date: Tue, 19 Dec 2006 13:26:07 UT
from: grlonline@agu.org
subject: Review Received by Geophysical Research Letters
to: t.osborn@uea.ac.uk

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   B2.11; Q2.03) Date: Tue, 19 Dec 2006 13:26:07 UT Message-Id: <116653476798@gems2>
   Dear Dr. Osborn:
   Thank you for your review of "Correction for proxy number and coherence in a large-scale
   temperature reconstruction" by David Frank, Jan Esper, and Edward Cook [Paper
   #2006GL028692], which we have safely received. A copy of this review is attached below for
   your reference.
   Thank you for your time and effort!
   Sincerely,
   James Famiglietti
   Editor
   Geophysical Research Letters
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   Presentation Category: Presentation Category A
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   Highlight:
   Formal Review:
   Review of Frank, Esper and Cook "Correction for proxy number and coherence in a large-scale
   temperature reconstruction"
   This manuscript addresses the important issue of artificial changes through time in the
   variance of a frequently-used reconstruction of Northern Hemisphere temperature changes
   during the last 1200 years or so. Artificial change in variance is just one of a number of
   issues that remain to be dealt with in relation to this (and indeed some other) temperature
   reconstructions, but nevertheless it is entirely appropriate for a manuscript to focus on
   this one issue. The method used to quantify, and to compensate for, the artifical changes
   in variance is mostly appropriate, though further consideration to the issue of
   non-stationarity should also be given. The magnitude of the artificial changes in variance
   is sufficiently large to warrant this publication of a revised reconstruction that removes
   them, though it should be made clearer throughout that this is not the only issue to
   consider when evaluating the information provided by this and other reconstructions
   Similar adjustments to remove artificial changes in variance, arising from changing sample
   size through time, have been made in some other climate reconstructions (which should be
   cited to accurately represent the research record), though perhaps not in such a systematic
   way as presented here.
   Overall, therefore, I consider that this manuscript is appropriate for publication, subject
   to minor revisions.
   Major comments
   (1) As suggested in the introductory paragraph above, it should be made clearer throughout
   the manuscript that there are other important sources of uncertainty/error in the
   reconstruction that are not addressed here. Related to this, I would suggest that the word
   "correction" be replaced with the word "adjustment" in the title and at other appropriate
   places within the text because use of the word "correction" suggests that the new record is
   now "correct" whereas it will still be subject to considerable error (the estimated
   reconstruction error has not been shown in any of the figures either).
   (2) On pages 4 and 11 of the manuscript, it is noted that the method used to modify the
   records is applicable in cases where the individual time series are stationary. On page 11,
   this is followed by a mention of Jones et al. (2001), who applied the adjustments only to
   the high-frequency residuals from a smoothing filter, under the assumption that the
   high-frequency component will be stationary, but allowing non-stationarity on longer time
   scales. This is followed by the statement that tree-ring data contain noise on short and
   long time scales, which I presume is their justification for applying the adjustments to
   all time scales. It should be made clear that just because some data may contain noise on
   all time scales, this does not mean that they are then non-stationary on all time scales.
   Certainly it means that some component of the variability may contain artificial changes in
   variance on all time scales, which should be removed where possible
   but it does not mean that the entire time series variation is a random stationary deviation
   from a mean. I appreciate that it is not possible to know the non-stationary signal, and
   hence identify the stationary deviations from the signal, but nevertheless the manuscript
   would be improved by greater consideration of this issue. On page 11 it is stated that "we
   approached a rough stationarity requirement by setting the long-term mean of series to
   zero". This is a strange statement, since adjusting the mean level of a non-stationary
   series yields a series that is still non-stationary!! Perhaps a more defensible statement
   would be "we continued our analysis under the assumption that the individual tree-ring
   series were stationary; if this assumption is invalid, then some real climate changes
   during periods with small samples of data (e.g., the MWP) may be adjusted towards the mean
   level of our reconstruction by a greater amount than they should be
   Climate deviations during periods of small sample size might, therefore, be underestimated.
   We did, however, adjust the long-term means of each series to equal zero prior to making
   the adjustments." Overall, the stationarity assumption and its implications must be given
   greater consideration in the manuscript.
   (3) The manuscript appropriately cites previous work that has discussed the dependence of
   variance on sample size, put forward the adjustment method, and applies it to instrumental
   temperature data (Wigley et al., 1984; Osborn et al., 1997; Jones et al., 2001; Brohan et
   al., 2006). It would be appropriate to cite the adjustment method paper more prominently at
   the start of the discussion section (page 11). More importantly, however, it is necessary
   to point out that the method has previously been applied to temperature reconstructions.
   Careful reading of Briffa et al. (1998; Nature, 393, 450-455) indicates that they made
   variance adjustments when making a large-scale average of chronologies or of regional
   series, though it is not stated that they adjusted the individual tree-ring chronologies
   for this effect. Similarly, Jones et al. (1998; Holocene, 8, 455-471) also state that they
   made such adjustments when forming a large-scale average of individual series
   as did Briffa et al. (2001; J. Geophys. Res., 106, 2929-2941). It must be made clear that
   the current manuscript is not the first example of this method being used with proxy-based
   temperature reconstructions.
   (4) Overall, the text of the manuscript needs to be improved. Although it appears to be
   mostly understandable, there are cases throughout where the text must be much more precise
   and tight, to avoid ambiguities arising from slightly sloppy wording. Make sure, for
   example, that you take into account words with specific statistical meanings. The
   "mean-value function" is mentioned a number of times, for example, but it should be
   clarified that you don't mean the time-average of the reconstructed values, but instead you
   mean the average of the sample of data in one particular year. A careful reading of the
   manuscript by all the authors, with a view to considering whether a reader of the
   manuscript could understand precisely and unambiguously what has been done, or what is
   meant, should easily find the various problematic sentences.
   Minor comments
   (1) Abstract, final sentence: perhaps change "appear to" to "may", since you haven't
   demonstrated that these other reconstructions do contain similar biases.
   (2) Page 2, line 45: would "measurement records" be better than the more technical
   "predictors"?
   (3) Page 2, line 50: replace "namely" by "typically", since not all records deteriorate
   monotonically into the past.
   (4) Page 3, line 66: you cannot "eliminate changes in the proxy network", but you can
   "eliminate the influence of changes in the proxy network".
   (5) Page 3, lines 68-69: I don't understand "while remaining fixed to the ECS dataset".
   (6) Page 3, line 78: replace "variance of a mean collection" with "variance of the mean of
   a collection".
   (7) Page 5, line 130: suggest you add "tree-ring" before "measurement" to remind the reader
   what data you are referring to.
   (8) Page 6: where RUNNINGr is discussed, you should also note and discuss the possibility
   that there could be real, climate-driven variations in inter-series correlation that you
   would want to retain, rather than considering them all as sources of artificial variance
   change. Also, why did you not consider a RUNNINGr adjustment when making the large-scale
   averaging set, where only the MEANr adjustment was shown. Did you try it and consider it to
   be inappropriate? For what reason?
   (9) Page 8, line 196: should "that make their mean highly sensitive" really be "that make
   their variance highly sensitive"?
   (10) Page 9, lines 226-229: the sentence about "self-consistent best estimates" needs to be
   explained more clearly.
   (11) Page 9, lines 229-231: expand this sentence about the overall temperature range (note
   "range" is better than "amplitude") to explain that the reason why the range is unaffected
   by the adjustments is that the "warmest" and "coldest" reconstructed temperatures occur
   (give dates) during periods with large samples of data.
   (12) Page 10, line 243: Jones et al. (1998) is not an EOF-based method, since they did
   composite plus scaling (they did show some EOFs of the proxy data, but didn't actually use
   them in making their NH reconstruction).
   (13) Page 12, line 289: I don't think the problem is just that the "site locations are
   generally less known", but rather that they may be more scattered with greater separation
   between them.
   (14) Figure 1C: Why do the standard deviations computed in running 100-year windows extend
   to each end of the record - I would expect them to stop 50 years from each end.
   (15) Figure 2D: The caption should say which line is which (red or black).
   (16) Figure 2E: The y-axis label says "Rel. variance" but should it be "Rel. St. Dev."?
   (17) Figure 3A: The caption states that this is a log scale, but the figure does not appear
   to have a log scale.
   (18) Figure 3B: The caption states that these are the "corrections", but I think that they
   are the "corrected series".
   (19) Supplementary material, lines 30-34: Make it clearer that the reason for the reduced
   variance earlier on is that they used many different regression models, each re-calibrated
   using the different subsets of proxy data that are available in each period.
   (20) Suppl. Mat., line 39: "speculative" not "speculate".
   (21) Suppl. Mat., line 48: what does "unadjusted MEANr adjusted" mean?
   (22) Figure S1: Are the units "degrees C" rather than "Z"?
