Tuesday, November 29, 2011

0199.txt

cc: Keith Briffa <k.briffaatXYZxyz.ac.uk>
date: Tue, 6 Nov 2007 14:43:29 -0800 (PST)
from: "David M. Ritson" <dmratXYZxyzc.stanford.edu>
subject: Re: RCS paleo reconstructions
to: Tim Osborn <t.osbornatXYZxyz.ac.uk>

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Dear Keith and Tim,

I wrote in the summer relative to the statement that underpins mmuch of the
discussions on the merits of RCS versus conventional standardizations of
paleo climate reconstructions of Cook et al 1995 that
`" ,,, the cross-dated annual changes in ring-width between trees due to
climate are forced out of alignment and effectively averaged out in the
creation of the mean regional curve."
This is commonly taken to imply that RCS methods largely circumvent the
segment-length curse. At the time I believed that, in most instances,
the systematics inherent in the actual data, such as the fractions of
juvenile and mature trees in the sites invalidated the cancellations implied
in the Cook et al. paragraph. There are cancellations, but in most
instances insufficient to better eliminate the segment curse. This appears
to be well known, by the professional dendrochronologist community. What is
disconcerting is that I find no clarifications or follow-up of the above
Cook statement either in the IPCC AR4 nor in the generally available climate
change literature. If indeed such clarifications are missing then I think it
is incumbent that you guys ensure that they are understood throughout the
climate community.

My own take on the current situation is that the only hard statements that can
be trusted should be based on `bounds'. Juvenile growth for the first century
or so, is likely to be variable and probably juvenile data should be ignored.
Subsequent to this an `upper' bound is provided by assuming that ring width
growth is independent of tree age. More speculatively a lower bound is
provided by assuming ring-area growth constancy. However nobody provides
such bounds.

Tim gave me some interim answers to the above, but promised me that Keith
would provide a more definitive summary of RCS status after the summer. I
certainly would appreciate your considered views as to the absolute precision
and trustworthiness of past millenial temperature reconstructions.
Obviously the North NAS committee had similar reservations .

Cheers

Dave Ritson


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