cc: esper@wsl.ch, k.briffa@uea.ac.uk, t.osborn@uea.ac.uk, p.jones@uea.ac.uk, tcrowley@duke.edu, rbradley@geo.umass.edu
date: Thu, 11 Apr 2002 10:57:15 -0400
from: Edward Cook <drdendro@ldeo.columbia.edu>
subject: Your letter to Science
to: "Michael E. Mann" <mann@multiproxy.evsc.virginia.edu>, "Malcolm Hughes" <mhughes@ltrr.arizona.edu>

   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.
   Regards,
   Ed

--

   =================================
   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
   Email: drdendro@ldeo.columbia.edu
   =================================
