From	Edward R. Cook	Date	Wed, 15 Mar 2006 19:33:48 +0700
To	Jonathan T. Overpeck, Keith Briffa, Eystein Jansen
CC	Edward R. Cook
Subject	Fwd: Comment on NRC Workshop
Hi Gents, Here is what I just sent off to NRC, cc'd to Gerry North and Bette Otto-Bliesner. Ed Begin forwarded message: > From: edwardcook > Date: March 15, 2006 7:23:23 PM GMT+07:00 > To: "Kraucunas, Ian" > Cc: edwardcook , g-north@tamu.edu, > ottobli@ucar.edu > Subject: Comment on NRC Workshop > > 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 naïve 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: drdendro@ldeo.columbia.edu > Phone: 845-365-8618 > Fax: 845-365-8152 > ================================== >    Hi Gents, Here is what I just sent off to NRC, cc'd to Gerry North and Bette Otto-Bliesner. Ed Begin forwarded message: From: edwardcook <[1]drdendro@ldeo.columbia.edu> Date: March 15, 2006 7:23:23 PM GMT+07:00 To: "Kraucunas, Ian" <[2]IKraucunas@nas.edu> Cc: edwardcook <[3]drdendro@ldeo.columbia.edu>, [4]g-north@tamu.edu, [5]ottobli@ucar.edu Subject: Comment on NRC Workshop 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 naïve 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 ================================== Attachment Converted: "c:\eudora\attach\2004_Cook_QSR3.pdf" Attachment Converted: "c:\eudora\attach\Cook_NRC.pdf" Attachment Converted: "c:\eudora\attach\Cook_QSR_Fig61.gif" References 1. mailto:drdendro@ldeo.columbia.edu 2. mailto:IKraucunas@nas.edu 3. mailto:drdendro@ldeo.columbia.edu 4. mailto:g-north@tamu.edu 5. mailto:ottobli@ucar.edu 6. mailto:drdendro@ldeo.columbia.edu