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This occurs when the different groups (a) update the primary source data used in the large-scale averages, (b) institute new adjustment procedures, or (c) adopt new spatial or temporal averaging techniques. Thus, a proxy record calibrated or verified using an early version of an instrumental record may be altered slightly if the instrumental data against which the proxy was calibrated changes. SPATIAL SAMPLING ISSUES Deducing the number of sites at which surface temperature needs to be sampled in order to represent variations in global (or hemispheric) mean temperature with a specified level of accuracy is a challenge no less formidable than deducing the temperature variations themselves.

Based on the analyses presented in the original papers by Mann et al. and this newer supporting evidence, the committee finds it plausible that the Northern Hemisphere was warmer during the last few decades of the 20th century than during any comparable period over the preceding millennium. D. 1600 lower our confidence in this conclusion compared to the high level of confidence we place in the Little Ice Age cooling and 20th century warming. Even less confidence can be placed in the original conclusions by Mann et al.

900 because of sparse data coverage and because the uncertainties associated with proxy data and the methods used to analyze and combine them are larger than during more recent time periods. Our confidence in the validity of large-scale surface temperature reconstructions is based, in part, on the fact that the individual proxy data series used to create these reconstructions generally exhibit strong correlations with local environmental conditions. In most cases, there is a physical, chemical, or physiological reason why the proxy reflects local temperature variations.

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