The claims of McIntyre and McKitrick regarding the Mann et al (1998) temperature reconstruction have recently been discredited by the following peer-reviewed article to appear in the American Meteorological Society journal, “Journal of Climate“:
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 (2005).
Key excerpts from the article are provided below:
1. page 13, 2nd paragraph (through top of page 14)
It should be noted that some falsely reported putative errors in the Mann et al.(1998) proxy data claimed by McIntyre and McKitrick (2003) are an artifact of (a) the use by these latter authors of an incorrect version of the Mann et al. (1998) proxy indicator dataset, and (b) their misunderstanding of the methodology used by Mann et al. (1998) to calculate PC series of proxy networks over progressively longer time intervals. In the Mann et al. (1998) implementation, the PCs are computed over different time steps so that the maximum amount of data can be used in the reconstruction.
For example, if a tree-ring network comprises 50 individual chronologies that extend back to AD 1600 and only 10 of those 50 extend to AD 1400 then calculating one set of PCs from 1400 to 1980 (the end of the Mann et al. (1998) calibration period) would require the elimination of 40 of the 50 chronologies available back to AD 1600. By calculating PCs for two different intervals in this example (1400-1980 and 1600-1980) and performing the reconstruction in a stepwise fashion, PCs of all 50 series that extend back to AD 1600 can be used in the reconstruction back to AD 1600 with PCs of the remaining 10 chronologies used to reconstruct the period from 1400-1600. The latter misunderstanding led McIntyre and McKitrick (2003) to eliminate roughly 70% of the proxy data used by Mann et al. (1998) prior to AD 1600, including 77 of the 95 proxy series used by Mann et al. (1998) prior to AD 1500. This elimination of data gave rise to spurious, anomalous warmth during the 15th century in their reconstruction, sharply at odds with virtually all other empirical and model-based estimates of hemispheric temperature trends in past centuries (see e.g. Jones and Mann, 2004).
2. page 32, beginning 2nd sentence, 3rd paragraph:
We compared the RegEM reconstruction with the Mann et al. (1998) surface temperature reconstruction employing the same predictor network, the same calendar annual target season, and same global target region as Mann et al. (1998). We eliminated the infilled values from AD 1400-1403 used by MBH98 to complete one of the Jacoby and D’Arrigo (1989) ‘Northern Treeline’ series back to AD 1400. This is easily done in the RegEM method by treating those values as missing, something that could not have been done in MBH98. We terminated the calibration period in 1971 to address the criticism by McIntyre and McKitrick (2003) of the use by MBH98 of a modest number of infilled missing proxy values in the PC/Multiproxy network between 1971 and 1980.
3. page 33, beginning 2nd paragraph (key passages highlighted for emphasis):
A remarkably close similarity is observed (Figure 3) between the RegEM and Mann et al. (1998) NH annual mean surface temperature reconstructions. The two reconstructions are indistinguishable well within their 2-sigma uncertainties. The RegEM NH reconstruction using all available individual proxy records (rather than replacing spatially dense tree-ring networks with their leading principal components as in the MBH98 Multiproxy/PC network) again yields nearly indistinguishable estimates (Figure 2). The close reproducibility of the MBH98 reconstruction based on both (a) the use of an independent CFR method and (b) the use of the individual proxies used by MBH98 rather than the Multiproxy/PC representation used by MBH98, discredits the arguments put forth by McIntyre and McKitrick (2003) in support of their putative correction to the MBH98 reconstruction.
Figure 1. From Fig. 2, Rutherford et al (2005). Comparisons of stepwise non-hybrid and hybrid-20 reconstructions for annual mean using the multiproxy/PC network and the hybrid-20 reconstruction using the full multiproxy network. Also shown is the instrumental record consisting of the few available data from 1750-1856 (Mann, 2002a) and the Jones et al. (1999) data for 1856-2000.
Figure 2. From Fig. 3, Rutherford et al (2005). Comparison between the RegEM-based hybrid-20 annual mean reconstruction (using the Mann et al. multiproxy/PC network) and Mann et al. (1998) showing overlapping uncertainties in both estimates. Instrumental record is shown for comparison (see Figure 1 for details).