cc: Keith Briffa date: Tue, 6 Nov 2007 14:43:29 -0800 (PST) from: "David M. Ritson" subject: Re: RCS paleo reconstructions to: Tim Osborn Dear Keith and Tim, I wrote in the summer relative to the statement that underpins mmuch of the discussions on the merits of RCS versus conventional standardizations of paleo climate reconstructions of Cook et al 1995 that `" ,,, the cross-dated annual changes in ring-width between trees due to climate are forced out of alignment and effectively averaged out in the creation of the mean regional curve." This is commonly taken to imply that RCS methods largely circumvent the segment-length curse. At the time I believed that, in most instances, the systematics inherent in the actual data, such as the fractions of juvenile and mature trees in the sites invalidated the cancellations implied in the Cook et al. paragraph. There are cancellations, but in most instances insufficient to better eliminate the segment curse. This appears to be well known, by the professional dendrochronologist community. What is disconcerting is that I find no clarifications or follow-up of the above Cook statement either in the IPCC AR4 nor in the generally available climate change literature. If indeed such clarifications are missing then I think it is incumbent that you guys ensure that they are understood throughout the climate community. My own take on the current situation is that the only hard statements that can be trusted should be based on `bounds'. Juvenile growth for the first century or so, is likely to be variable and probably juvenile data should be ignored. Subsequent to this an `upper' bound is provided by assuming that ring width growth is independent of tree age. More speculatively a lower bound is provided by assuming ring-area growth constancy. However nobody provides such bounds. Tim gave me some interim answers to the above, but promised me that Keith would provide a more definitive summary of RCS status after the summer. I certainly would appreciate your considered views as to the absolute precision and trustworthiness of past millenial temperature reconstructions. Obviously the North NAS committee had similar reservations . Cheers Dave Ritson