The Climate of the Last Millennium
We are living in unusual times. Twentieth century climate was dominated by near universal warming with almost all parts of the globe experiencing temperatures at the end of the century that were significantly higher than when it began (Figure 6.1) (Parker
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6.1 Introduction We are living in unusual times. Twentieth century climate was dominated by near universal warming with almost all parts of the globe experiencing temperatures at the end of the century that were significantly higher than when it began (Figure 6.1) (Parker et al. 1994, Jones et al. 1999). However the instrumental data provide only a limited temporal perspective on present climate. How unusual was the last century when placed in the longer-term context of climate in the centuries and millennia leading up to the 201h century? Such a perspective encompasses the period before large-scale contamination of the global atmosphere by human activities and global-scale changes in land-surface conditions. By studying the records of climate variability and forcing mechanisms in the recent past, it is possible to establish how the climate system varied under "natural" conditions, before anthropogenic forcing became significant. Natural forcing mechanisms will continue to operate in the 21 ' 1 century, and will play a role in future climate variations, so regardless of how anthropogenic effects develop it is essential to understand the underlying background record of forcing and climate system response. Sources of information on the climate of the last millennium include: historical documentary records, tree rings (width, density), ice cores (isotopes, melt layers, net accumulation, glaciochemistry), corals (isotopes and other geochemistry, growth rate), varved lake and marine sediments (varve thickness, sedimentology, geochemistry, biological content) and banded speleothems (isotopes). These are all paleoclimatic proxies that can provide continuous
records with annual to decadal resolution (or even higher temporal resolution in the case of documentary records, which may include daily observations, e.g. Bnizdil et al. 1999, Pfister et al. 1999a,b, van Engelen et al. 2001). Other information may be obtained from sources that are not continuous in time, and that have less rigorous chronological control. Such sources include geomorphological evidence (e.g. from former lake shorelines and glacier moraines) and macrofossils that indicate the range of plant or animal species in the recent past. In addition, ground temperature measurements in boreholes reflect the integrated history of surface temperatures, with temporal resolution decreasing with depth. These provide estimates of overall ground surface temperature changes from one century to the next (Pollack et al. 1998, Huang et al. 2000). Proxies of past climate are natural archives that have, in some way, incorporated a strong climatic signal into their structure (Bradley 1999). For some biological proxies, such as tree ring density or coral band width, the main factor might be temperatureor more specifically, the temperature of a particular season (or even just part of a season). Ring density and width can also be influenced by antecedent climatic conditions, and by other non-climatic factors. Similar issues are important in other proxies, such as the timing of sn
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