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Summary
Reconstructing past climates is important if we are to distinguish anthropogenic warming from natural variability in the climate system. Briffa and Osborn discuss a recent study by Mann et al. that has extended their Northern Hemisphere climatic reconstruction to A.D. 1000 and explain why there are large uncertainties in the reconstruction of medieval and earlier temperatures on large spatial scales. Thus, although the 20th century was almost certainly the warmest of the millennium, the amount of anthropogenic warming remains uncertain.
