cc: edwardcook , Jonathan Overpeck , Keith Briffa , ralley@geosc.psu.edu date: Sat, 11 Mar 2006 12:16:12 +0700 from: edwardcook subject: Re: Divergence to: Richard Alley Hi Richard, Thanks for the email. I admit to having thought you were on the NRC committee. I now stand corrected. I also hope you did not read into my email any unfair criticism of Rosanne, the NRC panel, or you for that matter. Certainly, none was intended and I tried to carefully word-smith my email to avoid any such suggestion. Perhaps I could have done better. I recognize the potentially serious implications of the divergence issue with IPCC etc. I also appreciate the concern you note about it possibly happening in the MWP or some similar past warm period. In my QSR paper I noted that when the northern chronologies were compared to the southern chronologies (QSR Fig. 6) used in the Esper et al. paper, the northern ones showed a clear downturn or divergence in the latter part of the 20th century and this did not show up in the southern chronologies. Yet back 1000 years in the past, no such clear separation between north and south was indicated. In fact, the late-20th century divergence is unique in the data back to AD 800. This, I argued, suggested an unspecified anthropogenic (i.e. pollution) cause for the 20th century divergence in the northern chronologies. Hence, the temperature estimates prior to about 1970 were probably reasonably accurate given the data and methods used. Until we understand the cause(s) of the 20th century divergence, I think that anthropogenic agents should be the working hypothesis for explaining it, not the other way around, because I have not seen evidence to support the other side of the argument. I do indeed appreciate your help here. Obviously I have a vested interest in seeing that tree rings are treated fairly, but I am quite open to the science pointing the way with regards to how the divergence issue affects the use of tree rings as records of past temperature. If someone can show that divergence has happened in the past in a large-scale sense as that for the 20th century, I will be happy to look at the evidence. Otherwise, I think we should err on the side of "no past divergence". Cheers, Ed On Mar 11, 2006, at 10:50 AM, <[1]ralley@geosc.psu.edu> <[2]ralley@geosc.psu.edu> wrote: Casting aspersions on Rosanne, on the NRC panel, or on me for that matter is not going to solve the underlying problem. ================================== Dr. Edward R. Cook Doherty Senior Scholar and Director, Tree-Ring Laboratory Lamont-Doherty Earth Observatory Palisades, New York 10964 USA Email: [3]drdendro@ldeo.columbia.edu Phone: 845-365-8618 Fax: 845-365-8152 ==================================