From	Michael E. Mann	Date	Fri, 12 Apr 2002 17:35:33 -0400
To	Edward R. Cook
CC	Malcolm K. Hughes, Malcolm K. Hughes, Jan Esper, Keith Briffa, Tim Osborn, Phil Jones, Tom Crowley, ray bradley, Jonathan T. Overpeck, Scott Rutherford
Subject	Re: Your letter to Science
Dear Ed, Tom, Keith, etc. In keeping w/ the spirit of Tom's and Keith's emails, I wanted to stress, before we all break for the weekend, that this is ultimately about the science, its not personal. If my comments seemed to assail e.g. Keith's motives or integrity, etc. I believe that they were misunderstood (as I tried to clarify that in my previous message), but I can see that there was a potential for misunderstanding of my message (precision in wording is very important) given the high levels of sensitivity in this debate. So I wanted to leave no uncertainty about that. And of course, I very much apologize to Keith (and Tim) if they took them my comments that way. They, again, were most decidedly not intended that way. I hope we can resolve the scientific issues objectively, and w/out injecting or any personal feelings into any of this. There are some substantial scientific differences here, lets let them play out the way they are supposed to, objectively, and in the peer reviewed literature. Enjoy the weekend all. cheers, Mike At 01:35 PM 4/12/02 -0400, Ed Cook wrote: Hi Mike, Tom, etc, Okay, I am quite happy to give this debate a rest, although I am sure that the issues brought up will still be grounds for scientific debate. I admit to getting a bit riled when I saw the ECS results on the MWP described as "perilous" because I regard that as being an unfair characterization of the work presented. Be that as it may, my reply to Science will be very carefully worded so as not to inflame the issues. Nuff said. Have a good weekend. I certainly intend to do so. Ed Ed and others, I thought I too should chime in here one last time... I'll leave it to you, Malcolm, Keith and others to debate out the issue of any additional uncertainties, biases, etc. that might arise from RCS in the presence of limited samples. That is beyond my range of expertise. But since this is a new and relatively untested approach, and it is on the basis of this approach that other estimates are being argued to be "underestimates", we would indeed have been remiss now to point this out in our letter. The wording "perilous" perhaps should be changed, by I very much stand by the overall sentiment expressed by Malcolm in our piece with regard to RCS. One very important additional point that Malcolm makes in his message is that conservative estimates of uncertainties, appropriate additional caveats, etc. were indeed all provided in MBH99, and I have always been careful to interpret our results in the context of these uncertainties and caveats. IPCC '2001 was careful to do so to, and based its conclusions within the context of the uncertainties (hence the choice of the conservative term "likely" in describing the apparently unprecedented nature of late 20th century warmth) and, moreover, on the collective results of many independent reconstructions. Briffa & Osborn would have you believe that IPCC '2001's conclusions in this regard rested on MBH99 alone. Frankly, Keith and Tim, I believe that is unfair to the IPCC, whether or not one cares about being fair to MBH or not. What is unfortunate here then is that Esper et al has been "spun" i to argue that MBH99 underestimates the quantity it purports to estimate, full Northern Hemisphere annual mean temperature. Given the readily acknowledged level of uncertainty in both estimates, combined with the "apples and oranges" nature of the comparison between the two (which I have sought to clarify in my letter to Science, and in my messages to you all, and the comparison plot I provided), I believe it is either sloppy or disingenuous reasoning to argue that this is the case. The fact that this sloppiness also readily serves the interests of the skeptics is quite unfortunate, but it is indeed beside the point! It would probably also be helpful for me to point out, without naming names, that many of our most prominent colleagues in the climate research community, as well government funding agency representatives, have personally contacted me over the past few weeks to express their dismay at the way they believe this study was spun. I won't get into the blame game, because there's more than enough of that to go around. But when the leaders of our scientific research community and our funding managers personally alert us that they believe the credibility of our field has been damaged, I think it is time for some serious reflection on this episode. that's my final 2 cents, Mike At 10:21 AM 4/12/02 -0400, Ed Cook wrote: Just a few comments here and then I'm done. Dear Ed and Mike and others, All of our attempts, so far, to estimate hemisphere-scale temperatures for the period around 1000 years ago are based on far fewer data than any of us would like. None of the datasets used so far has anything like the geographical distribution that experience with recent centuries indicates we need, and no-one has yet found a convincing way of validating the lower-frequency components of them against independent data. As Ed wrote, in the tree-ring records that form the backbone of most of the published estimates, the problem of poor replication near the beginnings of records is particularly acute, and ubiquitous. I would suggest that this problem probably cuts in closer to 1600 than 1400 in the several published series. Therefore, I accept that everything we are doing is preliminary, and should be treated with considerable caution. Therefore, I would guess that you would apply the word "perilous" to everyones' large-scale NH reconstructions covering the past 500-1000 years including those that you have been involved in. Why the sudden increase in caution now? It sounds very self-serving to me for you to call ECS "perilous" and not describe every other large-scale reconstruction in that way as well. I differ from Ed, and his co-authors, in believing that these problems have a special significance for the particular implementation of RCS they used, in the light of one of their conclusions that depends heavily on that implementation. As I understand what Ed, Keith and Hal Fritts have written at various times about RCS, and from my own limited experience with the method, it is extremely important to have strong replication, and I don't see 50-70 samples probably from 25-35 trees as a big sample. For reference, most chronologies used in dendroclimatology are based on 10-40 trees, that is 20-80 samples at 2 cores per tree for a single "site", usually a few hectares. Here are two passages from Briffa et al., 1992: page 114, column 1, last paragraph, "For a chronology composed of the same number of samples, one would therefore expect a larger statistical uncertainty using this approach than in a chronology produced using standardization curves fitted to the data from individual trees...............The RCS method therefore requires greater chronology depth (i.e. greater sample replication) to provide the same level of confidence in its representation of the hypothetical "true" chronology." ECS mention this issue. As I said in my previous email, we hid nothing in terms of the uncertainty concerning the pre-1200 interval. Are you suggesting that we should not have even shown those results? If so, that is ridiculous. page 114, column 1, third paragraph, there is a discussion of the problems arising from applying RCS when pith age is not known, "In the ring-width data, the final standardization curve probably slightly underestimates the width of young trees and could therefore impart a small positive bias to the standardized ring-width indices for young rings in a number of series. However, this effect will be insignificant when the biased indices are realigned according to calendar growth years and averaged with many other series." The problem here is that this latter condition is not met (in my view), and the "small positive bias" that may be retained could turn out to be important to the most controversial conclusion of ECS (the Medieval question). I can't speak for Jan here, but most of the data he used came from Schweingruber's lab. I believe that pains were taken to estimate the pith offset and that Jan used this information in his RCS analyses. Jan would be best to comment here. In any case, Jan has done a number of experiments in which he has artificially added large pith offset errors into the RCS analysis and the resulting bias is small. So, I do not believe that your "view" is correct. I also suspect that Keith and colleagues underestimated both the size and variability of the loss of years at the beginning of records, but the point stands even if this is not so. So far as I can see, ECS do not mention this issue, at least in the context of a possible positive bias. Are you claiming that the only possible bias is positive? I can show you examples of a probable negative bias using RCS. The discussion of RCS in the supplementary materials seems to assume good replication. It was a generic description of the method. The replication is clearly shown in the supplementary materials section as well as in the main paper. If you don't like the replication, that is your opinion. I would love to have more replication as well. Who wouldn't. But we did show the uncertainties, which you seem to ignore in your criticism. Ironically, the ECS estimates of warmth in the MWP are not that dissimilar to those seen in MBH, as ECS Fig. 3 shows. Are the MBH estimates of MWP warmth also similarly biased? ECS, as Ed rightly points out, clearly indicate, in both words and diagrams at several points in their paper and in the supplementary materials, that the number of sites and number of samples they used decreases sharply before 1200. Even so, ECS gives prominence (second sentence of the abstract, for example) to the reconstruction in that very period, and makes a comparison with the magnitude of 20th-century warming. All the methods, and their realizations so far, have significant problems. In our letter (Mike and I) we draw attention to a specific problem with this implementation of RCS that has a special bearing on the reconstruction of a period to which ECS have drawn attention. Hence the strong note of caution about the ECS conclusion on the comparison between the 10th/11th and late 20th centuries. I hope it's clear from this that I don't disagree with the general proposition that all existing reconstructions of hemipsphere-scale temperatures 1000 years ago (or even for all the first half of the second millennium AD) should be viewed as very preliminary. If anyone is interested I attach a short note on the replication in the year AD 1000 of records used in MBH99 to give an idea of what we are up against. There is obviously a lot more we can debate about here. I will simply stop here by saying that I stand by the results shown in ECS and will say so in my reply to your letter, pointing out that the use of the word "perilous" could be just as easily be applied to MBH. We all have a lot to do. I see four important tasks - 1) more investigation of the strengths and limitations of methods like RCS and age-banding - for example, how many samples would have been enough in this case, does the RC change through time? and so on; 2) use of tree- ring records where the loss of low-frequency information is least - those with long segments from open stands; 3) the search for tree-ring parameters without age/size related trend; 4) the development of completely independent proxies with intrinsically better low- frequency fidelity. Cheers, Malcolm The Briffa et al reference is to the 1992 paper, Climate Dynamics, 7:111-119 Hi Ed, OK--thanks for your response. I'll let Malcolm respond to the technical issues regarding RC. I'm not really qualified to do so myself anyway. Your other points are well taken... Cheers, Mike At 12:09 PM 4/11/02 -0400, Edward Cook wrote: Hi Mike, Thanks for the reply. I too do not want to see anything personal in our disagreements. It would be a shame if it got to that and it shouldn't. I don't think that the science we are talking about is sufficiently known yet to claim the "truth", which is why we are having some of our disagreements. I mainly wanted to clarify some issues relating to some criticisms of the ECS results that I thought were not totally fair. My biggest complaint is with Malcolm's contribution to your letter because it really isn't fair to use such words as "perilous". ECS did not hide anything and the uncertainties are clearly indicated in EGS > Figs. 2 and 3. So, you can make your own judgement. However, Malcolm's opinion does not invalidate the ECS record. If Malcolm's statement is correct, than ALL previous estimates of NH temperature over the past 1000 years are "perilous", especially before AD 1400 when the number of series available declines significantly in most records. Ed Ed, It will take some time to digest these comments, but my initial response is one of some disappointment. I will resist the temptation to make the letter to Science available to the others on this list, because of my fears of violating the embargo policy (I know examples of where doing so has led to Science retracting a piece form publication). So thanks for also resisting the temptation to do so... But I must point out that the piece by Malcolm and me is very similar in its content to the letter of clarification that you and I originally crafted to send to Science some weeks ago, before your co-author objected to your involvement! If there is no objection on your part, I'd be happy to send that to everyone, because it is not under consideration in Science (a quite unfortunate development, as far as I'm concerned). The only real change from that version is the discussion of the use of RCS. That is in large part Malcolm's contribution, but I stand behind what > Malcolm says. I think there are some real sins of omission with regard to the use of RCS too, and it would be an oversight on our part now to comment on these. Finally, with regard to the scaling issues, let me simply attach a plot which speaks more loudly than several pages possibly could The plot takes Epser et al (not smoothed, but the annual values) and scales it against the full Northern Hemisphere instrumental record 1856-1990 annual mean record, and compares against the entire 20th century instrumental record (1856-1999), as well as with MBH99 and its uncertainties. Suppose that Esper et al is indeed representative of the fullNorthern Hemisphere annual mean, as MBH99 purports to be. To the extent that differences emerge between the two in assuming such a scaling, I interpret them as differences which exist due to the fact that the extratropical Northern Hemisphere series and full Northern Hemisphere series likely did not co-vary in the past the same way they co-vary in the 20th century (when both are driven predominantly, in a relative sense, by anthropogenic forcing, rather than natural forcing and internal variability). What the plot shows is quite remarkable. Scaled in this way, there is remarkably little difference between Esper et al and MBH99 in the first place (the two reconstructions are largely within the error estimates of MBH99!)!, but moreover, where they do differ, this could be explainable in terms of patterns of enhanced mid-latitude continental response that were discussed, for example, in Shindell et al (2001) in Science last December. So I think this plot says a lot. Its say that there are some statistically significant differences, but certainly no grounds to use Esper et al to contradict MBH99 or IPCC '2001 as, sadly, I believe at least one of the published pieces tacitly appears to want to do. It is shame that such a plot, which I think is a far more meaningful comparison of the two records, was not shown in either Esper et al or the Briffa & Osborn commentary. I've always given the group of you adequate opportunity for commentary on anything we're about to publish in Nature or Science. I am saddened that many of my colleagues (and, I have always liked to think friends) didn't affort me the same opportunity before this all erupted in our face. It could have been easily avoided. But that's water under the bridge. > Finally, before any more back-and-forths on this, I want to make sure that everyone involved understands that none of this was in any way ever meant to be personal, at least not on my part (and if it ever has, at least on my part, seemed that way, than I offer my apologies--it was never intended that way). This is completely about the "science". To the extent that I (and/or others) feel that the science has been mis-represented in places, however, I personally will work very hard to make sure that a more balanced view is available to the community. Especially because the implications are so great in this case. This is what I sought to do w/ the NYT piece and my NPR interview, and that is what I've sought to do (and Malcolm to, as far as I'm concerned) with the letter to Science. Being a bit sloppy w/ wording, and omission, etc. is something we're all guilty of at times. But I do consider it somewhat unforgivable when it is obvious how that sloppiness can be exploited. And you all know exactly what I'm talking about! So, in short, I think are some fundamental issues over which we're in disagreement, and where those exist, I will not shy away from pointing them out. But I hope that is not mis-interpreted as in any way personal. I hope that suffices, > Mike p.s. It seemed like an omission to not cc in Peck and Scott Rutherford on this exchange, so I've done that. I hope nobody minds this addition... At 10:57 AM 4/11/02 -0400, Edward Cook wrote: Hi Mike and Malcolm, I have received the letter that you sent to Science and will respond to it here first in some detail and later in edited and condensed form in Science. Since much of what you comment and criticize on has been disseminated to a number of people in your (Mike's) somewhat inflammatory earlier emails, I am also sending this lengthy reply out to everyone on that same email list, save those at Science. I hadn't responded in detail before, but do so now because your criticisms will soon be in the public domain. However, I am not attaching your letter to Science to this email since that is not yet in the public domain. It is up to you to send out your submitted letter to everyone if you wish. I must say at the beginning that some parts of your letter to Science are as "flawed" as your claims about Esper et al. (hereafter ECS). The Briffa/Osborn perspectives piece points out an important scaling issue that indeed needs further examination. However, to claim as you do that they show that the ECS 40-year low-pass temperature reconstruction is "flawed" begs the question: "flawed" by how much? It is not at all clear that scaling the annually resolved RCS chronology to annually resolved instrumental temperatures first before smoothing is the correct way to do it. The ECS series was never created to examine annual, or even decadal, time-scale temperature variability. Rather, as was clearly indicated in the paper, it was created to show how one can preserve multi-centennial climate variability in certain long tree-ring records, as a refutation of Broecker's truly "flawed" essay. As ECS showed in their paper (Table 1), the high- frequency correlations with NH mean annual temperatures after 20-year high-pass filtering is only 0.15. That result was expected and it makes no meaningful difference if one uses only extra- tropical NH temperature data. So, while the amplitude of the temperature-scaled 40-year low- pass ECS series might be on the high end (but still plausible given the gridded borehole temperature record shown in Briffa/Osborn), scaling on the annually resolved data first would probably have the opposite effect of excessively > reducing the amplitude. I am willing to accept an intermediate value, but probably not low enough to satisfy you. Really, the more important result from ECS is the enhanced pattern of multi- centennial variability in the NH extra-tropics over the past 1100 years. We can argue about the amplitude later, but the enhanced multi-centennial variability can not be easily dismissed. I should also point out, again, that you saw Fig. 3 in ECS BEFORE it was even submitted to Science and never pointed out the putative scaling "flaw" to me at that time. With regards to the issue of the late 20th century warming, the fact that I did not include some reference to or plot of the up-to-date instrumental temperature data (cf. Briffa/Osborn) is what I regard as a "sin of omission". What I said was that the estimated temperatures during the MWP in ECS "approached" those in the 20th century portion of that record up to 1990. I don't consider the use of "approached" as an egregious overstatement. But I do agree with you that I should have been a bit more careful in my wording there. As you know, I have publicly stated that I never intended to imply that the MWP was as warm as the late 20th century (e.g., > my New York Times interview). However, it is a bit of overkill to state twice in the closing sentences of the first two paragraphs of your letter that the ECS results do not refute the unprecedented late 20th century warming. I would suggest that once is enough. ECS were also very clear about the extra-tropical nature of their data. So, what you say in your letter about the reduced amplitude in your series coming from the tropics, while perhaps worth pointing out again, is beating a dead horse. However, I must say that the "sin of omission" in the Briffa/Osborn piece concerning the series shown in their plot is a bit worrying. As they say in the data file of series used in their plot (and in Keith's April 5 email response to you), Briffa/Osborn only used your land temperature estimates north of 20 degrees and recalibrated the mean of those estimates to the same domain of land-only instrumental temperatures using the same calibration period for all of the other non- borehole series in the same way. I would have preferred it if they had used your data north of 30N to make the comparisons a bit more one-to- one. However, I still think that their results are interesting. In particular, they reproduce much of the reduced multi-centennial temperature variability seen in your complete NH reconstruction. So, if the amplitude of scaled ECS multi-centennial variability is far too high (as you would apparently suggest), it appears that it is also too low in your estimates for the NH extra-tropics north of 20N. I think that we have to stop being so aggressive in defending our series and try to understand the strengths and weaknesses of each in order to improve them. That is the way that science is supposed to work. I must admit to being really irritated over the criticism of the ECS tree-ring data standardized using the RCS method. First of all, ECS acknowledged up front the declining available data prior to 1200 and its possible effect on interpreting an MWP in the mean record. ECS also showed bootstrap confidence intervals for the mean of the RCS chronologies and showed where the chronologies drop out. Even allowing for the reduction in the number of represented sites before 1400 (ECS Fig. 2d), and the reduction in overall sample size (ECS Fig. 2b), there is still some evidence for significantly above average growth during two intervals that can be plausibly assigned to the MWP. Of course > we would like to have had all 14 series cover the past 1000-1200 years. This doesn't mean that we can't usefully examine the data in the more weakly replicated intervals. In any case, the replication in the MWP of the ECS chronology is at least as good as in other published tree-ring estimates of large-scale temperatures (e.g., NH extra-tropical) covering the past 1000+ years. It also includes more long tree-ring records from the NH temperate latitudes than ever before. So to state that "this is a perilous basis for an estimate of temperature on such a large geographic scale" is disingenuous, especially when it is unclear how many millennia-long series are contributing the majority of the temperature information in the Mann/Bradley/Hughes (MBH) reconstruction prior to AD 1400. Let's be balanced here. I basically agree with the closing paragraph of your letter. The ECS record was NEVER intended to refute MBH. It was intended, first and foremost, to refute Broecker's essay in Science that unfairly attacked tree rings. To this extent, ECS succeeded very well. The comparison of ECS with MBH was a logical thing to do given that it has been accepted by the > IPCC as the benchmark reconstruction of NH annual temperature variability and change over the past millennium. Several other papers have made similar comparisons between MBH and other even more geographically restricted estimates of past temperature. So, I don't apologize in the slightest for doing so in ECS. The correlations in Table 2 between ECS and MBH were primarily intended to demonstrate the probable large-scale, low-frequency temperature signal in ECS independent of explicitly calibrating the individual RCS chronologies before aggregating them. The results should actually have pleased you because, for the 20-200 year band, ECS and MBH have correlations of 0.60 to 0.68, depending on the period used. Given that ECS is based on a great deal of new data not used in MBH, this result validates to a reasonable degree the temperature signal in MBH in the 20-200 year band over the past 1000 years. Given the incendiary and sometimes quite rude emails that came out at the time when ECS and Briffa/Osborn were published, I could also go into the whole complaint about how the review process at Science was "flawed". I will only say that this is a very dangerous game to get into and complaints of this kind can easily cut both ways. I will submit an appropriately edited and condensed version of this reply to Science. Regards, Ed -- ================================= Dr. Edward R. Cook Doherty Senior Scholar Tree-Ring Laboratory Lamont-Doherty Earth Observatory Palisades, New York 10964 USA Phone: 1-845-365-8618 Fax: 1-845-365-8152 Email: drdendro@ldeo.columbia.edu ================================= _____________________________________________ __________________________ Professor Michael E. Mann Department of Environmental Sciences, Clark Hall University of Virginia Charlottesville, VA 22903 _____________________________________________ __________________________ e-mail: mann@virginia.edu Phone: (434) 924- 7770FAX: (434) 982-2137 [1]http://www.evsc.virginia.edu/faculty/people/mann.sht ml Attachment converted: Macintosh HD:esper- scaledcompare1980.jpg (JPEG/JVWR) (0008FDE3) -- ================================= Dr. Edward R. Cook Doherty Senior Scholar Tree-Ring Laboratory Lamont-Doherty Earth Observatory Palisades, New York 10964 USA Phone: 1-845-365-8618 Fax: 1-845-365-8152 Email: drdendro@ldeo.columbia.edu ================================= > ____________________________________________________ ___________________ Professor Michael E. Mann Department of Environmental Sciences, Clark Hall University of Virginia Charlottesville, VA 22903 ____________________________________________________ ___________________ e-mail: mann@virginia.edu Phone: (434) 924-7770FAX: (434) 982-2137 [2]http://www.evsc.virginia.edu/faculty/people/mann.shtml Malcolm Hughes Professor of Dendrochronology Laboratory of Tree-Ring Research University of Arizona Tucson, AZ 85721 520-621-6470 fax 520-621-8229 -- ================================== Dr. Edward R. Cook Doherty Senior Scholar 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 ================================== _______________________________________________________________________ Professor Michael E. Mann Department of Environmental Sciences, Clark Hall University of Virginia Charlottesville, VA 22903 _______________________________________________________________________ e-mail: mann@virginia.edu Phone: (434) 924-7770 FAX: (434) 982-2137 [3]http://www.evsc.virginia.edu/faculty/people/mann.shtml _______________________________________________________________________ Professor Michael E. Mann Department of Environmental Sciences, Clark Hall University of Virginia Charlottesville, VA 22903 _______________________________________________________________________ e-mail: mann@virginia.edu Phone: (434) 924-7770 FAX: (434) 982-2137 [4]http://www.evsc.virginia.edu/faculty/people/mann.[5]shtml References 1. http://www.evsc.virginia.edu/faculty/people/mann.sht 2. http://www.evsc.virginia.edu/faculty/people/mann.shtml 3. http://www.evsc.virginia.edu/faculty/people/mann.shtml%A0%A0%A0%A0%A0%A0%A0%A0%A0%A0%A0%A0 4. http://www.evsc.virginia.edu/faculty/people/mann.shtml 5. http://www.evsc.virginia.edu/faculty/people/mann.shtml