False Claims by McIntyre and McKitrick regarding the Mann et al. (1998) reconstruction

A number of spurious criticisms regarding the Mann et al (1998) proxy-based temperature reconstruction have been made by two individuals McIntyre and McKitrick ( McIntyre works in the mining industry, while McKitrick is an economist). These criticisms are contained in two manuscripts (McIntyre and McKitrick 2003 and 2004–the latter manuscript was rejected by Nature; both are collectively henceforth referred to as “MM”). MM claim that the main features of the Mann et al (1998–henceforth MBH98) reconstruction, including the “hockey stick” shape of the reconstruction, are artifacts of a) the centering convention used by MBH98 in their Principal Components Analysis (PCA) of the North American International Tree Ring Data Bank (‘ITRDB’) data, b) the use of 4 infilled missing annual values (AD 1400-1403) in one tree-ring series (the ‘St. Anne’ Northern Treeline series), and c) the infilling of missing values in some proxy data between 1972 and 1980. Each of these claims are demonstrated to be false below.

[McIntyre and McKitrick have additionally been discredited in a recent peer-reviewed article by Rutherford et al (2004)].

[Added 1/6/05: See also “On Yet Another False Claim by McIntyre and McKitrick” which discredits the claimed “Monte Carlo” experiment results from the rejected McIntyre and McKitrick comment to Nature]

Principal Components Analysis (PCA) is a traditional tool for representing a large spatiotemporal dataset in terms of a smaller number of leading patterns of variation in the data. The choice of how the data are ‘centered’ in PCA (i.e., what time interval is used to define the ‘zero’ baseline for the data series) in general simply changes the relative ordering of the leading patterns of variance (or linear combinations thereof), the means of the associated Principal Components (PC) time series, and the number of statistically significant patterns in the data. The centering convention does not influence important properties of PCA such as the orthornormality of the Empirical Orthogonal Functions (EOFS) or the completeness of the eigenvector basis set.

The MBH98 reconstruction is indeed almost completely insensitive to whether the centering convention of MBH98 (data centered over 1902-1980 calibration interval) or MM (data centered over the 1400-1971 interval) is used. Claims by MM to the contrary are based on their failure to apply standard ‘selection rules’ used to determine how many Principal Component (PC) series should be retained in the analysis. Application of the standard selection rule (Preisendorfer’s “Rule N’“) used by MBH98, selects 2 PC series using the MBH98 centering convention, but a larger number (5 PC series) using the MM centering convention. Curiously undisclosed by MM in their criticism is the fact that precisely the same ‘hockey stick’ pattern that appears using the MBH98 convention (as PC series #1) also appears using the MM convention, albeit slightly lower down in rank (PC series #4) (Figure 1). If MM had applied standard selection procedures, they would have retained the first 5 PC series, which includes the important ‘hockey stick’ pattern. The claim by MM that this pattern arises as an artifact of the centering convention used by MBH98 is clearly false.

FIGURE 1. Comparison of PC #1 of the North American ITRDB tree-ring data from MBH98 (red) and PC #4 resulting from a PCA of the same dataset using the MM centering convention (blue–for visual comparison the blue curve has been adjusted to have the mean and amplitude of the red curve, as only the relative pattern of variation in the predictors matters in the MBH98 methodology).

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