Paleoclimatologists face a dilemma. No sedimentary proxy is a pure recorder of quantitative climate information. Yet climate modelers and policy-makers increasingly seek quantitative comparisons between instrumentally documented, possibly anthropogenic, climate changes and those produced naturally in the past. In the second edition of his influential book, Bradley (1999, p. 6) discussed the calibration of proxy records to learn past climate changes: “Calibration involves using modern climatic records and proxy materials to understand how, and to what extent, proxy materials are climate-dependent. It is assumed that the modern relationships observed have operated, unchanged, throughout the period of interest (the principle of uniformitarianism).” In other words, one relates the characteristics of sediment to climate at different places for one time, or at a place for short times, and then uses that relation plus characteristics of older sediments to estimate the climatic conditions that produced those sedimentary characteristics. Bradley (1999) then extensively discussed the difficulties in applying this methodology in a complex world with imperfect recorders; nonetheless, the goal of using calibrated proxies for quantitative as well as qualitative paleoclimatic reconstruction is clear. A prominent recent use of calibrated paleoclimatic data is the assessment of whether the probably-anthropogenic warming of the latter 20th century moved beyond the band of natural variability of the prevailing climate. Bradley (2000) combined recent instrumental records with several longer proxy-based reconstructions of surface temperature including that of Mann et al. (1999). Based on this composite data set, Bradley (2000) argued that “temperatures in the late 20th century were unique in the context of the entire millennium”. The proxy records were primarily based on tree-ring data, but included isotopic and major-element geochemistry of corals, and occurrence of melt layers and isotopic ratios of water in ice cores. However, Broecker (2001) questioned the basis for the reconstruction of …
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Alley et al. (2001) studied this question.