(1) In drawing conclusions regarding past regional temperature changes from proxy records, it is essential to assess proxy data for actual sensitivity to past temperature variability. In some cases (Soon and Baliunas, 2003, Soon et al, 2003) a global 'warm anomaly' has been defined for any period during which various regions appear to indicate climate anomalies that can be classified as being either 'warm', 'wet', or 'dry' relative to '20th century' conditions. Such a criterion could be used to define any period of climate as 'warm' or 'cold', and thus cannot meaningfully characterize past large-scale surface temperature changes.
(2) It is essential to distinguish (e.g. by compositing or otherwise assimilating different proxy information in a consistent manner—e.g., Jones et al., 1998; Mann et al., 1998, 1999; Briffa et al., 2001) between regional temperature changes and changes in global or hemispheric mean temperature. Specific periods of cold and warmth differ from region to region over the globe (see Jones and Mann, 2004), as changes in atmospheric circulation over time exhibit a wave-like character, ensuring that certain regions tend to warm (due, for example, to a southerly flow in the Northern Hemisphere winter mid-latitudes) when other regions cool (due to the corresponding northerly flow that must occur elsewhere). Truly representative estimates of global or hemispheric average temperature must therefore average temperature changes over a sufficiently large number of distinct regions to average out such offsetting regional changes. The specification of a warm period, therefore requires that warm anomalies in different regions should be truly synchronous and not merely required to occur within a very broad interval in time, such as AD 800-1300 (as in Soon et al, 2003; Soon and Baliunas, 2003).
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