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date: Thu, 11 Apr 2002 11:36:44 -0400
from: "Michael E. Mann" <mannatXYZxyztiproxy.evsc.virginia.edu>
subject: Re: Your letter to Science
to: Edward Cook <drdendroatXYZxyzo.columbia.edu>, Malcolm Hughes <mhughesatXYZxyzr.arizona.edu>
It will take some time to digest these comments, but my initial response is one of some
disappointment. I will resist the temptation to make the letter to Science available to the
others on this list, because of my fears of violating the embargo policy (I know examples
of where doing so has led to Science retracting a piece form publication). So thanks for
also resisting the temptation to do so...
But I must point out that the piece by Malcolm and me is very similar in its content to
the letter of clarification that you and I originally crafted to send to Science some weeks
ago, before your co-author objected to your involvement! If there is no objection on your
part, I'd be happy to send that to everyone, because it is not under consideration in
Science (a quite unfortunate development, as far as I'm concerned). The only real change
from that version is the discussion of the use of RCS. That is in large part Malcolm's
contribution, but I stand behind what Malcolm says. I think there are some real sins of
omission with regard to the use of RCS too, and it would be an oversight on our part now to
comment on these.
Finally, with regard to the scaling issues, let me simply attach a plot which speaks more
loudly than several pages possibly could The plot takes Epser et al (not smoothed, but the
annual values) and scales it against the full Northern Hemisphere instrumental record
1856-1990 annual mean record, and compares against the entire 20th century instrumental
record (1856-1999), as well as with MBH99 and its uncertainties.
Suppose that Esper et al is indeed representative of the full Northern Hemisphere annual
mean, as MBH99 purports to be. To the extent that differences emerge between the two in
assuming such a scaling, I interpret them as differences which exist due to the fact that
the extratropical Northern Hemisphere series and full Northern Hemisphere series likely did
not co-vary in the past the same way they co-vary in the 20th century (when both are driven
predominantly, in a relative sense, by anthropogenic forcing, rather than natural forcing
and internal variability). What the plot shows is quite remarkable. Scaled in this way,
there is remarkably little difference between Esper et al and MBH99 in the first place (the
two reconstructions are largely within the error estimates of MBH99!)!, but moreover, where
they do differ, this could be explainable in terms of patterns of enhanced mid-latitude
continental response that were discussed, for example, in Shindell et al (2001) in Science
last December. So I think this plot says a lot. Its say that there are some statistically
significant differences, but certainly no grounds to use Esper et al to contradict MBH99 or
IPCC '2001 as, sadly, I believe at least one of the published pieces tacitly appears to
want to do.
It is shame that such a plot, which I think is a far more meaningful comparison of the two
records, was not shown in either Esper et al or the Briffa & Osborn commentary. I've always
given the group of you adequate opportunity for commentary on anything we're about to
publish in Nature or Science. I am saddened that many of my colleagues (and, I have always
liked to think friends) didn't affort me the same opportunity before this all erupted in
our face. It could have been easily avoided. But that's water under the bridge.
Finally, before any more back-and-forths on this, I want to make sure that everyone
involved understands that none of this was in any way ever meant to be personal, at least
not on my part (and if it ever has, at least on my part, seemed that way, than I offer my
apologies--it was never intended that way). This is completely about the "science". To the
extent that I (and/or others) feel that the science has been mis-represented in places,
however, I personally will work very hard to make sure that a more balanced view is
available to the community. Especially because the implications are so great in this case.
This is what I sought to do w/ the NYT piece and my NPR interview, and that is what I've
sought to do (and Malcolm to, as far as I'm concerned) with the letter to Science. Being a
bit sloppy w/ wording, and omission, etc. is something we're all guilty of at times. But I
do consider it somewhat unforgivable when it is obvious how that sloppiness can be
exploited. And you all know exactly what I'm talking about!
So, in short, I think are some fundamental issues over which we're in disagreement, and
where those exist, I will not shy away from pointing them out. But I hope that is not
mis-interpreted as in any way personal.
I hope that suffices,
p.s. It seemed like an omission to not cc in Peck and Scott Rutherford on this exchange, so
I've done that. I hope nobody minds this addition...
At 10:57 AM 4/11/02 -0400, Edward Cook wrote:
Hi Mike and Malcolm,
I have received the letter that you sent to Science and will respond to it here first in
some detail and later in edited and condensed form in Science. Since much of what you
comment and criticize on has been disseminated to a number of people in your (Mike's)
somewhat inflammatory earlier emails, I am also sending this lengthy reply out to
everyone on that same email list, save those at Science. I hadn't responded in detail
before, but do so now because your criticisms will soon be in the public domain.
However, I am not attaching your letter to Science to this email since that is not yet
in the public domain. It is up to you to send out your submitted letter to everyone if
I must say at the beginning that some parts of your letter to Science are as "flawed" as
your claims about Esper et al. (hereafter ECS). The Briffa/Osborn perspectives piece
points out an important scaling issue that indeed needs further examination. However, to
claim as you do that they show that the ECS 40-year low-pass temperature reconstruction
is "flawed" begs the question: "flawed" by how much? It is not at all clear that
scaling the annually resolved RCS chronology to annually resolved instrumental
temperatures first before smoothing is the correct way to do it. The ECS series was
never created to examine annual, or even decadal, time-scale temperature variability.
Rather, as was clearly indicated in the paper, it was created to show how one can
preserve multi-centennial climate variability in certain long tree-ring records, as a
refutation of Broecker's truly "flawed" essay. As ECS showed in their paper (Table 1),
the high-frequency correlations with NH mean annual temperatures after 20-year high-pass
filtering is only 0.15. That result was expected and it makes no meaningful difference
if one uses only extra-tropical NH temperature data. So, while the amplitude of the
temperature-scaled 40-year low-pass ECS series might be on the high end (but still
plausible given the gridded borehole temperature record shown in Briffa/Osborn), scaling
on the annually resolved data first would probably have the opposite effect of
excessively reducing the amplitude. I am willing to accept an intermediate value, but
probably not low enough to satisfy you. Really, the more important result from ECS is
the enhanced pattern of multi-centennial variability in the NH extra-tropics over the
past 1100 years. We can argue about the amplitude later, but the enhanced
multi-centennial variability can not be easily dismissed. I should also point out,
again, that you saw Fig. 3 in ECS BEFORE it was even submitted to Science and never
pointed out the putative scaling "flaw" to me at that time.
With regards to the issue of the late 20th century warming, the fact that I did not
include some reference to or plot of the up-to-date instrumental temperature data (cf.
Briffa/Osborn) is what I regard as a "sin of omission". What I said was that the
estimated temperatures during the MWP in ECS "approached" those in the 20th century
portion of that record up to 1990. I don't consider the use of "approached" as an
egregious overstatement. But I do agree with you that I should have been a bit more
careful in my wording there. As you know, I have publicly stated that I never intended
to imply that the MWP was as warm as the late 20th century (e.g., my New York Times
interview). However, it is a bit of overkill to state twice in the closing sentences of
the first two paragraphs of your letter that the ECS results do not refute the
unprecedented late 20th century warming. I would suggest that once is enough.
ECS were also very clear about the extra-tropical nature of their data. So, what you
say in your letter about the reduced amplitude in your series coming from the tropics,
while perhaps worth pointing out again, is beating a dead horse. However, I must say
that the "sin of omission" in the Briffa/Osborn piece concerning the series shown in
their plot is a bit worrying. As they say in the data file of series used in their plot
(and in Keith's April 5 email response to you), Briffa/Osborn only used your land
temperature estimates north of 20 degrees and recalibrated the mean of those estimates
to the same domain of land-only instrumental temperatures using the same calibration
period for all of the other non-borehole series in the same way. I would have preferred
it if they had used your data north of 30N to make the comparisons a bit more
one-to-one. However, I still think that their results are interesting. In particular,
they reproduce much of the reduced multi-centennial temperature variability seen in your
complete NH reconstruction. So, if the amplitude of scaled ECS multi-centennial
variability is far too high (as you would apparently suggest), it appears that it is
also too low in your estimates for the NH extra-tropics north of 20N. I think that we
have to stop being so aggressive in defending our series and try to understand the
strengths and weaknesses of each in order to improve them. That is the way that science
is supposed to work.
I must admit to being really irritated over the criticism of the ECS tree-ring data
standardized using the RCS method. First of all, ECS acknowledged up front the
declining available data prior to 1200 and its possible effect on interpreting an MWP in
the mean record. ECS also showed bootstrap confidence intervals for the mean of the RCS
chronologies and showed where the chronologies drop out. Even allowing for the reduction
in the number of represented sites before 1400 (ECS Fig. 2d), and the reduction in
overall sample size (ECS Fig. 2b), there is still some evidence for significantly above
average growth during two intervals that can be plausibly assigned to the MWP. Of course
we would like to have had all 14 series cover the past 1000-1200 years. This doesn't
mean that we can't usefully examine the data in the more weakly replicated intervals.
In any case, the replication in the MWP of the ECS chronology is at least as good as in
other published tree-ring estimates of large-scale temperatures (e.g., NH
extra-tropical) covering the past 1000+ years. It also includes more long tree-ring
records from the NH temperate latitudes than ever before. So to state that "this is a
perilous basis for an estimate of temperature on such a large geographic scale" is
disingenuous, especially when it is unclear how many millennia-long series are
contributing the majority of the temperature information in the Mann/Bradley/Hughes
(MBH) reconstruction prior to AD 1400. Let's be balanced here.
I basically agree with the closing paragraph of your letter. The ECS record was NEVER
intended to refute MBH. It was intended, first and foremost, to refute Broecker's essay
in Science that unfairly attacked tree rings. To this extent, ECS succeeded very well.
The comparison of ECS with MBH was a logical thing to do given that it has been accepted
by the IPCC as the benchmark reconstruction of NH annual temperature variability and
change over the past millennium. Several other papers have made similar comparisons
between MBH and other even more geographically restricted estimates of past
temperature. So, I don't apologize in the slightest for doing so in ECS. The
correlations in Table 2 between ECS and MBH were primarily intended to demonstrate the
probable large-scale, low-frequency temperature signal in ECS independent of explicitly
calibrating the individual RCS chronologies before aggregating them. The results should
actually have pleased you because, for the 20-200 year band, ECS and MBH have
correlations of 0.60 to 0.68, depending on the period used. Given that ECS is based on
a great deal of new data not used in MBH, this result validates to a reasonable degree
the temperature signal in MBH in the 20-200 year band over the past 1000 years.
Given the incendiary and sometimes quite rude emails that came out at the time when ECS
and Briffa/Osborn were published, I could also go into the whole complaint about how the
review process at Science was "flawed". I will only say that this is a very dangerous
game to get into and complaints of this kind can easily cut both ways. I will submit an
appropriately edited and condensed version of this reply to Science.
Dr. Edward R. Cook
Doherty Senior Scholar
Lamont-Doherty Earth Observatory
Palisades, New York 10964 USA
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
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