Heterogeneous warming of Northern Hemisphere surface temperatures over the last 1200 years

The relationship between the mean and spatial variability of Northern Hemisphere surface temperature anomalies over the last 1200 years is examined using instrumental and proxy records. Nonparametric statistical tests applied to 14 well-studied, annually resolved proxy records identify two centuries roughly spanning the Medieval Climate Anomaly as characterized by increased spatial variability relative to the preinstrumental baseline climate, whereas two centuries spanning the Little Ice Age are characterized by decreased spatial variability. Analysis of the instrumental record similarly indicates that the late and middle twentieth century warm intervals are generally associated with increased spatial variability. In both proxy and instrumental records an overall relationship between the first two moments is identified as a weak but significant positive correlation between time series of the spatial mean and spatial standard deviation of temperature anomalies, indicating that warm and cold anomalies are respectively associated with increased and reduced spatial variability. Insomuch as these historical patterns of relatively heterogeneous warming as compared with cooling hold, they suggest that future warming will feature increased regional variability.