Mann et al. have recently argued that they can salvage MBH98-type results, even with correct PC calculations by also using “the standard selection rule (Preisendorfer’s Rule N) used by MBH98″. They say that this method permits them to retain 5 PCs in the North American network. The reason why this matters to them is that the bristlecones are represented in the PC4 in correct PC calculations and the expanded roster would permit them to imprint the NH temperature reconstruction even from the PC4 position.
We have discussed elsewhere many issues regarding the robustness and statistical significance of this calculation, arguing that the bristlecones are not a valid proxy, that the MBH98 reconstruction is not robust to the presence/absence of the bristlecones and the MBH98 reconstruction has no statistical significance in its early portion. Here I consider the narrow issue of whether the PC selection method illustrated in the Nov. 22, 2004 diagram was actually “used by MBH98″ for tree ring networks.
I have been able to closely replicate the Nov. 22, 2004 diagram published at realclimate.org – which is now said to demonstate the PC selection method used in MBH98 (although this method was not reported in MBH98). I have tested the 19 network/calculation step combinations used in MBH98 to verify the archived PC selections at the Corrigendum SI against this diagram.
In 18 of 19 cases, the selections from the Preisendorfer-type calculation are inconsistent with the the Corrigendum SI archive. In some cases, more PCs are selected; in some cases, fewer. In 3 calculations, different selections are taken from the same network in different calculation steps – a result inconsistent with the stated policy.
This is strong evidence that the method of the Nov. 22, 2004 diagram, as illustrated and without still undisclosed modifications, was not used to determine the number of retained PCs in MBH98. We believe that climate scientists, who have relied on MBH98, should ask Nature and/or the U.S. National Science Foundation to require the disclosure of all source code used in MBH98, including the selection of PC series.