Saturday, December 10, 2011


From: Tim Osborn <>
To: "Michael E. Mann" <>, Ed Cook <>
Subject: Re: Your letter to Science
Date: Mon Apr 15 13:57:54 2002
Cc: Malcolm Hughes <>,,,,,,,

Dear all,
well, the time zone may let you have the last word before the weekend, but we can get the
first word in on a Monday morning!
At 22:35 12/04/02, Michael E. Mann wrote:

In keeping w/ the spirit of Tom's and Keith's emails, I wanted to stress, before we all
break for the weekend, that this is ultimately about the science, its not personal. If
my comments seemed to assail e.g. Keith's motives or integrity, etc. I believe that they
were misunderstood (as I tried to clarify that in my previous message), but I can see
that there was a potential for misunderstanding of my message (precision in wording is
very important) given the high levels of sensitivity in this debate. So I wanted to
leave no uncertainty about that. And of course, I very much apologize to Keith (and Tim)
if they took them my comments that way. They, again, were most decidedly not intended
that way.

Thanks for clarifying that, Mike. I think that both Keith and I interpreted your earlier
e-mail as being more critical of us than you actually meant it to be.
Most issues surrounding the recent Esper et al. and Briffa & Osborn pieces seem to have
been covered adequately already. There are just a couple of issues on which I'd like to
add a few comments, hopefully clarifying the situation rather than opening up more avenues
for debate.
The first relates to the purpose and style of the Briffa & Osborn piece. Perspectives are
brief, non-technical and not peer-reviewed. Our instructions were: "The Perspective should
provide an overview of recent research in the field and explain to the general reader why
the work is particularly exciting." Is it any surprise then that we should focus on the
new insights provided by the Esper et al. work, and that it suggests a different climate
history than earlier work? And that the constraints of the perspectives format (in terms
of length, audience and style) prevented us from listing ALL the caveats and uncertainties
related to this and earlier reconstructions and that might be of relevance to their
intercomparison? I don't think it is surprising, nor do I think we should be criticised
for it.
Moreover, despite the constraints of the perspectives format, I think we were very careful
with our wording to avoid misleading the reader. The reference to the IPCC, for example,
was not at all sloppy - the opposite, in fact, since it was very carefully worded: the IPCC
Synthesis Report is referred to, rather than the full TAR, and it is quite true that there
is a focus on the reconstruction of Mann et al. in the former. As Mike says, IPCC
conclusions were based on other work too. But I'd guess that many of the readers of our
perspective won't have read the full IPCC report, so we thought it valid to focus on the
difference between the new work and that shown in the Synthesis Report (which more will
have seen). To do this is certainly not unfair to the IPCC. It would only have been
unfair if we had implied that the IPCC had ignored this new work - but of course we weren't
doing that, because how could one expect the TAR to consider work that is published a year
after the TAR itself? We were similarly careful with our wording in our brief mention of
the MWP, by saying it is "more pronounced" in Esper et al. - this doesn't mean it is warmer
than the others (and thus has no implications for the IPCC conclusion of recent unusual
warmth), rather it is pronounced because it is followed by stronger cooling.
The second issue is our re-calibration of the reconstructions. While it hasn't been
explicitly stated, I get the impression that this is considered by some to be a poor thing
to do. The particular re-calibration we do has a number of effects, including making the
Mann et al. reconstruction appear more consistently at the top of the range of
alternatives. But please let me assure you (Mike, Ray and Malcolm) that the reason for
re-calibrating the records is definitely *not* to make your record appear as an outlier,
and I hope you believe me. Indeed, in Jones, Osborn & Briffa (2001: Science 292, 662-667)
we showed various NH records *without* applying our re-calibration.
We produced our first comparison of records for an earlier Science perspectives piece in
1999 (Briffa & Osborn, 1999) and thought it would be useful to do a re-calibration to
remove some of the reasons for inter-reconstruction differences (which can be due to:
different proxy data, different statistical methods, different calibration target and
different calibration period). The latter two reasons were removed by re-calibrating
against a common target series and over a common period. We updated this in Briffa et al.
(2001) and acknowledged that the target series (in terms of its spatial and seasonal
definition) may not be optimal in all cases. Indeed, it may be especially sub-optimal for
Mann et al., because their reconstruction approach combines the proxy records to optimally
reconstruct full NH, annual mean T (whereas we have selected land north of 20N, warm-season
T as our target for the recalibration). Despite this, we felt justified in doing the
recalibration because the Mann et al. series still outperformed the others in terms of its
correlation with the instrumental record over the calibration period! In our latest piece,
we have updated the intercomparison in two ways (as well as including new series): (i) we
took the spatially-resolved gridded reconstructions of Mann et al. and extracted only land
boxes north of 20N; and (ii) we used annual, not warm-season, temperature as the target.
The first of these (as explained by Keith and I in an earlier e-mail, which is repeated
below because it didn't get sent to all of you firs time round) deals with all the points
raised by Mike about tropical versus extratropical differences. I would again argue that
we were not sloppy, because these changes to our intercomparison were carefully thought
So that explains what we have done and why. There is some sensitivity, clearly, to
calibration choices, which implies to me that the true uncertainty ranges are probably
larger than those estimated solely from the statistical properties of calibration residuals
(as used by Briffa et al., and [I think] by Mann et al.). There is clearly more progress
to be made!
Best regards to you all

Date: Fri, 05 Apr 2002 17:17:55 +0100
To: "Michael E. Mann" <>,,,,,,,
From: Keith Briffa <>
Subject: Re: Briffa & Osborn piece
Cc: Tim Osborn <>
Dear Mike, (and interested colleagues)
Given the list of people to whom you have chosen to circulate your message(s), we
thought we should make a short, somewhat formal, response here. I am happy to reserve
my informal response until we are face to face! We did not respond earlier because we
had more pressing tasks to deal with. This is not the place to go into a long or
over-detailed response to all of your comments but a few brief remarks might help to
clear up a couple of misconceptions.
You consider our commentary on Ed and Jan's paper
"more flawed than even the paper itself"
on the basis that scaling the relationship between full Northern Hemisphere and
extratropical Northern Hemisphere is *much* more problematic than even any of the
seasonal issues we discuss. In fact we did not do this. The curve labelled Mann99 in
our figure was, in fact, based on the average of only the land areas, north of 20
degrees N, extracted from your spatially-resolved reconstructions. We then scaled it by
calibration against the instrumental annual temperatures from the same region. This is,
just as you stress in your comments on the Esper et al. paper, what should have been
done. We think that this single point addresses virtually of all your concerns. We
can, of course, argue about what this means for the pre-1400 part of your
reconstruction, when only 1 EOF was reconstructed, but the essential message is that we
did our best to exclude the tropics (and the oceans too!) from your series so that it
could more readily be compared with the other records.
The fact that we have used only the extra-tropical land from your data is not clear from
the text, so we can see why you may not have appreciated this, but we think you will
concede that this fact negates much of what you say and that we acted "more correctly"
than you realised. Blame *Science* for being so mean with their space allocation if you
want! Remember that this was an unrefereed piece and we felt justified in concentrating
on one issue; that of the importance of the method of scaling and its effect on apparent
"absolute" reconstruction levels. In our draft, we went on to say that this was crucial
for issues of simple model sensitivity studies and climate detection, citing the work of
Tom Crowley and Myles Allen, but this fell foul of the editor's knife.
You also express concerns about the calibration of Esper et al. (e.g., you say "if the
authors had instead used the actual (unsmoothed) instrumental record for the
extratropical northern hemisphere to scale their record, their reconstruction would be
much closer to MBH99").
This point is wholly consistent with our discussion in the perspective piece, and indeed
we show that in absolute terms the records are closer when Esper et al. is calibrated
using unsmoothed data but since the variance is also reduced, the significance of the
differences may be just as high.
Finally, we have to say that we do not feel constrained in what we say to the media or
write in the scientific or popular press, by what the sceptics will say or do with our
results. We can only strive to do our best and address the issues honestly. Some
"sceptics" have their own dishonest agenda - we have no doubt of that. If you believe
that I, or Tim, have any other objective but to be open and honest about the
uncertainties in the climate change debate, then I am disappointed in you also.
Best regards
Keith (and Tim)


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