key is that McKitrick and McIntyre appear to have incorrectly eliminated the PC that did show the hockey stick, whereas MBH did not! So Wegman is correct in his point, but that does not mean that McKitrick and McIntyre’s point is correct. Interestingly enough, if you throw out all the fancy PCA statistics altogether YOU STILL GET A HOCKEYSTICK! Rutherford et al (2005)[2] did this very thing and still got a hockeystick! Harvard-Smithsonian Meta Study of Soon et al. (2003) This been largely discredited in a number of journals such as EOS and Science. In fact the editor of the journal in which Soon and Baliunus published this went so far as to publish the following statement: “While these statements may be true, the critics point out that they cannot be concluded convincingly from the evidence provided in the paper. CR should have requested appropriate revisions of the manuscript prior to publication.” The Soon et al article even lead to one of the lead editors of CR to resign in protest. A detailed rebuttal to Soon et al. as published in EOS can be found in reference [4] below. There is still much debate within the climate community about the nature of the Medieval Warm Period. The primary problem with this is that the MWP doesn’t seem to show “synchronous” warming on a hemispherical scale. It appears to show localized warming in different areas at different times. This is why this is still not a settled issue. And certainly not to the level of decreeing it somehow more extreme than the current warming which so far has not been possible to explain without explicit reliance on anthropogenic factors along with natural factors. It is clearly possible to disagree on statistical and analytical methodology as we all know. The Hockey Stick was done using appropriate statistical methods (and indeed the critics appear to have used inappropriate methods) but even if there is a difference in opinion on the choices of analytical methods does not change the quality of the science. The hockey stick is robust. And, indeed, seen in numerous reconstructions by different groups using different methods with and without PCA and using different proxies. Good science is reproducible science and this is reproducible science. SOURCES [1] McIntyre, S., McKitrick, R., 2003, Corrections to the Mann et. al. (1998) Proxy Data Base and Northern Hemispheric Average Temperature Series, Energy and Environment, v14, pp751-771 [2]Rutherford, S., Mann, M.E., Osborn, T.J., Bradley, R.S., Briffa, K.R., Hughes, M.K., Jones, P.D., Proxy-based Northern Hemisphere Surface Temperature Reconstructions: Sensitivity to Method, Predictor Network, Target Season, and Target Domain, 2005, J. Climate, V18 [3] Wahl,E., Ammann, C.M, 2007, Robustness of the Mann, Bradley, Hughes reconstruction of Northern Hemisphere surface temperatures: Examination of criticism based on the nature and processing of proxy climate evidence, Climate Change, v85 pp33-69 [4] http://holocene.meteo.psu.edu...