date: Thu, 6 Jan 2005 16:39:48 -0700 from: Jonathan Overpeck subject: Fwd: [Wg1-ar4-ch06] Section 6.3.2.1 to: Keith Briffa , Eystein Jansen , trond.dokken@bjerknes.uib.no, "Ricardo Villalba" Keith and Ricardo - you have these covered, correcto (you've seen most/all before?)- use as appropriate and save any debates for after ZOD is my advice. But if you have time to debate before, feel free to do so. I'm sure David is up to the task. Hope things are going well. Will send input from Julie before you wake up, and also a note on sorting out glacier stuff. Thanks, more soon, peck X-Sieve: CMU Sieve 2.2 X-Sender: drind@4dmail.giss.nasa.gov Date: Thu, 6 Jan 2005 17:28:46 -0500 To: wg1-ar4-ch06@joss.ucar.edu From: David Rind Subject: [Wg1-ar4-ch06] Section 6.3.2.1 X-BeenThere: wg1-ar4-ch06@joss.ucar.edu X-Mailman-Version: 2.1.1 List-Id: List-Help: List-Post: List-Subscribe: , List-Archive: List-Unsubscribe: , Sender: wg1-ar4-ch06-bounces@joss.ucar.edu X-Virus-Scanned: by amavisd-new at email.arizona.edu X-Spam-Status: No, hits=-4.5 required=7.0 tests=BAYES_00, HTML_FONT_BIG, HTML_MESSAGE X-Spam-Level: Hi, these suggestions are primarily for Keith who is the lead, but it might be of general interest, because this is such an important (and big) part of Chapter 6. I tried to review it as if I were an outside reviewer; for what it's worth, here are some comments: Section 6.3.2.1 Figure 1 should be of the last 2000 years, with appropriate caveats, not just since 1860 (which will undoubtedly be in other chapters). pp. 8-18: The biggest problem with what appears here is in the handling of the greater variability found in some reconstructions, and the whole discussion of the 'hockey stick'. The tone is defensive, and worse, it both minimizes and avoids the problems. We should clearly say (e.g., page 12 middle paragraph) that there are substantial uncertainties that remain concerning the degree of variability - warming prior to 12K BP, and cooling during the LIA, due primarily to the use of paleo-indicators of uncertain applicability, and the lack of global (especially tropical) data. Attempting to avoid such statements will just cause more problems. In addition, some of the comments are probably wrong - the warm-season bias (p.12) should if anything produce less variability, since warm seasons (at least in GCMs) feature smaller climate changes than cold seasons. The discussion of uncertainties in tree ring reconstructions should be direct, not referred to other references - it's important for this document. How the long-term growth is factored in/out should be mentioned as a prime problem. The lack of tropical data - just a few corals prior to 1700 - has got to be discussed. The primary criticism of McIntyre and McKitrick, which has gotten a lot of play on the Internet, is that Mann et al. transformed each tree ring prior to calculating PCs by subtracting the 1902-1980 mean, rather than using the length of the full time series (e.g., 1400-1980), as is generally done. M&M claim that when they used that procedure with a red noise spectrum, it always resulted in a 'hockey stick'. Is this true? If so, it constitutes a devastating criticism of the approach; if not, it should be refuted. While IPCC cannot be expected to respond to every criticism a priori, this one has gotten such publicity it would be foolhardy to avoid it. In addition, there are other valid criticisms to the PC approach. Assuming that the PC structure stays the same was acknowledged in the Mann et al paper as somewhat risky, given the possibility of altered climate forcing (e.g., solar). Attempting to reconstruct tropical temperatures using high latitude PCs assumes that the PCs are influenced only by global scale processes. In a paper we now have in review in JGR, and in other papers already published, it is shown that high latitude climate changes can directly affect the local expression of the modes of variability (NAO in particular). So attempting to fill in data at other locations from PCs that could have local influences may not work well; at the least, it has large uncertainties associated with it. The section from p.18-20 - simulations of temperature change over the last millennium , including regional expressions - should not be in this section. It is covered in the modeling section (several different times), and will undoubtedly be in other chapters as well. And the first paragraph on p. 19 is not right - only by using different forcings have models been able to get similar responses (which does not constitute good agreement). The discussion in the first paragraph of p. 20 is not right - the dynamic response is almost entirely in winter, which would not have affected the 'warm season bias' paleoreconstructions used to prove it. It also conflicts with ocean data (Gerard Bond, personal communication). Anyway, it's part of the section that should be dropped. pp. 20-28: The glacial variations should be summarized in a coherent global picture. Variations as a function of time should be noted - not just lumped together between 1400 and 1850 - for example, it should be noted where glaciers advanced during the 17th century and retreated during the 19th century, for that is important in understanding possible causes for the Little Ice Age (as well as the validity of the 'hockey stick'). The discussion on the bottom of p.25-27 as to the causes of the variations is inappropriate and should be dropped - note if solar forcing is suspect, every paragraph that relates observed changes to solar forcing will be equally suspect (e.g., see also p. 44, first paragraph). Bottom of p. 27: Greene et al. (GRL, 26, 1909-1912, 1999) did an analysis of 52 glaciated areas from 30-60N and found that the highest correlation between their ELA variations in the last 40 years was with summer season freezing height and winter season precip. The warm season freezing height was by far more important. Therefore, the relationship of glacier variations to NAO changes (which are important only in winter), as discussed in this paragraph, while perhaps valid for a period of time in southern Norway, is not generally applicable. p. 36: 6 ppm corresponds to a temperature response of 0.3 to 0.6°K using the IPCC sensitivity range. p. 36, last paragraph: one could equally well conclude that the reconstructions are showing temperature changes that are too small. This is the essence of the problem with the last 2000 years: if the reconstructions are right, either there was no solar forcing, or climate sensitivity is very low. If the real world had more variability, either there was solar forcing, or climate sensitivity is high (as is internal variability). I've tried to say this in the climate sensitivity sub-chapter (5.8). -- /////////////////////////////////////////////////////////////////////////// /////////////////////////////////////////////////////////////////////////// _______________________________________________ Wg1-ar4-ch06 mailing list Wg1-ar4-ch06@joss.ucar.edu http://www.joss.ucar.edu/mailman/listinfo/wg1-ar4-ch06 -- Jonathan T. Overpeck Director, Institute for the Study of Planet Earth Professor, Department of Geosciences Professor, Department of Atmospheric Sciences Mail and Fedex Address: Institute for the Study of Planet Earth 715 N. Park Ave. 2nd Floor University of Arizona Tucson, AZ 85721 direct tel: +1 520 622-9065 fax: +1 520 792-8795 http://www.geo.arizona.edu/ http://www.ispe.arizona.edu/