date: Thu, 6 Jan 2005 16:39:48 -0700
from: Jonathan Overpeck <jto@u.arizona.edu>
subject: Fwd: [Wg1-ar4-ch06] Section 6.3.2.1
to: Keith Briffa <k.briffa@uea.ac.uk>, Eystein Jansen <eystein.jansen@geo.uib.no>, trond.dokken@bjerknes.uib.no, "Ricardo Villalba" <ricardo@lab.cricyt.edu.ar>

   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

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     From: David Rind <drind@giss.nasa.gov>
     Subject: [Wg1-ar4-ch06] Section 6.3.2.1
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     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 0.6K 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).

     --

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--

   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
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   http://www.ispe.arizona.edu/
