Wednesday, May 16, 2012


date: Thu, 11 Apr 2002 10:57:15 -0400
from: Edward Cook <>
subject: Your letter to Science
to: "Michael E. Mann" <>, "Malcolm Hughes" <>

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 you wish.
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
Tree-Ring Laboratory
Lamont-Doherty Earth Observatory
Palisades, New York 10964 USA
Phone: 1-845-365-8618
Fax: 1-845-365-8152

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