From	Michael E. Mann	Date	Thu, 11 Apr 2002 11:36:44 -0400
To	Edward R. Cook, Malcolm K. Hughes
CC	Jan Esper, Keith Briffa, Tim Osborn, Phil Jones, Tom Crowley, ray bradley, Jonathan T. Overpeck, Scott Rutherford, Michael E. Mann
Subject	Re: Your letter to Science
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 full Northern 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) 924-7770 FAX: (434) 982-2137 [1]http://www.evsc.virginia.edu/faculty/people/mann.[2]shtml Attachment Converted: "c:\eudora\attach\esper-scaledcompare1980.jpg" References 1. http://www.evsc.virginia.edu/faculty/people/mann.shtml 2. http://www.evsc.virginia.edu/faculty/people/mann.shtml