Figure 3. Reconstructed Northern Hemisphere temperatures since 1000 A.D. (Mann et al., 1999), referenced to 1902-1980 mean. Preinstrumental data are derived from a spatially weighted combination of twelve proxy climate indices based on tree ring and ice core data, calibrated against twentieth century instrumental record (red curve). Yellow shading shows an envelope of uncertainty (2 s) of annual temperature estimates. A modest cooling trend is suggested for most of the millennium, although uncertainties are large prior to 1400 A.D. Twentieth century warming is large, abrupt, and significant in comparison with variability exhibited in earlier centuries.