Climates of the past: evidence from natural and documentary archives

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  • Editorial de and Env nd viden nce he la ch (I n of enta climate records include the interpretation of documentary ns might also be of use to the climate modelling community in their attempts to reduce northern Europe, the eastern Mediterranean and the Sahel JOURNAL OF QUATERNARY SCIENCE (2009) 24(5) 411–414 Copyright � 2009 John Wiley & Sons, Ltd. Published online in Wiley InterScience ( DOI: 10.1002/jqs.1300 E-mail: Contract/grant sponsor: European Commission (Millennium project; Contract/ Loader, 2004). This can be facilitated through the novel analysis of conventional archives, through examination of a Linderholm et al. (2009) use a 550-year annually resolved dendroclimatological reconstruction of the summer North Atlantic Oscillation (SNAO) to investigate links between the SNAO and drought across the eastern North Atlantic. They found an association between the SNAO and drought in * Correspondence to: I. Robertson, Department of Geography, Swansea Univer- sity, Swansea SA2 8PP, UK. records and the measurement of the physical and chemical properties of natural archives such as tree rings, corals, ice cores, terrestrial sedimentary sequences and marine records. One of the problems with existing proxy-based reconstructions is that relatively few records cover the entire last 1000 years (Jones et al., 2009) and the most well known of such reconstructions are heavily dominated by a relatively small number of records (Jones et al., 1998; Mann et al., 1998, 1999; Briffa, 2000; Esper et al., 2002), many of which may systematically underestimate climatic variability (Osborn and Briffa, 2004; von Storch et al., 2004, 2006), although this is still the subject of considerable debate (Wahl et al., 2006; Wahl and Ammann, 2007). A more reliable picture of past natural climate variability may be obtained by establishing new, longer-term proxy records and through a combination of independent records (Lough and Barnes, 1997) where averaging multiple time series can dampen the non-climatic signal (McCarroll and uncertainty in estimates of future climate change. Since the calibration of the relationship between proxy and climate is usually dominated by high-frequency variability (Esper et al., 2004), the reconstruction of lower-frequency trends has remained an elusive challenge. Recently, lower- frequency climatic variability has been retained successfully from proxies at a lower temporal resolution or by detrending tree proxies with novel methods (Briffa et al., 1996, 2001; Osborn and Briffa, 2000; Esper et al., 2002; Mann and Jones, 2003; Moberg et al., 2005; Grudd, 2008). One of the advantages of using sedimentary archives with a lower temporal resolution is that the maximum period of proxy information is not as limited by the segment length of the constituent time series (Cook et al., 1995). This special issue presents some recent developments from both tree ring research and the development of proxies with a lower temporal resolution. Although there has been considerable success in reconstructing Northern Hemisphere temperatures (IPCC, 2007), there is still uncertainty associated with past climatic variability and the reconstruction of lower-frequency (decadal to multi- centennial) changes. As instrumental climatic records rarely extend back further than a few centuries, additional climatic wider range of climatic variables, and through improvements in chronological control which permit the incorporation of traditionally non-annually resolved records into climatic reconstructions. New multi-proxy archives, with increased replication, would allow greater confidence in the early part of millennial length climate reconstructions. In addition, statisti- information is required from other indirect sources. These proxy cally robust climatic reconstructio KEYWORDS: climatic reconstruction; multi-proxy; millennium; time series. Climates of the past: evi documentary archives IAIN ROBERTSON,1* CYNTHIA A. FROYD,2 MARY GAGEN1 1 Department of Geography, Swansea University, Swansea, UK 2 Long-Term Ecology Laboratory, School of Geography and the 3 Department of Geosciences, University of Oulu, Oulu, Finla Robertson, I., Froyd, C. A., Gagen, M. and Hicks, S. 2009. Climates of the past: e 414. ISSN 0267-8179. Received 22 April 2009; Accepted 5 May 2009 ABSTRACT: This special issue of the Journal of Quaternary Scie upon a thematic session aimed at reconstructing climate of t archives, held at the International Union for Quaternary Resear Australia. New techniques are presented to enable the extractio and documentary archives with the aim of preserving environm scales. Copyright # 2009 John Wiley & Sons, Ltd. grant sponsor number: 017008-2) nce from natural and SHEILA HICKS3 ironment, University of Oxford, Oxford, UK ce from natural and documentary archives. J. Quaternary Sci., Vol. 24 pp. 411– contains a set of 12 papers based st 1000 years from multi-proxy NQUA) XVII Congress in Cairns, proxy climatic data from natural l information across all temporal region of northern Africa. The relationship in the eastern
  • 412 JOURNAL OF QUATERNARY SCIENCE Mediterranean was clearest at the low-frequency, centennial scale, and both low- and high-frequency signals were observed in the Sahel. Buhay et al. (2009) use the analysis of stable oxygen and carbon isotopes from lake sediments to provide a 1000-year record of dry conditions in the eastern Canadian Prairies, finding evidence of four distinct drought periods. Understanding the frequency, severity and causal mechanisms driving drought events in the past is important for projecting regional impacts under future climatic change scenarios. The analysis of extreme climatic events and their impacts is also made possible through the inclusion of hermeneutic datasets in reconstructions, as demonstrated by Glaser and Riemann (2009). They use documentary evidence to examine climatic variability in Germany and central Europe over the last 1000 years. Documentary sources have the unique advantage in that they provide information from the warm and cold seasons unlike many of the biological proxies, allowing the reconstruction of climatic variables that cannot typically be obtained from natural archives, variables such as winter temperature, precipitation and pressure patterns. The resulting dataset for Germany and central Europe is one of the few continuous annually resolved documentary records covering the last 1000 years (Bra´zdil et al., 2005), making it a valuable source for inclusion in multi-proxy reconstructions. Helama et al. (2009) used tree rings to reconstruct regional- scale climatic variability over the last ca. 1200 years in northern Finland, postulating that a multi-decadal oscillation observed in the regional climate throughout the Medieval Warm Period (MWP) can be attributed to instability in the formation of North Atlantic Deep Water (NADW). They suggest that this may provide an explanation for discrepancies in timing of the MWP onset between proxy-based and modelled reconstructions. Knudsen et al. (2009) present a high-resolution analysis of palaeoclimatic proxies within marine sedimentary sequences from the North Icelandic Shelf, an area sensitive to climatic change as a result of its position at the boundary region separating temperate and Arctic conditions. Benthic and planktonic foraminifera, ice-rafted debris concentrations and diatoms were analysed and the findings compared with instrumental and documentary data over a 130-year calibration period, which was then used to reconstruct palaeoenviron- mental conditions of the last 1000 years. Knudsen et al. (2009) found a strong connection between atmospheric and oceanic changes, with shifts in reconstructed sea surface temperatures and ice-rafted debris concentrations interpreted as resulting from past migrations of the Polar Front. Increased influence of Atlantic water masses was shown to have occurred during the MWP, whereas the period encompassing the Little Ice Age was characterised by the increasing influence of Arctic waters. The predominance of annually resolved temperature recon- structions for the Northern Hemisphere is based on the analysis of the physical characteristics of tree growth rings, such as ring width and relative density. In many regions, however, tree ring measures alone do not provide enough information to fully capture climatic variability (Hilasvuori et al., 2009). A novel proxy measure that can be used to provide annually resolved records of past environmental conditions is the analysis of stable isotopes in tree rings (McCarroll and Loader, 2004). Hilasvuori et al. (2009) demonstrate the application of this technique for Scots pine (Pinus sylvestris) growing at two different sites in Finland. They reconstructed summer tempera- ture over the last 400 years based upon the relationship between carbon isotopes from pooled tree ring cellulose and summer temperature. As with several other high-frequency proxies, they also noted that the temporal variability of the climate signal has changed in recent years (Aykroyd et al., Copyright � 2009 John Wiley & Sons, Ltd. 2001), with serious implications for calibration exercises. This technique has progressed far beyond the pioneering work of the 1970s and now offers the potential to extract lower-frequency (decadal to multi-centennial) climatic signals (Gagen et al., 2007) over millennia (Treydte et al., 2006). Radiocarbon dating uncertainties, over periods such as the modern radiocarbon plateau (ca. AD 1650–1955), have restricted the adoption of many low-frequency proxies, such as terrestrial sedimentary sequences, as the dating errors have been prohibitively large in the modern era. However, Goslar et al. (2009) present an elegant method for improving age– depth modelling in sedimentary archives, using a free-shape algorithm and the incorporation of pollen concentration results. Although this pioneering method is partly subjective, it is a considerable improvement over a visual fitting of age–depth profiles (Goslar et al., 2005). Wastega˚rd and Davies (2009) highlight the potential use of tephrochronology as a correlation tool for improving chronological models in northern European sedimentary archives over the last 1000 years, identifying key tephra horizons. Increased timescale precision will facilitate the more effective inclusion of non-annually resolved archives, such as peat and non-laminated lake sediments, in palaeocli- matic reconstructions with a higher degree of confidence. Kuoppamaa et al. (2009) describe the potential use of arboreal fossil pollen accumulation rates in peat sequences as a record of inter-annual temperature variation. This work is supported by the findings of Huusko and Hicks (2009), who demonstrate the correlation, over a 23-year period, between pollen production of pine and spruce at the boreal forest limit and July temperatures of the preceding year. Kamenik et al. (2009) present a pollen-based climate inference model for the last 150 years from a near-annually resolved peat sequence in the Swiss Alps, using Goslar’s aforementioned dating algorithm in combination with chronological ’tie points’ for increased precision. They found that pollen percentage measurements of six key taxa provided the best predictive model for reconstruct- ing temperature, and that the model best reflected decadal- scale variability. The most powerful tools available to help us try and predict how Earth’s climate will evolve in the future are general circulation models (GCMs). These models are complicated and not without their flaws, but they contain all that we know about the way the climate system works and they are the best tool that we have for predicting the future. The accuracy of the GCM runs is checked against instrumental climate data over a relatively short period of time (usually ca. AD 1850–present). At the moment the comparisons with the instrumental data do not place a tight enough constraint on the models and as such our estimates of future change have very large uncertainties. These uncertainties are large enough to be problematic for those who are planning how our societies will adapt in the future. If the palaeoclimate community can provide clear evidence of the climate changes of the past, we can help to strengthen the way that the models are weighted and potentially reduce the uncertainty estimate. Comparing palaeoclimate data with model data is a significant quantitative challenge beyond the scope of this issue but it would be an effort well rewarded. Tan et al. (2009) highlight that longer climate records spanning a wider range of climate states might usefully help in assessing the skill of the GCMs for simulating climates different from the present. In their comparison of a tree ring/stalagmite record from China with a 1000-year model run they highlight several significant chal- lenges in the effort to bring together the proxy and modelling worlds; first, in their discussion of how best to combine proxy records from different sources and, second, in their highlighting of potential weaknesses in both proxy and model data. J. Quaternary Sci., Vol. 24(5) 411–414 (2009) DOI: 10.1002/jqs
  • ing the message of ancient trees. Quaternary Science Reviews 19: 6. w- frequency signals. In Climatic Variations and Forcing Mechanisms of the Last 2000 Years, Jones PD, Bradley RS, Jouzel J (eds). Springer: Berlin; 9–41. EDITORIAL 413 Briffa KR, Osborn TJ, Schweingruber FH, Harris IC, Jones PD, Shiyatov SG, Vaganov EA. 2001. Low frequency temperature variations from a northern tree-ring density network. Journal of Geophysical Research 106: 2929–2941. Buhay WM, Simpson S, Thorleifson H, Lewis M, King J, Telka A, Wilkinson P, Babb J, Timsic S, Bailey D. 2009. A 1000-year record of dry conditions in the eastern Canadian prairies reconstructed from oxygen and carbon isotope measurements on Lake Winnipeg sedi- ment organics. Journal of Quaternary Science 24: 426–436. Cook ER, Briffa KR, Meko DM, Graybill DA, Funkhouser G. 1995. The ’segment length curse’ in long tree-ring chronology development for palaeoclimatic studies. The Holocene 5: 229–237. Esper J, Cook ER, Schweingruber FH. 2002. Low frequency signals in long tree-ring chronologies for reconstructing past temperature varia- bility. Science 295: 2250–2253. Esper J, Frank DC, Wilson RJS. 2004. Climate reconstructions: low frequency ambition and high frequency ratification. EOS 85: 113, 120. Esper J, Wilson RJS, Frank DC, Moberg A, Wanner H, Luterbacher J. 2005. Climate: past ranges and future changes. Quaternary Science Reviews 24: 2164–2166. Gagen M, McCarroll D, Loader NJ, Robertson I, Jalkanen R, Anchukaitis KJ. 2007. Exorcising the ’segment length curse’: summer temperature reconstruction since AD 1640 using non-detrended stable carbon isotope ratios from pine trees in northern Finland. The Holocene 17: 435–446. 87–105. Briffa KR, Jones PD, Schweingruber FH, Karlen W, Shiyatov G. 199 Tree ring variables as proxy climate indicators: problems with lo The 12 papers (from 27 presentations) selected from the Climate of the last 1000 Years thematic session from the International Union for Quaternary Research (INQUA) XVII Congress held in Cairns, Australia, in 2007 reflect the immense interest and relevance of past environmental change and its potential for elucidating future trends. Numerous authors (e.g. IGBP, 1990; Maslin and Berger 1997; Esper et al., 2005) have stressed the need for annually resolved proxy climatic records over one or two millennia to capture accurately natural climatic variability. As recent warming in the Northern Hemisphere appears to be anomalous in the last 1300 years (Jansen et al., 2007; Mann et al., 2008), additional proxy data are required (Jones et al., 2009) to improve the spatiotemporal range and enhance the climatic signal at different frequencies to put the anthropogenic influences upon climate into perspective. The studies presented in this special issue start to address these concerns. Acknowledgements The authors are grateful to the European Com- mission (Millennium project; grant 017008-2), the Quaternary Research Association (IR, CAF) and the Royal Society (IR) for financial assistance. CAF thanks the Natural Environment Research Council for continuing support (NE/C510667/1). References Aykroyd RG, Lucy D, Pollard AM, Carter AHC, Robertson I. 2001. Temporal variability in the strength of proxy-climate correlations. Geophysical Research Letters 28: 1559–1562. Bra´zdil R, Pfister C, Wanner H, von Storch H, Luterbacher J. 2005. 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