From: Jonathan Overpeck <jto@u.arizona.edu>
To: k.briffa@uea.ac.uk
Subject: Fwd: Re: [Wg1-ar4-ch06] IPCC last 2000 years data
Date: Tue, 4 Jan 2005 21:52:47 -0700
Cc: Eystein Jansen <eystein.jansen@geo.uib.no>, cddhr@giss.nasa.gov

   Hi Keith - Happy new year. Hopefully, you had a good holiday. I've had a chance to read
   your section and hopefully you've had a chance to read what I sent just before the
   holidays. The purpose of this email is to help get a focus on the finish line (just a few
   days away) and to get a dialog going that will hopefully help you finish section 6.3.2.1.
   If you'd like to talk on the phone, just let me know.

   Please see my email from right before xmas holidays for original comments. Plus, here are
   the new ones from both me and David Rind:

   0) as leader of this KEY section, we need you to take the lead integrating everything you
   think should be integrated, editing and boiling it down to just ca 4 pages of final text
   (e.g., 8 pages of typed text plus figs). This means cutting some material (e.g., forcings
   and simulations) and perhaps moving glacier record (MUCH boiled down) to a box. See below.
   00) note that we can also perhaps move some of the details to the appendix (although we
   won't write this until after the current ZOD crunch, save an outline of what you might want
   in there).
   1) I like your figure ideas, with the comments:
   1a) I don't think you need figure 1d - the SH recons are sketchy since not much data, and
   it might be better to just discuss in a sentence or three. Any space saved is good too. Not
   sure about your proposed 1e - have to see it, I guess.
   1b) Figure 2 looks interesting. I'm trying to get the latest Arctic recon from Konrad
   Hughen - it is quite robust and a significant multi-proxy update. Should be published in
   time, though not sure thing since he's still hot on including his (our) AO recon which is
   more sketchy
   1c) I think we can save space and improve organization if we DO NOT include Fig 3. However,
   this is open for debate - see David's comments below.
   2) I agree with David's comments in general - so see them below. The prickly issue is where
   to put the forcings and simulated changes. I am close to having the prose from the
   radiation chapter, including the latest Lean and Co's view on solar - this will make many
   of the existing simulations involving inferred past solar forcing suspect (I will send in a
   day or so I hope). This means that we might be best saving space and downplaying this work
   some. I'm not sure, but wanted to debate it with you. Also, Chap 9 will have simulations in
   spades, so we can save space by letting them do it. Also, as David points out, we can focus
   on it elsewhere in our chapter more concisely - leaving you to focus on the VERY important
   obs record of temp and other changes. Can you tell, I'm still not 100% sure? I'll send
   another email to you and others about this in a bit.
   3) Your section is too long and needs to be condensed. Thus, you need to think through
   what's most important and what's less so. For example, we need to figure out how to
   condense the glacier record of change. David thinks it should be a separate section that
   cuts across time scales (i.e., Holocene and last 2000 years). Perhaps we should try to make
   it into a box - 3 to 5 short paragraphs and a figure or two. Either way we have to really
   wack it. What do you think - you and I should be on the same page with Eystein before
   discussing w/ Olga perhaps. Or you can discuss with her - you're the lead on this section.
   4) you're doing an impressive job! Lots to keep track of.
   Next, here is what David has offered. Take it all with a grain of salt, but I have read it
   and he has many good points. On the structural or any other points, I'm happy to discuss on
   the phone, or you can just debate with him and me on email.
   ******* From David Rind 1/4/05 ****************
   6.3 Understanding Past Climate System Change (forcing and response)
   6.3.1 Introduction (0.5 pages)
   6.3.2 The Current Interglacial
   6.3.2.1 Last 2000 years (4 pages)
   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 - 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 coherentglobal  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. 34-36 on forcings: note that this is redundant to what is discussed in several later
   sections (e.g., 6.5.2); and other chapters), and that is true of forcing in general for the
   whole of section 6.2. I would strongly suggest dropping forcing from section 6.3.2.1, at
   least, and perhaps giving it its own number, or referring to othersubsections for it. It
   has a different flavor from the responses, and the section is already very big. Forcing
   does need to be discussed in the paleoclimate chapter, for reasons of climate sensitivity
   and explaining observations, but that is what Chapter 6.5 is about.
   (In summary - 6.3.2.1 already is taking on one controversy - paleotemperatures, which is
   needs to do better,  It should not have to deal with the forcing problems as well, and
   especially not in an off-handed way.)
   Specific comments: 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.
   pp. 37-41: obviously a lot of overlap, but it shouldn't be hard to combine these.
   p. 39, first paragraph: but can the models fully explain what is thought to have happened?
   Quantification is important here, because many of the same climate/veg models are being
   used to assess future changes in vegetation.
   p. 42 - first full paragraph: what are the implications of the methane drop without a CO2
   drop?
   p. 43, middle paragraph: obviously should mention solar-orbital forcing in this paragraph.
   p. 44, first paragraph: again, assuming a solar forcing
   p. 45, first paragraph: overlap with pp. 20-28.
       Second paragraph: overlap with p.39, last full paragraph
   p. 52 - repeat of p. 43.
   ******* END From David Rind 1/4/05 ****************

   Thanks! Cheers, peck

--

   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
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