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

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).

As discussed above, MM incorrectly truncated the PC basis set at only 2 PC series based on a failure to apply standard selection rules to determine the number of PC series that should be retained in the analysis. Five, rather than two PC series, are indicated by application of standard selection rules if using the MM, rather than MBH98, centering convention to represent the North American ITRDB data. If these five series are retained as predictors, essentially the same temperature reconstruction as MBH98 is recovered (Figure 2).

To address the other spurious criticisms of MM, a 1902-1971 calibration interval was employed in this analysis to avoid any infilled missing proxy data between 1972 and 1980, and the ‘St Anne River ‘ series challenged by MM (due to 4 infilled missing values from AD 1400-1403) was eliminated. The standard measure of reconstructive skill, the ‘reduction of error’ metric (‘RE’) used by MBH98, was used to evaluate the fidelity of the resulting reconstruction using 19th century instrumental data that are independent of the calibration (RE<0 exhibits no skill, while RE= -1 is the average value for a random estimate). This evaluation indicates a clearly skillful though modestly lower verification resolved variance than in MBH98 (RE=0.22 vs. RE=0.51).

FIGURE 2. Reconstruction using MM centering convention and retaining first 5 PCs of North American ITRDB data (red: only AD 1400-1600 interval during which MM claim to produce a significantly different reconstruction from MBH98 is shown) , along w/ MBH98 reconstruction (blue).

We further show that the entire issue raised by MM regarding the centering convention used in PCA is spurious by demonstrating that similar results are produced whether or not proxy networks are represented using PCA at all (Figure 3). We performed the reconstruction employing all 95 proxy indicators available in the MBH98 proxy network back to AD 1400 individually (rather than using any PC series to represent these indicators). We begin the reconstruction at the date AD 1404, avoiding the infilled missing values from AD 1400-1403 in the ‘St. Anne’ Northern Treeline tree ring series. As above, a 1902-1971 calibration period was used to avoid address the issue of missing values in some proxy series between 1972 and 1980. Each proxy record was standardized to have unit variance and zero mean over a 1902-1971 calibration period, and equal weight was applied to all proxy records in the analysis. We performed an additional analysis back to AD 1400 eliminating the ‘St. Anne’ Series entirely (Figure 3). Both analyses yield a reconstructed Northern Hemisphere mean temperature history quite similar to that of MBH98, and demonstrate skill against independent 19th century instrumental (2) data (RE=0.39, 0.33, respectively – only moderately lower than the MBH98 result RE=0.51). The central ‘hockey stick’ result of MBH98 is thus quite clearly seen to be robust to the specious criticisms raised by MM with regard to the centering convention used in the PCA of proxy data networks or the infilling of missing values in certain proxy series.

FIGURE 3. Comparison of MBH98 reconstruction from AD 1400-1980 with alternative reconstructions from AD 1400-1971 based on the direct use of the individual 95 proxy series available back to AD 1404 (yellow), and the 94 proxy series (all 95 series except the ‘St Anne’ series discussed above) available back to AD 1400 (green). Shown for comparison is the instrumental NH annual mean record 1856-1980 (red). The MBH98 reconstruction was based on a ’stepwise’ approach employing increasingly more proxy data over time, while the other two reconstructions, for simplicity, are performed with the same (’frozen’) proxy network over the entire interval.

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