Timothy J. Osborn and Keith R. Briffa
26 October 2009
In a related comment, we noted that the final years of the Yamal ring-width chronology (Briffa, 2000; Briffa et al., 2008) should be used cautiously on the basis that the values for the most recent part of this chronology are based on relatively few individual measurement series and this smaller available sample emphasises the faster growing trees. Despite this caveat, it should not be assumed that the Briffa (2000) and Briffa et al. (2008) chronologies are "wrong": a reworked version of the Yamal chronology, using more data than were used in the published chronologies, shows essentially the same picture of comparatively high tree growth throughout most of the 20th century (i.e. 1901-1990). The only available data, from two separate locations in the area, provide mutually consistent indications of an even higher level of growth in the 6 years after 1990. Nevertheless, it is still worth considering whether the published Yamal RCS chronology has been used cautiously.
The Yamal chronology of Briffa (2000) has been used in a number of related studies.
In Osborn and Briffa (2006), the Yamal chronology was one of 14 proxy series used to investigate the spatial extent of warming and cooling over the last 1200 years across the Northern Hemisphere. One aspect of our study was that the results cannot be strongly biased by extreme values in any individual proxy series.
Our approach was simply to count how many of the proxy records exceeded a chosen threshold (e.g. in Figure 1 the threshold is one standard deviation from the long-term mean). A particular proxy series is only counted once, no matter how far it exceeds the chosen threshold — even if a series reaches 4 or 5 standard deviations above its long-term mean, it makes no additional contribution to our final result.
This was a deliberate objective when we were designing this study, to reduce the sensitivity of our results to particularly extreme values in any individual proxy series.
Figure 1 (extracted from Figure 3B of Osborn and Briffa, 2006). Difference between the fraction of the records available in each year that have normalized values > 1 and < -1. The difference series are shown for 800 to 1995 and have been filtered to remove variations on time scales less than 20 years. Zero indicates that the number of series exceeding the upper threshold equals those with values below the lower threshold. The red and blue lines show the 95th and 5th percentiles of distributions obtained by repeating the analysis 10,000 times with each proxy time series shifted randomly in time; dark red and blue shading indicates times when the difference series exceeds the 95th or 5th percentile.
Our use of smoothed (i.e., low-pass filtered) series could, however, spread the influence of extreme values that are short-lived or occur near the ends of the series over a broader range of years. We undertook additional sensitivity tests, therefore, to assess the impact on our results of completely excluding one, two or three out of the 14 proxy series (using all possible combinations of which proxies to exclude).
Figure 2 shows the results in terms of the statistical significance of widespread warmth or coldness, for exclusion of each proxy series in turn. The Yamal ring-width chronology is number 10 in our analysis. Figure 3 shows the results of this sensitivity analysis in a comparable format to Figure 1.
As expected from our study design, exclusion of an individual proxy record (e.g., the Yamal ring-width chronology) does not have a large impact on our results.
Figure 2 (reproduced from Figure S3 in the Supporting Online Material of Osborn and Briffa, 2006). Sensitivity of the results shown in Figure 1 to the exclusion of each of the 14 proxy records in turn (as indicated above each line of results; proxy 10 is the Yamal ring-width chronology). The bars indicate the years for which the analysis shown in Figure 1, when repeated for each combination of 13 records, exceeds the 90, 95 or 99 percentiles (small, medium or large red bars above the line) or the 10, 5 or 1 percentiles (small, medium or large blue bars below the line).
Figure 3 (reproduced from Figure S4 in the Supporting Online Material of Osborn and Briffa, 2006). Difference between the fraction of the available records in each year that have normalised values >1 and <-1, filtered to remove variations on time scales less than 20 years. The red and blue shading shows the result using all 14 proxy records. The thin black lines indicate results obtained when each of the 14 proxies is systematically excluded from the analysis.
The Palaeoclimate chapter (Jansen et al., 2007) of the Working Group 1 contribution to the IPCC's Fourth Assessment Report used the Yamal ring-width chronology of Briffa (2000) in constructing Figure 1 of Box 6.4 (reproduced here as Figure 4). This series was labelled "NW Russia" in this Figure and the data are available at http://www.cru.uea.ac.uk/~timo/datapages/ipccar4.htm.
In this analysis, the Yamal chronology was used cautiously because the series was truncated in 1985 for the purposes of constructing this Figure. Thus, the high recent values from Yamal were not shown in this Figure.
Figure 4 (reproduced from Figure 1 of Box 6.4 of Jansen et al., 2007, including the original caption).
The full chronology was used when 20-year smoothing was applied, which could (as noted earlier) spread the influence of the post-1985 data across a broader range of years. To evaluate this influence, we have created an alternative to this Figure by truncating the Yamal chronology in 1985 prior to applying the smoothing filter (Figure 5). The impact on the NW Russia series is minor and the IPCC conclusions that this Figure was used to illustrate are not affected.
Figure 5. As Figure 4, but with the NW Russia (Yamal ring-width chronology) truncated at 1985 prior to applying the 20-year smoothing filter.
The Palaeoclimate chapter (Jansen et al., 2007) of the IPCC's Fourth Assessment Report also showed (Figure 6.10, reproduced here as Figure 6) Northern Hemisphere temperature reconstructions from 12 published studies. These studies were listed in Table 6.1 of Jansen et al. (2007) and are also listed here in Table A.
Table A also notes which of these studies used the Yamal ring-width chronology of Briffa (2000). Of these 12 reconstructions, eight did not use Yamal and thus their results are entirely independent of the Yamal series. Of the other four studies, one used only the high-frequency information from the Yamal record (the temperature variations on time scales of centuries to millennia were estimated only from non-tree-ring proxy records). Two studies composited the Yamal series with records from other regions before interpreting them as evidence of average Northern Hemisphere temperature changes. The influence of the Yamal series on the reconstruction of Northern Hemisphere temperatures is likely to be relatively small in these studies. Only in Briffa (2000), where the Yamal data are used along with only two other northern Eurasian series is the result likely to be heavily influenced by the inclusion of the Yamal chronology.
|Study/reconstruction||Contribution of the Briffa (2000) Yamal chronology|
|Jones et al. (1998)||Not used|
|Mann et al. (1999)||Not used|
|Briffa et al. (2001)||Not used|
|Esper et al. (2002)||Not used|
|Briffa (2000)||Briffa (2000) Yamal was used|
|Mann and Jones (2003)||Briffa (2000) Yamal was used in a composite of three ring-width chronologies from northern Eurasia|
|Rutherford et al. (2005)||Not used|
|Moberg et al. (2005)||Only high-frequency information from the Briffa (2000) Yamal chronology was used|
|D'Arrigo et al. (2006)||Briffa (2000) Yamal was used, though possibly labelled as Polar Urals|
|Hegerl et al. (2006)||Not used|
|Pollack and Smerdon (2004)||Not used|
|Oerlemans (2005)||Not used|
Thus, with the exception of Briffa (2000), the reconstructions shown by the IPCC (Figure 6) either do not use the Yamal record or they combine the Yamal record with many others and this reduces their sensitivity to the inclusion of any individual records. The IPCC then takes this aggregation one step further, by considering multiple reconstructions (Figure 6). The overall conclusions of the Palaeoclimate chapter in the IPCC report (Jansen et al., 2007) are not, therefore, strongly dependent on the Yamal record of Briffa (2000).
Figure 6 (reproduced from Figure 6.10 of Jansen et al., 2007). Records of NH temperature variation during the last 1300 years listed in Table 1 and the HadCRUT2v instrumental temperature record in black. All series have been smoothed with a Gaussian-weighted filter to remove fluctuations on time scales less than 30 years; smoothed values are obtained up to both ends of each record by extending the records with the mean of the adjacent existing values. All temperatures represent anomalies (degrees C) from the 1961 to 1990 mean.
Briffa, K. R. 2000. Annual climate variability in the Holocene: interpreting the message of ancient trees. Quaternary Science Reviews 19:87-105.
Briffa, K. R., V. V. Shishov, T. M. Melvin, E. A. Vaganov, H. Grudd, R. M. Hantemirov, M. Eronen, and M. M. Naurzbaev. 2008. Trends in recent temperature and radial tree growth spanning 2000 years across northwest Eurasia. Philosophical Transactions of the Royal Society B-Biological Sciences 363:2271-2284.
Jansen et al. (2007) Palaeoclimate. Chapter 6 in Climate change 2007: the physical science basis; contribution of Working Group 1 to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (eds. Solomon et al.), Cambridge University Press, Cambridge, UK and New York, USA.
Osborn TJ and Briffa KR (2006) The spatial extent of 20th century warmth in the context of the last 1200 years. Science 311, 841-844 (doi:10.1126/science.1120514).