cc: edwardcook <drdendro@ldeo.columbia.edu>, Jonathan Overpeck <jto@u.arizona.edu>, Keith Briffa <k.briffa@uea.ac.uk>, ralley@geosc.psu.edu
date: Sat, 11 Mar 2006 12:16:12 +0700
from: edwardcook <drdendro@ldeo.columbia.edu>
subject: Re: Divergence
to: Richard Alley <ralley@essc.psu.edu>

   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

   ==================================

