date: Thu, 13 Jan 2005 17:06:58 -0500 from: Henry Pollack subject: Re: near final 6.3.2.1 to: Keith Briffa Hi Keith, Thanks for the latest version, which in the borehole discussion differs notably from the earlier version you sent, presumably due to comments from others. Because I am traveling again (leave Friday, back on Tuesday), I will send only a few general comments today, in no particular order. 1. Figure 1b, as discussed on pp.1,2 . This figure shows borehole results, and 'pseudo' borehole results, but the borehole curves are never mentioned explicitly in the text. The pseudo borehole results are the so-called 'optimal' results of Mann. Mike has never analyzed borehole temperatures; he simply manipulated reconstructions that I sent him, in a manner that dealt with spatial noise in a totally inadequate way. I think it is inappropriate to show the so-called 'optimal' reconstruction because it is flawed (even after the correction of gridding errors); the gridded curve should be the one shown in Pollack & Smerdon 2004, not the Briffa and Osborn 2002. Tim has told me of how that was constructed, and the P&S 04 version should be substituted. 2. Your discussion of uncertainty on pages 4,5 is good. The paragraph following acknowledges that there is generally greater centennial-scale variability in the ensemble of reconstructions than is displayed in the hockey stick. I think the hockey stick is effectively an end-member reconstruction, rather than a 'centrist' whose uncertainty bounds encompass all other reconstructions. 3. The statistical efforts (yours, others) to retain more long period variability in the dendro series have led to greater variability in the reconstructions, all toward a cooler past in the 16th-17th centuries. There is no a priori reason that the improved dendro series should have led to a cooler past; retention of more long period variability might conceivably have led to a warmer past, but it did not. That makes me think that the 16th-17th centuries were indeed cooler than the hockey stick portrays. 4. page 7. The Von Storch, Zorita model results suggest that the regression reconstruction techniques do not extract signals well in the presence of noise. There is a new paper soon to be submitted by Tom Crowley, Gabi Hegerl et al that reaches a similar conclusion. I believe that this is an important issue, one that is at the heart of my criticism of how Mike Mann re-analyzed the borehole results. May I suggest that in the FOD (first order draft) that you include a discussion of field reconstruction methodologies, particularly as to how they fare in the presence of noise. This would compare and contrast the regression techniques, the von Storch approach, the Crowley approach, the revised Rutherford Mann (2005)methodology, and perhaps the wavelet approach that Anders Moberg has proposed. I do not think that the problem that you cite in the last paragraph on p.7 is strictly with the particular simulation of Von Storch. The same general conclusions are reached in the new Crowley, Hegerl paper, which use different inputs. 5. It is not clear what you are saying on p.8, 3rd paragraph when you say that the GST histories imply a very much higher level of 'recent' warmth when compared to the general picture of proxy reconstructions. If 'recent' means the period of the instrumental record, the GST reconstructions compare very well, and if anything are a little muted. Moreover, both the von Storch and the new Crowley results show a very good overlap in the 17th century of borehole reconstructions with proxy reconstructions that use non-regression reconstruction methods. 6.GCM simulations are new players in the reconstruction arena. The work of Von Storch, Simon Tett, Caspar Ammann are examples. I think a discussion of how these are being integrated into the historical debate would be useful. You make the point that there are few new proxy data since the TAR. But what is new are the methods of reconstructing the climate field , and the role of the GCMs in testing certain methodologies with pseudo-proxies. The field has come quite a way from what is in the TAR, but your summary seems to convey the feeling that little has changed. 7. I am not offering any comments on the rest of the material (hydrological, etc.), as I have not had time to digest it yet. Thanks for the opportunity to comment! The comments are offered in the most constructive of spirit. You have undertaken a task of which I have not the slightest of envy. I am confident that the final product will be fair and balanced. Please let me know the time frame of when this document will undergo further shaping and refinement. I am grateful to have had the opportunity to participate. Cheers, Henry ___ ___ Henry N. Pollack [ \ / ] Professor of Geophysics | \/ | Department of Geological Sciences |MICHIGAN| University of Michigan [___]\/[___] Ann Arbor, Michigan 48109-1063, U.S.A. Phone: 734-763-0084 FAX: 734-763-4690 e-mail: hpollack@umich.edu URL: http://www.geo.lsa.umich.edu/~hpollack URL: www-personal.umich.edu/~hpollack/book.html ------------------------------------------------------------------------ Quoting Keith Briffa : > > Henry > latest version and would appreciate comments > Keith > >> -- > Professor Keith Briffa, > Climatic Research Unit > University of East Anglia > Norwich, NR4 7TJ, U.K. > > Phone: +44-1603-593909 > Fax: +44-1603-507784 > > http://www.cru.uea.ac.uk/cru/people/briffa/