date: Tue, 1 Apr 1997 13:26:01 -0500 (EST) from: ray bradley subject: standardisation to: k.briffa@uea.ac.uk Hiya Keef: Hope you survived the journey back from Cape Town. I hear you got hit with a large bill at the hotel...maybe you covered my bar bill which did not appear on my statement!!! Could you look over this section which I propose to add to my book, dealing with the problems of standardisation.....& let me know if I got it wrong...(The proposed Figures 10.9-10.11 are Figs 2- 4 from your Climate Dynamics paper)...All comments would be appreciated. I've enjoyed reading your papers on this & other matters.... ".......It is clear that this standardization procedure is not easy to apply and may actually remove important low-frequency climatic information. It is not possible, a priori, to decide if part of the long-term change in ring width is due to a coincident climatic trend. The problem is exacerbated if one is attempting to construct a long-term dendrochronological record, when only tree fragments or historical timbers are available and the corresponding growth function may not be apparent. ***added*** The consequences of different approaches to standardisation are well illustrated by the studies of long tree-ring series (Scots pine, Pinus sylvestris) from northern Fennoscandia by Briffa et al. (1990, 1992). In order to produce a long dendroclimatic reconstruction extending over 1500 years, Briffa et al. (1990) constructed a composite chronology made up of many overlapping cores which varied in their individual length, from less than 100 to more than 200 years (Figure 10.9). In the shorter segments, the growth function is significant over the entire segment length, but in longer segments the growth factor becomes less significant (cf Figure 10.7, upper panel). In Briffa et al. (1990) each segment was standardised individually (the procedure used in almost all dendroclimatic studies) in this case by the use of a spline function which retains variance at periods less than ~2/3 of the record length. Thus, in a 100 year segment, variance at periods >66 years would be lost whereas in a 300 year segment, variance at periods up to 200 years would be retained. All standardised cores were then averaged together, producing the record shown in Figure 10.10c. This shows considerable interannual to decadal scale variability, but little long-term, low frequency variability. In fact, since the mean segment length varies over time (Figure 10. 9) so too will the low frequency variance represented in the composite series. In Briffa et al. (1992) the standardisation procedure was revised by first aligning all core segments by their relative age, then averaging them (i.e. all values of the first year in each segment (t1) were averaged, then all values of t2 etc.... to tn.; this assumes that in each segment sampled, t1 was at (or very close to) the center [pith?] of the tree). The resulting 'regional curve' then provided a target for deriving a mean growth function which could be applied to all of the individual core segments, regardless of length (Figure 10.11). Averaging together the core segments, standardised in this way by the regional curve, produced the record shown in Figure 10.10b. This has far more low frequency information than the record produced from individually standardised cores (Figure 10.10c) and retains many of the characteristics seen in the original data. From this series, low growth from the late 1500s to the early 1800s is clearly seen, corresponding to other European records which record a "Little Ice Age" during this interval. Also seen is a period of enhanced growth from ~950-1100 A.D., during a period which Lamb (1965) characterised as the "Medieval Warm Epoch". It is apparent from a comparison of Figures 10.9b and 10.9c that any conclusions drawn about which were the warmest or coldest years and decades of the past can be greatly altered by the standardisation procedure employed. All of the high frequency variance of Figure 10.10c is still represented in the record produced by regional curve standardisation (Figure 10.10b) but potentially important climatic information at lower frequencies is also retained. The problem of extracting low frequency climatic information from long composite records made up of many individual short segments is addressed explicitly by Cook et al. (1995) who refer to this as the 'segment length curse'! Although it is of particular concern in dendroclimatology, it is in fact an important problem in all long-term paleoclimatic reconstructions which utilise limited duration records (e.g. historical data, composite coral records etc) yet it has received relatively little attention in the literature so far.******