cc: "Michael E. Mann" , esper@ldeo.columbia.edu, Jonathan Overpeck , Keith Briffa date: Wed, 2 May 2001 10:59:37 -0400 from: Edward Cook subject: Re: hockey stick to: tom crowley >Ed, > >heard some rumor that you are involved in a non-hockey stick reconstruction >of northern hemisphere temperatures. I am very intrigued to learn about >this - are these results suggesting the so called Medieval Warm Period may >be warmer than the early/mid 20th century? > >any enlightenment on this would be most appreciated, Tom > > > >Thomas J. Crowley >Dept. of Oceanography >Texas A&M University >College Station, TX 77843-3146 >979-845-0795 >979-847-8879 (fax) >979-845-6331 (alternate fax) Hi Tom, As rumors often are, the one you heard is not entirely accurate. So, I will take some time here to explain for you, Mike, and others exactly what was done and what the motivation was, in an effort to hopefully avoid any misunderstanding. I especially want to avoid any suggestion that this work was being done to specifically counter or refute the "hockey stick". However, it does suggest (as do other results from your EBM, Peck's work, the borehole data, and Briffa and Jones large-scale proxy estimates) that there are unresolved (I think) inconsistencies in the low-frequency aspects of the hockey stick series compared to other results. So, any comparisons with the hockey stick were made with that spirit in mind. What Jan Esper and I are working on (mostly Jan with me as second author) is a paper that was in response to Broecker's Science Perspectives piece on the Medieval Warm Period. Specifically, we took strong exception to his claim that tree rings are incapable of preserving century time scale temperature variability. Of course, if Broecker had read the literature, he would have known that what he claimed was inaccurate. Be that as it may, Jan had been working on a project, as part of his post-doc here, to look at large-scale, low-frequency patterns of tree growth and climate in long tree-ring records provided to him by Fritz Schweingruber. With the addition of a couple of sites from foxtail pine in California, Jan amassed a collection of 14 tree-ring sites scattered somewhat uniformly over the 30-70 degree NH latitude band, with most extending back 1000-1200 years. All of the sites are from temperature-sensitive locations (i.e. high elevation or high northern latitude. It is, as far as I know, the largest, longest, and most spatially representative set of such temperature-sensitive tree-ring data yet put together for the NH extra-tropics. In order to preserve maximum low-frequency variance, Jan used the Regional Curve Standardization (RCS) method, used previously by Briffa and myself with great success. Only here, Jan chose to do things in a somewhat radical fashion. Since the replication at each site was generally insufficient to produce a robust RCS chronology back to, say, AD 1000, Jan pooled all of the original measurement series into 2 classes of growth trends: non-linear (~700 ring-width series) and linear (~500 ring-width series). He than performed independent RCS on the each of the pooled sets and produced 2 RCS chronologies with remarkably similar multi-decadal and centennial low-frequency characteristics. These chronologies are not good at preserving high-frquency climate information because of the scattering of sites and the mix of different species, but the low-frequency patterns are probably reflecting the same long-term changes in temperature. Jan than averaged the 2 RCS chronologies together to produce a single chronology extending back to AD 800. It has a very well defined Medieval Warm Period - Little Ice Age - 20th Century Warming pattern, punctuated by strong decadal fluctuations of inferred cold that correspond well with known histories of neo-glacial advance in some parts of the NH. The punctuations also appear, in some cases, to be related to known major volcanic eruptions. Jan originally only wanted to show this NH extra-tropical RCS chronology in a form scaled to millimeters of growth to show how forest productivity and carbon sequestration may be modified by climate variability and change over relatively long time scales. However, I encouraged him to compare his series with NH instrumental temperature data and the proxy estimates produced by Jones, Briffa, and Mann in order bolster the claim that his unorthodox method of pooling the tree-ring data was producing a record that was indeed related to temperatures in some sense. This he did by linearly rescaling his RCS chronology from mm of growth to temperature anomalies. In so doing, Jan demonstrated that his series, on inter-decadal time scales only, was well correlated to the annual NH instrumental record. This result agreed extremely well with those of Jones and Briffa. Of course, some of the same data were used by them, but probably not more than 40 percent (Briffa in particular), so the comparison is based on mostly, but not fully, independent data. The similarity indicated that Jan's approach was valid for producing a useful reconstruction of multi-decadal temperature variability (probably weighted towards the warm-season months, but it is impossible to know by how much) over a larger region of the NH extra-tropics than that produced before by Jones and Briffa. It also revealed somewhat more intense cooling in the Little Ice Age that is more consistent with what the borehole temperatures indicate back to AD 1600. This result also bolsters the argument for a reasonably large-scale Medieval Warm Period that may not be as warm as the late 20th century, but is of much(?) greater significance than that produced previously. Of course, Jan also had to compare his record with the hockey stick since that is the most prominent and oft-cited record of NH temperatures covering the past 1000 years. The results were consistent with the differences shown by others, mainly in the century-scale of variability. Again, the Esper series shows a very strong, even canonical, Medieval Warm Period - Little Ice Age - 20th Century Warming pattern, which is largely missing from the hockey stick. Yet the two series agree reasonably well on inter-decadal timescales, even though they may not be 1:1 expressions of the same temperature window (i.e. annual vs. warm-season weighted). However, the tree-ring series used in the hockey stick are warm-season weighted as well, so the difference between "annual" and "warm-season weighted" is probably not as large as it might seem, especially before the period of instrumental data (e.g. pre-1700) in the hockey stick. So, they both share a significant degree of common interdecal temperature information (and some, but not much, data), but do not co-vary well on century timescales. Again, this has all been shown before by others using different temperature reconstructions, but Jan's result is probably the most comprehensive expression (I believe) of extra-tropical NH temperatures back to AD 800 on multi-decadal and century time scales. Now back to the Broecker perspectives piece. I felt compelled to refute Broecker's erroneous claim that tree rings could not preserve long-term temperature information. So, I organized a "Special Wally Seminar" in which I introduced the topic to him and the packed audience using Samuel Johnson's famous "I refute it thus" statement in the form of "Jan Esper and I refute Broecker thus". Jan than presented, in a very detailed and well espressed fashion, his story and Broecker became an instant convert. In other words, Wally now believes that long tree-ring records, when properly selected and processed, can preserve low-frequency temperature variability on centennial time scales. Others in the audience came away with the same understanding, one that we dendrochronologists always knew to be the case. This was the entire purpose of Jan's work and the presentation of it to Wally and others. Wally had expressed some doubts about the hockey stick previously to me and did so again in his perspectives article. So, Jan's presentation strongly re-enforced Wally's opinion about the hockey stick, which he has expressed to others including several who attended a subsequent NOAA meeting at Lamont. I have no control over what Wally says and only hope that we can work together to reconcile, in a professional, friendly manner, the differences between the hockey stick and other proxy temperature records covering the past 1000 years. This I would like to do. I do think that the Medieval Warm Period was a far more significant event than has been recognized previously, as much because the high-resolution data to evaluate it had not been available before. That is much less so the case now. It is even showing up strongly now in long SH tree-ring series. However, there is still the question of how strong this event was in the tropics. I maintain that we do not have the proxies to tell us that now. The tropical ice core data are very difficult to interpret as temperature proxies (far worse than tree rings for sure and maybe even unrelated to temperatures in any simple linear sense as is often assumed), so I do not believe that they can be used alone as records to test for the existence of a Medieval Warm Period in the tropics. That being the case, there are really no other high-resolution records from the tropics to use, and the teleconnections between long extra-tropical proxies and the tropics are, I believe, far too tenuous and probably unstable to use to sort out this issue. So, at this stage I would argue that the Medieval Warm Period was probably a global extra-tropical event, at the very least, with warmth that was persistent and probably comparable to much of what we have experienced in the 20th century. However, I would not claim (and nor would Jan) that it exceeded the warmth of the late 20th century. We simply do not have the precision or the proxy replication to say that yet. This being said, I do find the dismissal of the Medieval Warm Period as a meaningful global event to be grossly premature and probably wrong. Kind of like Mark Twain's commment that accounts of his death were greatly exaggerated. If, as some people believe, a degree of symmetry in climate exists between the hemispheres, which would appear to arise from the tropics, then the existence of a Medieval Warm Period in the extra-tropics of the NH and SH argues for its existence in the tropics as well. Only time and an enlarged suite of proxies that extend into the tropics will tell if this is true. I hope that what I have written clarifies the rumor and expresses my views more completely and accurately. Cheers, 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 ==================================