Shifting Distribution of Northern Hemisphere Summer Temperature Anomalies, 1951-2011

As the graph moves forward in time, the bell curve shifts to the right, representing an increase in the frequency of the various hot anomalies. It also gets wider and shorter, representing a wider range of temperature extremes. As the graph moves beyond 1980, the temperatures are still compared to the seasonal mean of the 1951-1980 base period, so that as it reaches the 21st century, there is a far greater frequency of temperatures that once fell 3 standard deviations beyond the mean.

As the graphic indicates, each bell curve shown through the time series represents the distribution of anomalies over an 11-year period.