Thursday, December 15, 2011


From: Jonathan Overpeck <>
Subject: Fwd: Re: [Wg1-ar4-ch06] IPCC last 2000 years data
Date: Tue, 4 Jan 2005 21:52:47 -0700
Cc: Eystein Jansen <>,

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
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 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
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

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, 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 - 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
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
University of Arizona
Tucson, AZ 85721
direct tel: +1 520 622-9065
fax: +1 520 792-8795

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