cc: Eystein Jansen <eystein.jansen@geo.uib.no>, Keith Briffa <k.briffa@uea.ac.uk>
date: Wed, 15 Mar 2006 21:40:32 -0700
from: Jonathan Overpeck <jto@u.arizona.edu>
subject: Re: Fwd: Comment on NRC Workshop
to: edwardcook <drdendro@ldeo.columbia.edu>

   Great! Any chance Keith can follow up with an independent document? Think this would be
   very helpful, and save much effort down the line.

   Thanks again, peck

     Hi guys,

     Gerry North has what I sent. Hopefully it will have a positive impact.

     Cheers,

     Ed

     Begin forwarded message:

     From: "Gerald R. North" <[1]g-north@tamu.edu>

     Date: March 15, 2006 9:17:16 PM GMT+07:00

     To: edwardcook <[2]drdendro@ldeo.columbia.edu>

     Cc: Ian Kraucunas <[3]IKraucunas@nas.edu>, Bette Otto-Bliesner <[4]ottobli@ucar.edu>,
     Mike Wallace <[5]wallace@atmos.washington.edu>

     Subject: Re: Comment on NRC Workshop

     Dear Dr. Cook,

     Your information will be distributed to the entire committee, and it will be given full
     consideration in our discussions. Your additional information does add to what Dr.
     D'Arrigo said in her excellent presentation to the Committee.

     Sincerely,

     Gerald R. North

     On Mar 15, 2006, at 6:23 AM, edwardcook wrote:

     Ian Kraucunas, Ph.D.

     Board on Atmospheric Sciences and Climate

     National Research Council of The National Academies

     500 Fifth Street NW, Keck 705

     Washington, DC 20001

     Dear Dr. Kraucunas,

     I request that this document (also attached as Cook_NRC.pdf) and the attached scientific
     paper (2001_Cook_QSR.pdf) be forwarded to all NRC committee members who participated in
     the recent NRC workshop "Surface Temperature Reconstructions for the Past 2,000 Years:
     Synthesis of Current Understanding and Challenges for the Future", ideally with a cc to
     me when this is done.  I have heard via emails and telephone conversations about a
     serious concern raised about tree rings by some committee members and invited
     participants at the NRC workshop.  This concern could have an unfairly negative impact
     on the use of tree rings for reconstructing past climate, especially that related to
     surface air temperatures, hence my letter to you and the committee.  As part of her
     talk, Dr. Rosanne D'Arrigo mentioned the discovery of "divergence" between instrumental
     temperatures and tree growth during the last few decades of the 20th century at selected
     boreal sites in the Northern Hemisphere.  The affected trees systematically
     under-responded to increasing temperatures, i.e. they grew more slowly than they should
     have based on a well-fitted linear response model applied to the data prior to the onset
     of "divergence".  The large-scale occurrence of this change in responsiveness has also
     been described by Keith Briffa (Briffa et al., 1998) in Nature.  A number of hypotheses
     have been proposed to explain it, which range from natural (climatic change) to
     anthropogenic (pollution related), but the actual cause is still unknown.  This
     phenomenon needed to be mentioned by Dr. D'Arrigo, but it appears to have taken on a
     level of specious importance that is not justified by the evidence.

     Perhaps not surprisingly, but also somewhat alarmingly, it is my understanding that some
     NRC committee members and other influential participants have come to the conclusion
     that the observed 20th century "divergence" calls into serious question the value of the
     tree-ring reconstructions of temperatures over the past millennium.  The implicit
     assumption apparently being made is that the "divergence" being caused by environmental
     conditions in the 20th century could have also prevailed back during times like the
     Medieval Warm Period (MWP) some 800-1000 years in the past.  If this were the case, then
     the concern raised by some at the workshop would be justified.  However, the available
     evidence does not support such a conclusion.  In a paper I published in Quaternary
     Science Reviews in 2004 (Cook et al., 2004), I reviewed the properties and
     interpretation of the tree-ring data used in the Esper et al. (2002) paper published in
     Science.  The reasonably well distributed set of tree-ring data in both boreal and more
     temperate latitude sites around the Northern Hemisphere allowed me to split up the data
     into sub-regional ensembles, including 8 sites in the 55-70 north band and 6 sites in
     the 30-55 south band.  The purpose was to demonstrate the overall robustness of the
     multi-centennial temperature signal in the tree-ring data.  This plot from the QSR paper
     is embedded below and the paper is sent being sent as an attachment.  The importance of
     this plot to the "divergence" debate follows next.

     In their paper, Briffa et al. (1998) showed that the "divergence" between tree growth
     and temperatures was largely restricted to the region covered by the north band
     described in Cook et al. (2004).  Consistent with that finding, the north ensemble mean
     shown below (blue curve) reveals a serious downturn in growth after about 1950.  This is
     an expression of the large-scale "divergence" described by Briffa et al. (1998) and also
     by Dr. D'Arrigo in her NRC talk.  In contrast, the south ensemble mean (red curve) shows
     the opposite growth trajectory after 1950, i.e. a substantial growth increase that is
     much more consistent with 20th century warming.  If one then follows the plots back in
     time, all sub-region ensemble means track each other remarkably well at multi-centennial
     time scales even when they enter the putative MWP 800-1000 years ago.  In fact, at no
     time prior to the 20th century is there a separation between north and south that is at
     all comparable to that found after 1950.  This result indicates that no large-scale
     "divergence" of the order found during the 20th century occurred during the MWP even
     though that period is suggested to have been somewhat warmer than average overall.  It
     thus refutes the argument that "divergence" of the kind found in the 20th century could
     very well have happened in the past, thus implying that tree rings cannot produce
     reliable reconstructions of past temperatures.  It also supports the existence of an
     admittedly unknown anthropogenic cause of the 20th century "divergence".  The lack of
     any known cause is unfortunate, but this would be true regardless of how the importance
     of "divergence" is interpreted.

     I am not aware of ANY evidence that demonstrates the occurrence of large-scale
     "divergence" between tree growth and climate prior to the 20th century.  Indeed, the
     available evidence indicates just the opposite.  In my opinion it is therefore
     unjustified to call into question the use of tree rings for reconstructing temperatures
     over the past millennium based on a nave and inappropriate extrapolation of the growth
     "divergence" problem into the past when it appears to be unique to the 20th century.
     The NRC committee members must consider this in their report if it is to have the
     necessary scientific credibility that is expected of it.

     References

     Briffa, K.R., Schweingruber, F.H., Jones, P.D., Osborn, T.J., Shiyatov, S.G., Vaganov,
     E.A. 1998. Reduced sensitivity of recent tree-growth to temperature at high northern
     latitudes. Nature 391: 678-682.

     Esper, J., Cook, E.R., Schweingruber, F.H. 2002. Low-frequency signals in long tree-ring
     chronologies for reconstructing past temperature variability. Science 295: 2250-2253.

     Cook, E.R., Esper, J., D'Arrigo, R.D. 2004. Extra-tropical Northern Hemisphere land
     temperature variability over the past 1000 years. Quaternary Science Reviews 23(20-22):
     2063-2074.

     Sincerely,

     Edward R. Cook

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

     Dr. Edward R. Cook

     Doherty Senior Scholar and

     Director, Tree-Ring Laboratory

     Lamont-Doherty Earth Observatory

     Palisades, New York 10964  USA

     Email:    [6]drdendro@ldeo.columbia.edu

     Phone:    845-365-8618

     Fax:    845-365-8152

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

     <2004_Cook_QSR.pdf>


     <Cook_NRC.pdf>

     <Cook_QSR_Fig6.gif>

--

   Jonathan T. Overpeck
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