Keith R. Briffaa and Thomas M. Melvina
With thanks to Timothy J. Osborna, Philip D. Jonesa, Rashit M. Hantemirovb, Stepan Shiyatovb and Vladimir V. Shishovc,d.
Climatic Research Unit, School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, U.K.
b Laboratory Of Dendrochronology, Institute of Plant and Animal Ecology, Ural Branch of Russian Academy of Sciences, 8 Marta St., Ekaterinburg, 620144, Russia
c Dendroecology Department, Sukachev Institute of Forest, Siberian Branch of Russian Academy of Sciences, Akademgorodok St., Krasnoyarsk, 660036, Russia
d IT and Math. Modelling Department, Krasnoyarsk State Trade-Economical Institute, L. Prushinskoi St., Krasnoyarsk, 660075, Russia
AbstractAt http://www.climateaudit.org/?p=7168, Steve McIntyre reports an analysis he undertook to test the "sensitivity" of the "Regional Curve Standardised" tree-ring chronology (Briffa, 2000; Briffa at al., 2008) to the selection of measurement data intended to provide evidence of long-term changes of tree growth, and, ultimately inferred temperature variation through two millennia in the Yamal region of northern Russia. It would be a mistake to conclude that McIntyre's sensitivity analysis provides evidence to refute our current interpretation of relatively high tree growth and summer warmth in the 20th century in this region. A reworked chronology, based on additional data, including those used in McIntyre's analysis, is similar to our previously published chronologies. Our earlier work thus provides a defensible and reasonable indication of tree growth changes during the 20th century and in the context of long-term changes reconstructed over the last two millennia in the vicinity of the larch tree line in southern Yamal. McIntyre's use of the data from a single, more spatially restricted site, to represent recent tree growth over the wider region, and his exclusion of the data from the other available sites, likely represents a biased reconstruction of tree growth. McIntyre's sensitivity analysis has little implication, either for the interpretation of the Yamal chronology or for other proxy studies that make use of it.
Update: 14 June 2013. Our most recent work in this area has now been published in the peer-reviewed literature (Briffa et al., 2013; Quaternary Science Reviews) and interested readers should refer to that. The paper, together with supporting data and software, are freely available from our website.
Subsequently, a slightly different version of the RCS Yamal chronology was produced using the same data set but processed using an improved implementation of the RCS technique (Briffa et al. 2008). However, the difference between the 2000 and 2008 versions of the chronology is small.
The data set used in the above-cited work was assembled and supplied by our colleagues Rashit Hantemirov and Stepan Shiyatov (Institute of Plant and Animal Ecology, Ekaterinburg, Russia) who also published a version of this chronology (Hantemirov and Shiyatov 2002) based on a different processing method (corridor standardisation).
The AD portion of the data set, common to these three analyses consisted of measurements from 235 sub-fossil larch samples collected at numerous sites (see map) adjacent to the Porzayakha, Yadayakhodyyakha, Tanlovayakha and Khadytayakha rivers (Hantemirov and Shiyatov, 2002) and samples from 17 living trees growing at 5 sites in the vicinity of the sub-fossil trees.
McIntyre's analysis involved removing the measurement data for 12 trees (from 3 of these sites POR, YAD and JAH), data that make up the most recent part of our chronology, and replacing them with measurements from 18 trees growing at a different single site (KHAD), slightly to the south of the locations of the removed trees and originating from a different source. This alternative "modern" set was not considered or used by Hantemirov and Shiyatov or in the previous analyses (Briffa, 2000; Briffa at al., 2008)
McIntyre produced two alternative versions of his chronology, both constructed using a simple RCS technique similar to that employed in Briffa (2000). Both versions retained all of the sub-fossil material and the measurements from 5 'living' trees (from two sites) sampled in or before 1983. In one version of the chronology McIntyre included recent data only from the KHAD site. In the second version he retained the data from the 12 trees used in the original versions of the RCS chronology as well as the data from KHAD. The two versions of his chronology exhibit different patterns of tree growth over the 20th century: one with a positive trend and one (that with only the KHAD data) indicating a clear negative trend in tree growth after 1950. If one accepts that in this area, warmer temperatures are associated with enhanced tree growth, the "KHAD only" version could be interpreted as indicating generally reducing tree growth or a general cooling during the last 40 years. Both of these interpretations are largely at odds with the inference we have drawn from our published analyses.
We wish to stress that McIntyre himself has made no such assertions. At no time does he suggest that either of his versions of the chronology represents general Yamal tree-growth changes "more realistically" than in our earlier work. However, his original posting has been interpreted in this way by others, both on the Climate Audit website and elsewhere. Some postings on Climate Audit, notably that by Ross McKitrick (comment no. 7), strongly imply that the data used in the published versions of the Yamal chronology were deliberately selected in order to manufacture misleading evidence of a recent tree-growth increase in this region. Subsequent reports of McIntyre's blog (e.g. in The Telegraph, The Register and The Spectator) amount to hysterical, even defamatory misrepresentations, not only of our work but also of the content of the original McIntyre blog, by using words such as 'scam', 'scandal', 'lie', and 'fraudulent' with respect to our work.
Our current practice when selecting data to incorporate in a regional chronology, is to include data exhibiting high levels of common high-frequency variability (i.e. on the basis of high inter-site correlations, where these are calculated using high-pass filtered data). Judged according to this criterion it is entirely appropriate to include the data from the KHAD site (used in McIntyre's sensitivity test) when constructing a regional chronology for the area. However, we simply did not consider these data at the time, focussing only on the data used in the companion study by Hantemirov and Shiyatov and supplied to us by them.
We would never select or manipulate data in order to arrive at some preconceived or regionally unrepresentative result. However, as we will show here, the fact that we did not incorporate the KHAD data has no serious implications for the general validity of our published work.
In his implementation that included recent data from only the single KHAD site, McIntyre produces a chronology whose recent character is dominated by the growth behaviour of trees observed in a narrower region slightly to the south of the original sites.
We have now undertaken a more extensive sensitivity test, using the RCS approach, to examine the relative growth rates of trees at each of the 3 original locations removed by McIntyre, as well as the KHAD site. We have also taken the opportunity to acquire and incorporate additional data from the 3 original sites, in this analysis.
The rates of recent tree growth differ somewhat between the sites (see figure C and figure D). When compared to average growth during the previous century, each site chronology exhibits greater ring growth after 1920, noticeably in the mid 1920s, during the late 1930s/early 1940s, during the mid 1950s and the mid 1960s. The relative increase in tree growth, above the long-term mean, during the 20th century is greatest at the POR and YAD sites. The mean level of growth is also clearly above the long-term mean at JAH. The relative level of growth is lowest at KHAD. While trees at each of the sites that we originally used show high average growth after 1970, the KHAD trees upon which McIntyre focused exhibit anomalously low growth, below the long-term mean, at this time.
This reveals that exclusive use of the KHAD data (i.e. the most 'extreme' of McIntyre's alternative chronologies) likely provides an atypical representation of the more general long-term course of changing tree growth, as represented by the data from the other sites.
A chronology using only the recent data from either POR or YAD will exhibit a greater 20th century increase in growth than one based on JAH, but one based only on KHAD, as in McIntyre's experiment, is the most anomalous and, therefore, arguably the least defensible. With no additional information with which to justify the exclusion of any of these data, we have produced a chronology using the measurements from all 4 sites (see Figure E and Figure F).
When this Yamal_All chronology is plotted alongside the Briffa et. al. (2008) chronology and the McIntyre versions (i.e. using the data selected by McIntyre but produced using our current implementation of the RCS), the "KHAD-only" chronology is clearly anomalous in the recent period compared to the other series and is very unlikely to present a realistic indication of comparative tree-growth changes representative of the wider region in a way that is consistent through time. The 'living' chronologies, at POR, JAH and YAD and the alternative McIntyre version (where the KHAD data are included along with the original Hantemirov and Shiyatov data), all show similar notably increasing growth around the beginning of the 20th century and relatively high growth maintained thereafter.
We note that in the previously published Yamal chronologies, the last 2 years (1995 and 1996) were made up of data from only 5 trees, all from the YAD site. The previous 6 years (1989 to 1994) were based on data from only 10 trees, originating from the POR and YAD sites. Therefore, the most recent section of the chronology was somewhat biased by the proportionately greater growth shown at these two sites compared to that at JAH which ended in 1988. The reworked chronology now contains data from 20 trees up to 1994 and 10 trees in 1995 and 1996, but again for the last 5 years they come only from the POR and YAD sites which show higher growth rates in recent decades compared with JAH (which now ends 1991) and, particularly, KHAD. The slower-growth KHAD site data end in 1990, so that our published and new versions of the chronology (the latter containing KHAD data) are directly comparable only up until 1990. This does not mean that our published or reworked Yamal_All versions of the chronology present an incorrect impression of relatively high tree growth after 1990. It means that the only available evidence comes from the (faster growing) POR and YAD sites, so the magnitude of the post-1990 increase may be slightly elevated.
This Figure shows the two earlier versions of the Yamal RCS larch chronology in red (published in Briffa, 2000) and blue (Briffa et al., 2008) compared to the new version, based on all of the currently available data (Yamal_All) for the original (POR, YAD and JAH) sites and including the additional data from the KHAD site (in black). Tree sample counts for this 'new' chronology are shown by the grey shading. The upper panel shows the data smoothed with a 40-year low-pass cubic smoothing spline. The lower panel shows the yearly data from 1800 onwards. All series have been scaled so the yearly data have the same mean and standard deviation as the Yamal_All series over the period 1-1600.
The new chronology based on all of the available data up until 1990 can be considered as a more 'conservative' indicator of the likely history of wider-regional tree growth in the southern Yamal area. This new Yamal_All series exhibits a slightly lower level of mean tree growth over the 20th century than is shown in previously published series (see Figure above). It nevertheless provides essentially the same picture of relatively high tree growth in the 20th century in the context of the last two millennia. A number of other studies have also reported enhanced germination and growth of larch trees during the 20th century in north central Russia (Devi et al. 2008, MacDonald et al. 2008, Shiyatov 2009).
If one accepts a link between enhanced tree-growth and warmer summers in this region, the re-examined evidence indicates that the "20th century was unusually warm" as was concluded in our previous publications (e.g. Briffa et al. 2008, p2281).
McIntyre states "If the non-robustness observed here prove out .. this will have an important impact on many multiproxy studies ...". We have shown here that the "KHAD only" example constructed by McIntyre itself represents a biased chronology, contradicted by the evidence of other chronologies constructed using additional and more representative site data. The evidence does not support a conclusion that our previous work was in any way seriously flawed. The last 8 years of our chronology ARE based on data from a decreasing number of sites and trees and this smaller available sample does emphasise the faster growing trees, so this section of the chronology should be used cautiously. The reworked chronology, based on all of the currently available data is similar to our previously published versions of the Yamal chronology demonstrating that our earlier work presents a defensible and reasonable indication of tree growth changes during the 20th century, and in the context of long-term changes reconstructed over the last two millennia in the vicinity of the larch treeline in southern Yamal.
This does not mean that these chronologies will not change as additional data become available and as the RCS processing technique evolves, but the results we show here do suggest that McIntyre's sensitivity analysis has little implication for those other proxy studies that make use of the published Yamal chronology data.
When using the RCS technique, it is important to examine the robustness of RCS chronologies, involving the type of sensitivity testing that McIntyre has undertaken and that we have shown in this example. Indeed, we have said so before and stressed in our published work that possible chronology biases can come about when the data used to build a regional chronology originate from inhomogeneous sources (i.e. sources that would indicate different growth levels under the same climate forcing). In the case of the Yamal chronology. However, such tests (i.e. those described here) do not change the interpretation that tree growth has been unusually high and summers warm during the 20th century compared to the evidence of the previous two millennia.
While we re-emphasise that further experimentation with the RCS technique is warranted we would also stress that it is imperative that much more sub-fossil and recent data are collected in this and other high-latitude and high-elevation (near tree-line) sites to facilitate more detailed research into the character and causes of tree growth changes. The IPCC Fourth Assessment Report has also emphasised this: "... this assessment would be improved with extensive networks of proxy data that run right up to the present day. This would help measure how the proxies responded to the rapid global warming observed in the last 20 years and it would also improve the ability to investigate the extent to which other, non-temperature, environmental changes may have biased the climate response of proxies in recent decades." (Jansen et al. 2007, page 483)
The Climatic Research Unit has never been a prolific producer of tree-ring records, focussing mainly on the collaborative analysis of data generously provided by other institutions. We will continue to respect restrictions placed upon the dissemination of data by those colleagues who provide them. All of the data produced at CRU (sampled from living oaks or pines at various sites around the UK and Scandinavia) have been provided on request. (All of the data used or produced in the analysis described here are provided on the Data page.)
We would normally aim to present our work in the peer-reviewed literature. We intend to submit some of this material for publication as part of a wider discussion updating our interpretation of Yamal and other published chronologies.
Update: 14 June 2013. Our intention to present our work in the peer-reviewed literature has fulfilled by the publication of Briffa et al. (2013).
Briffa K.R., Melvin T. M., Osborn T. J., Hantemirov R. M., Kirdyanov A. V., Mazepa V., Shiyatov S. G. and Esper J. (2013) Reassessing the evidence for tree-growth and inferred temperature change during the Common Era in Yamalia, northwest Siberia. Quaternary Science Reviews 72, 83-107. doi: 10.1016/j.quascirev.2013.04.008
This paper, together with supporting data and software, are freely available from our website.
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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.
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