date: Sun, 19 Mar 2006 16:53:00 -0500 from: "Michael E. Mann" subject: Re: Trees to: Richard Alley Hi Richard, Thanks for your email, and for your earnest views. There was indeed considerable discussion of thes issues on friday, the day after your talk. Both Malcolm Hughes and I discussed these issues in some detail with the committee. Please feel free to take a look at the presentation I gave to the committee: [1]http://www.meteo.psu.edu/~mann/Mann/lectures/lectures.html There is no doubt that there are issues with the potential non-stationarity of tree responses to climate, and this introduces caveats. As I pointed out to the committee, these issues were actually stressed in our '99 article which produced the millennial temperature reconstruction, the title of which was (emphasis added) "[2]Northern Hemisphere Temperatures During the Past Millennium: Inferences, Uncertainties, and Limitations". The underlying assumption of our own work has always been that each of the proxies have their own potential problems, and "multiproxy" approaches are probably the most robust. I don't have a particular axe to grind about any particular proxy, and recognize that there are some pretty serious potential problems with all proxies, including ice core delta o18 (as you're aware, these are not clean paleotemperature proxies at all), and Sr/Ca or o18 from corals. There is a good discussion of the strengths and weaknesses in all of the proxies in Jones and Mann (2004): Jones, P.D., Mann, M.E., [3]Climate Over Past Millennia, Reviews of Geophysics, 42, RG2002, doi: 10.1029/2003RG000143, 2004. I won't try to defend Rosanne D'Arrigo's analysis, because frankly many in the tree-ring community feel it was not very good work.You should be aware that her selection criteria were not as rigorous as those used by other researchers, and the conclusions she comes to reflect only the data and standardization methods she used--they don't speak for many other, in my mind, more careful studies. If you want the views of the leading experts in this community, I would refer you to my colleagues Malcolm Hughes and Keith Briffa, who have been carefully researching these issues for decades. With your permission, I'd like to forward your email to them for a more informed response--would that be ok? >From the questions asked by the community, I really only sensed from one individual the sort of extreme tree-ring skepticism that you describe. And I frankly think the individual proved himself to be not especially informed. The committee appeared to be convinced by the responses I provided to that individual. In short (and please see my presentation for further information) I made the following points: 1) multiproxy reconstructions that don't use tree-ring information at all for the long-term variability (Moberg et al, 2005) agree w/ all other (roughly a dozen now) reconstructions that late 20th century warmth is anomalous in the context of the past 2000 years at the hemispheric scale. This is one key point (i.e, the take home conclusion doesn't depend on tree-rings at all!). Another point I made is that the criticism (by some) that tree-rings underestimate the low-frequency variability is seriously challenged by the fact that temperature reconstructions based on data such as northern hemisphere glacier mass balance inversions (i.e. Oerlemans et al, 2005) show less hemispheric LIA cooling than many of the purely tree-ring based reconstructions. Another point I made in response to this line of criticism is that many of the long-term tree-ring series used in these reconstructions (see e.g. the recent Science article by Osborn and Briffa) show late 20th century conditions that are unprecedented in at least a millennial context. That is to say, if there is some upper temperature threshold in the past beyond which trees do not record, they do not appear to have encountered that threshold prior to the late 20th century, because the most positive anomalies in more than a thousand years are encountered in the last 20th century for most regions. The Osborn and Briffa science paper (attached) shows that the conclusion of anomalous 20th century warmth is spatially robust in a pan-hemispheric data set, it does not just reflect one region (in fact, they show that their conclusion of anomalous late 20th century warmth is robust to the elimination of any three data series used). Of course, Lonnie Thompson comes to the same conclusion using composites of his tropical ice cores. i.e., that the late 20th century behavior is anomalous in a greater-than-millennial context. Of course, tropical ice core delta o18 is difficult to defend as a pure paleothermometer too, but in this case it is difficult to see where non-climatic impacts could enter into the anomalous late 20th century behavior. So, in short, while the issues you mention are real (and have been emphasized by those actually working in this area for decades, as well as by us in all of our key publications), the primary conclusions (i.e. that late 20th century warmth is robust in at least a millennial context) appears robust, and is common to reconstructions whether or not they use tree-rings to reconstruct the low-frequency variability. . There is yet another study (embargoed right now in Science) that comes precisely to this same conclusion yet again. I'll actually be quite surprised if the committee comes to a *different* conclusion from that. Nonetheless, I appreciate your comments and your concerns, and your message does highlight a few issues which would be useful for us to clarify for the committee in case there is still any misunderstanding of the key points I have raised. As I said, I'd like to be able to forward your message to Keith and Malcolm, if this is ok w/ you, so that they can provide a perhaps even better informed response to the criticisms you raise. So please let me know if that would be ok... thanks, mike Richard Alley wrote: Mike--Just a quick follow-up on what I was saying Wednesday. Reconstruction of temperatures for the last millennium is not my problem, I'm not doing anything with it, I have no ax to grind, and I'm not tapped into the deliberations of the NRC committee. But, I think it is highly likely that the committee will end up casting doubts on the use of tree rings as paleothermometers. (I actually don't expect the committee to get very excited about supposed issues with EOF/PCA or similar statistical red herrings; I think the committee will focus on the indicators rather than on the methods of aggregating the indicators.) (I also rather expect that the committee will ask some pointed questions of the ground-temperature paleothermometers, and especially of groundwater-motion signals.) The triggering issue was the "divergence" problem as raised by Rosanne D'Arrigo, that a spatially and temporally complex difference has arisen between many of the long tree-ring records and the instrumental record more recently than the calibration period in many cases. This has been in the literature for a while, as you know much better than I do, and was not highlighted by Rosanne in her talk, but some committee members jumped on it in questions, and she was not convincing that trees were thermometers when it was warm a millennium ago but are not thermometers when it is warm now. She mentioned the existence of hypotheses (ozone or other pollution damage, for example), and I believe she tried some of the arguments about spatial coherence/incoherence of divergence versus nondivergence and of recent warmth versus medieval warmth, but overall was not convincing to me. (I'm happy to go into details as to why the arguments were not convincing, insofar as I captured the arguments, but they were not convincing to me, and looking around the committee room, I don't think they were convincing to important members of the committee.) Under one of my other hats, I raised this issue in a very non-public way with one individual and a request to keep it quiet, but my questions were rather quickly circulated well beyond and elicited some fairly warm replies from some well-respected tree-ring people--I am undoubtedly persona non grata in some quarters right now. But, having read the arguments, I still find them non-convincing. I don't believe the tree-ring community knows with any confidence whether there is a pollution signal, whether there could have been CO2 fertilization during the calibration period and falling off since in the way CO2 fertilization seems to behave in natural systems, whether there is now moisture stress or a snow-cover signal, whether the most-temperature-sensitive trees simply become less temperature-sensitive when temperatures become sufficiently high, or whether something else is going on. Clearly, if some of these are correct, then the "divergence problem" has no bearing on thermometry of a millennium ago; if others are correct, then thermometry of a millennium ago is affected. Nor do I believe that the community has nearly enough data to dismiss the recent signal as just a northern problem, or as being so anomalous in spatial pattern as to demonstrate that it must be anthropogenic. (The Cook-Esper 2004 QSR paper shows the North and South reconstructions as being about 5 normal deviates apart now but much closer medieval, but the east and west are close now and were over 3 normal deviates apart in the medieval, so it seems a stretch to argue that the recent must be anthropogenically anomalous for non-climatic reasons.) My suspicion is that the committee will end up noting that the peak twentieth century warmth in most reconstructions exceeds that of the tenth/eleventh century interval, and thus that recent warmth being higher is the leading estimate; however, the confidence in that may not rise much above 50%. I will be surprised if the committee is much friendlier to the millennial reconstructions than that, although I obviously could be completely off base. I don't want to stir up trouble, I don't want to piss off the tree-ring people yet again, but I do think that the tree-ring workers (and by association, all of us who do climate change) have a serious problem, and have not answered it very well yet. If better answers are out there, I hope that they come out soon. --Richard -- Michael E. Mann Associate Professor Director, Earth System Science Center (ESSC) Department of Meteorology Phone: (814) 863-4075 503 Walker Building FAX: (814) 865-3663 The Pennsylvania State University email: [4]mann@psu.edu University Park, PA 16802-5013 [5]http://www.met.psu.edu/dept/faculty/mann.htm Attachment Converted: "c:\eudora\attach\OsbornBriffaScience06.pdf"