From: Jonathan Overpeck <jto@u.arizona.edu>
To: edwardcook <drdendro@ldeo.columbia.edu>
Subject: Re: NRC Committee on Surface Temperature Reconstructions
Date: Tue, 14 Mar 2006 21:29:28 -0700
Cc: Eystein Jansen <eystein.jansen@geo.uib.no>, Keith Briffa <k.briffa@uea.ac.uk>

   Hi Ed (and Keith) - this looks good. For what it's worth, here are some comments:

   1. I agree Keith should send in an independent letter by email too (I'd put both on
   letterhead or at least include as pdf attachments, so email forwarding wouldn't have the
   chance of messing it up) .

   2. I would say right up front - first line that you'd like your letter (s) to go to all
   committee members, if possible with a cc to you. Don't leave any wriggle room.

   3.  cc to G. North and B. Otto-Bliesner - again, so there is no doubt that this gets to
   everyone

   4. no need to mention IPCC. Focus on the science and the NRC review. Don't want to
   introduce extra politics.

   Thanks both for doing this - I agree there is a real need to ensure that the panel has the
   science from the experts.

   Best, peck

     Hi everyone,
     Here is a draft of what I want to quickly send to
     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
     Email: ikraucunas@nas.edu
     Phone: (202) 334-2546
     Fax: (202) 334-3825
     He originally invited me to talk before the NRC. I do not have any other information on
     who to send it too. Please let me know what you think, but don't be too pedantic or
     critical at this stage. I get the feeling we have very little time to make an impact on
     the NRC committee and its report. I personally think that I am correct as far as I can
     take the argument. Let me know if I should send this on to Richard as well.
     Ed
     Dear Ian,
     I have heard via emails and telephone conversations about some rather serious
     developments that could have an unfairly negative impact on the use of tree rings for
     reconstructing past climate and the upcoming IPCC assessment, especially that related to
     surface temperatures. Apparently as part of her talk Rosanne D'Arrigo mentioned the
     phenomenon of "divergence" between instrumental temperatures and tree growth in the
     latter few decades of the 20th century. The large-scale nature of this phenomenon was
     first described in Nature by Keith Briffa back in 1998 (Briffa et al., 1998) and to this
     day its cause is not well understood at all. A number of hypotheses have been mentioned,
     which range from natural (climatic change) to anthropogenic (i.e. pollution related),
     but the actual cause is still unknown.
     Somewhat alarmingly, it is my impression now the the NRC committee members and other
     influential participants of the meeting 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 being
     made is that the "divergence" is being caused by climatic change related to 20th century
     warming, conditions that could have also prevailed back during the Medieval Warm Period
     (MWP) some 800-1000 years in the past. If this were the case, then the concerns of the
     committee 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 latititude 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
     show the overall robustness of the multi-centennial temperature signal in the tree-ring
     data. This plot from the QSR paper is attached below as is the paper itself.
     In his 1998 paper, Briffa showed that the divergence 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 reveals a serious downturn in growth after
     about 1950. This is an expression of the "divergence" that has been described first by
     Briffa and also by D'Arrigo in her NRC talk. In contrast, the south ensemble mean shows
     the opposite, i.e. a substantial growth increase which is much more consistent with 20th
     century warming. If one than follows the plots back in time, all of the 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 separation between north and south that is remotely comparable to
     that found after ca. 1950. This result suggests 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. This result clearly refutes
     the argument that "divergence" of the kind noted in the 20th century happened in the
     past. It also suggests a unique anthropogenic cause to the 20th century divergence.

     I am not aware of ANY evidence that demonstrates the occurrence of large-scale
     "divergence" in the past. It is therefore unjustified to call into question the use of
     tree rings for reconstructing temperatures over the past millennium based on a naive
     extrapolation of growth "divergence" into the past when it appears to be unique to the
     20th century. The NRC committee members must be made aware of this if their report is to
     have the necessary scientific credibility that is expected of it.
     Sincerely,
     Edward R. Cook
     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.
     ?
     ?

     Hi everyone,

     Here is a draft of what I want to quickly send to

     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

     Email: [1]ikraucunas@nas.edu

     Phone: (202) 334-2546

     Fax: (202) 334-3825

     He originally invited me to talk before the NRC. I do not have any other information on
     who to send it too. Please let me know what you think, but don't be too pedantic or
     critical at this stage. I get the feeling we have very little time to make an impact on
     the NRC committee and its report. I personally think that I am correct as far as I can
     take the argument. Let me know if I should send this on to Richard as well.

     Ed

     Dear Ian,

     I have heard via emails and telephone conversations about some rather serious
     developments that could have an unfairly negative impact on the use of tree rings for
     reconstructing past climate and the upcoming IPCC assessment, especially that related to
     surface temperatures. Apparently as part of her talk Rosanne D'Arrigo mentioned the
     phenomenon of "divergence" between instrumental temperatures and tree growth in the
     latter few decades of the 20th century. The large-scale nature of this phenomenon was
     first described in Nature by Keith Briffa back in 1998 (Briffa et al., 1998) and to this
     day its cause is not well understood at all. A number of hypotheses have been mentioned,
     which range from natural (climatic change) to anthropogenic (i.e. pollution related),
     but the actual cause is still unknown.

     Somewhat alarmingly, it is my impression now the the NRC committee members and other
     influential participants of the meeting 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 being
     made is that the "divergence" is being caused by climatic change related to 20th century
     warming, conditions that could have also prevailed back during the Medieval Warm Period
     (MWP) some 800-1000 years in the past. If this were the case, then the concerns of the
     committee 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 latititude 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
     show the overall robustness of the multi-centennial temperature signal in the tree-ring
     data. This plot from the QSR paper is attached below as is the paper itself.

     In his 1998 paper, Briffa showed that the divergence 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 reveals a serious downturn in growth after
     about 1950. This is an expression of the "divergence" that has been described first by
     Briffa and also by D'Arrigo in her NRC talk. In contrast, the south ensemble mean shows
     the opposite, i.e. a substantial growth increase which is much more consistent with 20th
     century warming. If one than follows the plots back in time, all of the 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 separation between north and south that is remotely comparable to
     that found after ca. 1950. This result suggests 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. This result clearly refutes
     the argument that "divergence" of the kind noted in the 20th century happened in the
     past. It also suggests a unique anthropogenic cause to the 20th century divergence.

     I am not aware of ANY evidence that demonstrates the occurrence of large-scale
     "divergence" in the past. It is therefore unjustified to call into question the use of
     tree rings for reconstructing temperatures over the past millennium based on a naive
     extrapolation of growth "divergence" into the past when it appears to be unique to the
     20th century. The NRC committee members must be made aware of this if their report is to
     have the necessary scientific credibility that is expected of it.

     Sincerely,

     Edward R. Cook

     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.

     Attachment converted: Macintosh HD:2004_Cook_QSR 1.pdf (PDF /«IC») (0011FEF2)

     Attachment converted: Macintosh HD:Cook_QSR_Fig6.gif (GIFf/«IC») (0011FEF5)

--

   Jonathan T. Overpeck
   Director, Institute for the Study of Planet Earth
   Professor, Department of Geosciences
   Professor, Department of Atmospheric Sciences
   Mail and Fedex Address:
   Institute for the Study of Planet Earth
   715 N. Park Ave. 2nd Floor
   University of Arizona
   Tucson, AZ 85721
   direct tel: +1 520 622-9065
   fax: +1 520 792-8795
   http://www.geo.arizona.edu/
   http://www.ispe.arizona.edu/

References

   1. mailto:ikraucunas@nas.edu

