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

Given that each of the criticisms of MBH98 raised by MM are demonstrably false, one might well be led to wonder how MM, using the MBH98 method and their putative ‘corrected’ version of the MBH98 proxy dataset, were able to obtain a reconstruction so at odds with the MBH98 reconstruction and virtually all existing reconstructions (in particular, in its apparent indication of anomalous 15th century warmth). Rather than ‘correcting’ the MBH98 proxy data set, we demonstrate that the reconstruction of MM resulted, instead, from their selective censoring of key indicators from the MBH98 proxy dataset. Indeed, we are able to reproduce the MM reconstruction of anomalous 15th century warmth when the entire ITRDB North American data set (and the ‘Queen Anne’ series) are censored from the proxy network (Figure 4). These data (in fact, 70% of all of the proxy data used by MBH98 prior to AD 1600) were unjustifiably censored from the MBH98 dataset by McIntyre and McKitrick (2003) in their original analysis (see Jones and Mann, 2004, and Rutherford et al, 2004 for a discussion).

MM in their more recent rejected submission to Nature, instead filtered out the ‘hockey stick’ pattern of low-frequency variability in the North American ITRDB data through the incorrect PCA truncation described above, which censors this pattern by retaining too few Principal Components series in the data. As discussed above, the MBH98 reconstruction and the variants of the reconstruction (i.e., Figures 2 and 3) that address the various spurious criticisms raised by MM, each pass statistical verification. In stark contrast, our reproduction of the MM reconstruction demonstrates that their reconstruction dramatically fails statistical verification (see Figure 5) with an RE score ( -0.76) that is statistically indistinguishable from the results expected for a purely random estimate (as a reminder, RE<0 exhibits no skill, and RE= -1 is the average value expected for a purely random estimate). In short, the supposed ‘correction’ of MBH98 by MM is seen to represent little more than a statistically meaningless, botched application of the MBH98 procedure that relies upon censoring key indicators from the MHB98 proxy data set.

FIGURE 4. Comparison of MBH98 reconstruction (blue) from AD 1400-1980 (thick black curve is 40 year smoothed version) with a reconstruction over AD 1400-1600 (yellow) based on the ‘censored’ network described in the text, arising from the censoring of the entire North American ITRDB data set and the ‘St. Anne’ Northern Treeline series from the predictor network prior to AD 1600. As MM censored the MBH98 dataset primarily prior to AD 1600, a suitable approximation to the ’stepwise’ MM reconstruction is provided by a splice of the yellow curve from AD 1400-1600 with the actual MBH98 reconstruction (blue) from AD 1600-1971. The resulting series is shown smoothed on a 40 year timescale (thick orange).

FIGURE 5. Comparison as in Figure 4, but showing the continuation of our reproduction of the MM reconstruction (based on their ‘censored’ version of the MBH98 network discussed in the text) through the end of the calibration interval (green curve). The full instrumental Northern Hemisphere temperature record (1856-2003) is shown for comparison. The failure of the MM reconstruction to verify against independent instrumental temperature is evident from the extreme discrepancy in variance between the green and red series during the 1854-1901 verification interval.

References:

Jones, P.D., Mann, M.E., Climate Over Past Millennia, Reviews of Geophysics., 42, RG2002, doi: 10.1029/2003RG000143, 2004.

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

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 Methodology, Predictor Network, Target Season and Target Domain, Journal of Climate, in press, 2004.

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