cc: anders@misu.su.se, Eduardo.Zorita@gkss.de, hegerl@duke.edu, esper@wsl.ch, weber@knmi.nl, t.osborn@uea.ac.uk
date: Thu Nov 16 11:57:09 2006
from: Keith Briffa <k.briffa@uea.ac.uk>
subject: Re: Mitrie: Bristlecones
to: Martin Juckes <m.n.juckes@rl.ac.uk>, "Myles Allen" <allen@atm.ox.ac.uk>

   Martin and all,
   I know Franco very well - but he has not worked extensively with the Bristlecones. I still
   believe that it  would be wise to involve Malcolm Hughes in this discussion - though I
   recognise the point of view that says we might like to appear (and be) independent of the
   original Mann, Bradley and Hughes team to avoid the appearance of collusion. In my opinion
   (as someone how has worked with the Bristlecone data hardly at all!) there are undoubtedly
   problems in their use that go beyond the strip bark problem (that I will come back to
   later).
   The main one is an ambiguity in the nature and consistency of their sensitivity to
   temperature variations. It was widely believed some 2-3 decades ago, that high-elevation
   trees were PREDOMINANTLY responding to temperature and low elevation ones  to available
   water supply (not always related in a simple way to measured precipitation) . However,
   response functions ( ie sets of regression coefficients on monthly mean temperature and
   precipitation data derived using principal components regression applied to the tree-ring
   data) have always shown quite weak and temporally unstable associations between chronology
   and climate variations (for the high-elevations trees at least). The trouble is that these
   results are dominated by inter-annual (ie high-frequency) variations and apparent
   instability in the relationships is exacerbated by the shortness of the instrumental
   records that restrict analyses to short periods, and the large separation of the climate
   station records from the sites of the trees. Limited comparisons between tree-ring density
   data (which seem to display less ambiguos responses) imply that there is a reasonable
   decadal time scale association and so indicate a real temperature signal , on this time
   scale .The bottom line though is that these trees likely represent a mixed temperature and
   moisture-supply response that might vary on longer timescales.
   The discussion is further complicated by the fact that the first PC of "Western US" trees
   used in the Mann et al. analyses is derived from a mixture of species (not just
   Bristlecones ) and they are quite varied in their characteristics , time span, and
   effective variance spectra . Many show low interannual variance and a long-term declining
   trend , up until about 1850 , when the Bristlecones (and others) show the remarkable
   increasing trend up until the end of the record. The earlier negative trend could be
   (partly or more significantly) a consequence of the LACK of detrending to allow for age
   effects in the measurements (ie standardisation) - the very early sections of relative high
   growth were removed in their analysis, but no explicit standardistion of the data was made
   to account for remaining slow width changes resulting from tree aging.  This is also
   related to the "strip bark" problem , as these types of trees will have unpredictable
   trends as a consequence of aging  and depending on the precise nature of each tree's
   structure .
    Another serious issue to be considered relates to the fact that the PC1 time series in the
   Mann et al. analysis was  adjusted to reduce the positive slope in the last 150 years (on
   the assumption - following an earlier paper by Lamarche et al. - that this incressing
   growth was evidence of carbon dioxide fertilization) , by differencing the data from
   another record produced by other workers in  northern Alaska and Canada (which
   incidentally was standardised in a totally different way). This last adjustment obviously
   will have a large influence on the quantification of the link between these Western US
   trees and N.Hemisphere temperatures. At this point , it is fair to say that this adjustment
   was arbitrary and the link between Bristlecone pine growth and CO2 is , at the very least,
   arguable. Note that at least one author (Lisa Gaumlich) has stated that the recent growth
   of these trees could be temperature driven and not evidence of CO2 fertilisation.
   The point of this message is to show that that this issue is complex , and I still believe
   the "Western US" series and its interpretation in terms of Hemispheric mean temperature is
   perhaps a "Pandora's box" that we might open at our peril!
   What does Jan say about this - he is very acquainted with these issues?
   cheers
   Keith
   At 15:01 15/11/2006, Martin Juckes wrote:

     Hi,
     Concerning Bristlecones, I had a sympathetic reply from Prof. North, but he
     deferred to the person who wrote the relevant paragraph in the NAS report
     (Franco Biondi) who is firmly of the view that strip-bark bristlecones should
     not be used. I've read a few of the articles cited to back up this statement
     and I am surprised by the extreme weakness of the evidence. There is one
     study of 27 strip-bark pines which shows that they clearly developed
     anomalous growth around 1850. Attributing this to CO2 is odd, to say the
     least. I'm writing a brief review of the literature which I'll send round in
     a few days time.
     cheers,
     Martin

     On Sunday 12 November 2006 22:21, Myles Allen wrote:
     > Although it probably doesn't feel like it, it seems to me you're doing
     > rather well...
     >
     > -----Original Message-----
     > From: Martin Juckes [[1]mailto:m.n.juckes@rl.ac.uk]
     > Sent: 10 November 2006 15:24
     > To: anders@misu.su.se; Eduardo.Zorita@gkss.de; hegerl@duke.edu;
     > esper@wsl.ch; k.briffa@uea.ac.uk; Myles Allen; weber@knmi.nl;
     > t.osborn@uea.ac.uk
     > Subject: Mitrie
     >
     > Hello,
     >
     > well, I've had a few exchanges on climateaudit, and decided to leave
     > them to
     > it for a few days.
     >
     > I'm going to send an email to Prof. North of the NAS panel to ask if he
     > really
     > meant "don't use bristlecones", as he is quoted by McIntyre. I believe
     > it
     > would be incorrect to select sites on the basis of what the data from
     > the
     > sites looks like, and this makes up a substantial part of the argument
     > in
     > Graybill and Idso (1993).
     >
     > Does anyone know where I can get hold of the categorisation of the Sheep
     >
     > Mountain trees used by Graybill and Idso (ca534.rwl from the WDC for
     > paleoclimatology I think) into "strip-bark" and "full-bark"? I've sent
     > an
     > email to the WDC query address.
     >
     > I've also sent of for a publication which is cited by co2science as
     > using
     > Sargasso Sea data with the dating shifted by 50 years (Loehle,  2004,
     > Ecological Modelling). This appears to be a source of considerable
     > confusion
     > among the climate sceptics. The shifted series fits nicely with the idea
     > that
     > the Medieval Warm Period was warmer than the 20th century, so there is a
     >
     > widespread perception that it is being ignored to fudge the results.
     >
     > Apart from a couple of oversights in the documentation of the data files
     >
     > McIntyre hasn't come up with much yet. I need to read up a bit more on
     > the
     > different Tornetraesk/Fennoscandia series.  There was an interesting
     > discussion on "cherrypicking", with contributors suggesting that testing
     > the
     > effect of removing each proxy series in turn was "cherrypicking" and
     > that
     > selecting series based on subjective analysis of what the series look
     > like
     > would be much better!
     >
     > I've had a comment from the editor saying that responses to non-refereee
     >
     > comments are optional, especially if the comments are not relevant to
     > the
     > paper.
     >
     > cheers,
     > Martin
     >
     >
     >

   --
   Professor Keith Briffa,
   Climatic Research Unit
   University of East Anglia
   Norwich, NR4 7TJ, U.K.

   Phone: +44-1603-593909
   Fax: +44-1603-507784
   [2]http://www.cru.uea.ac.uk/cru/people/briffa/

