cc: Malcolm Hughes <mhughesatXYZxyzr.arizona.edu>, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, mannatXYZxyzginia.edu
date: Mon, 15 Apr 2002 12:44:53 -0400
from: "Michael E. Mann" <mannatXYZxyztiproxy.evsc.virginia.edu>
subject: Re: Your letter to Science
to: Tim Osborn <t.osbornatXYZxyz.ac.uk>, Ed Cook <drdendroatXYZxyzo.columbia.edu>
Thanks for your message. Yes, you guys have us beat on the early monday end of things!
Your points are all taken. I think we all agree there is much work left to be done, more
than enough for all of us to continue to be involved in constructive collaboration, etc.
Scott and I, for example, are almost done writing up the work based on your visit w/ us
last year, and will send the initial draft on to you, Keith, and the others involved in the
near future. It will be a good chance to try to address a lot of these questions in an
article of adequate length to discuss the nuances that unfortunately cannot be addressed in
a shorter piece.
I also appreciate your more detailed comments about the comparisons, etc. Your points are
all reasonable ones. We can maintain an honest difference about how well those points were
conveyed in the Science piece (for example, you can imagine how the statement in your piece
"This record has a smaller amplitude of century-to-century variability, and is consistently
at or near the upper limit of alternate records produced by other researchers" might indeed
have been interpreted as setting MBH99 apart as, in your words, an "outlier").
We have good reason to believe that our reconstruction *will* in fact nderestimate
extratropical temperature means but far less so full globe/hemisphere-means prior to the
18th century because the basis functions that primarily set the extratropics apart from the
full hemispheric patterns (e.g., NAO type patterns and other anomaly patterns largely
carried by EOFs #2 and #3) start to drop out from our basis set prior to the 18th century,
while the pattern that best resolves the full global and/or hemispheric mean (with note
from MBH98, particularly large loadings primarily in the tropics and subtropics) still
remains. That is why we have never published an *extratropical* temperature reconstruction
prior to the 18th century. I would be happy to discuss this point with you and Keith and
others in more detail. Thus, I have compared Esper et al w/ our records in the manner
described in my previous email, which I think allows us to diagnose the extent to which
differing high-latitude and full-hemispheric patterns may, at times, explain the somewhat
modest differences between the records when similarly scaled to the full hemispheric
1856-1990 mean, and always, within the context of the diagnosed uncertainties. There is no
guarentee, as you say, that the uncertainties are correct, but I personally believe they'll
stand up over time. You can call me on this 10 years from now, and somebody will owe
somebody a beer...
In any case, I hope and fully expect we can all continue to all be engaged in constructive
interaction & hopefully continued collaboration. It will require some sensitivity on all
our part to the larger issues surrounding our work, and the way it gets presented to the
broader community, but I don't think that should be all that difficult.
I look forward to these more constructive interactions. I'll do my best to foster them,
At 01:57 PM 4/15/02 +0100, Tim Osborn wrote:
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
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 out.
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" <firstname.lastname@example.org>,email@example.com,
From: Keith Briffa <k.briffaatXYZxyz.ac.uk>
Subject: Re: Briffa & Osborn piece
Cc: Tim Osborn <t.osbornatXYZxyz.ac.uk>
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.
Keith (and Tim)
Dr Timothy J Osborn | phone: +44 1603 592089
Senior Research Associate | fax: +44 1603 507784
Climatic Research Unit | e-mail: t.osbornatXYZxyz.ac.uk
School of Environmental Sciences | web-site:
University of East Anglia __________| http://www.cru.uea.ac.uk/~timo/
Norwich NR4 7TJ | sunclock:
UK | http://www.cru.uea.ac.uk/~timo/sunclock.htm
Professor Michael E. Mann
Department of Environmental Sciences, Clark Hall
University of Virginia
Charlottesville, VA 22903
e-mail: mannatXYZxyzginia.edu Phone: (434) 924-7770 FAX: (434) 982-2137