From	Jonathan T. Overpeck	Date	Tue, 14 Mar 2006 21:29:28 -0700
To	Edward R. Cook
CC	Eystein Jansen, Keith Briffa
Subject	Re: NRC Committee on Surface Temperature Reconstructions
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