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Temperatures over most of the past 2000 years compared to the 1961-1990 average, based on proxy data (tree rings, ice cores, corals) and modern thermometer-based data. Over the past two millenia, climate warmed and cooled, but no previous warming episodes appear to have been as large and abrupt as recent global warming. NOAA Climate.gov graph by Fiona Martin, adapted from Figure 34.5 in the National Climate Assessment, based on data from Mann et al., 2008.