The above charts are snapshots from an animation (see below) which shows the evolution of the summer temperature distribution in the Northern Hemisphere from the 1950s to present where “normal” temperatures are shaded in gray (in the middle), cold temperatures in blue (to the left), and warm temperatures in red (to the right). The height of these temperature designations indicates their frequency over any given period.
The green curve illustrates the distribution of temperatures between 1951-1980 against which the evolving temperatures (in 11-year increments) are compared.
The charts above plainly show how much statistically warmer the last decade (2001-2011) was compared to the 1950s (and the 1951-1980 average or green curve). Notice how red and blue were balanced in the 1950s. Now red dominates.
Animation of the summer temperature distrubition in the Northern Hemisphere since the 1950s from NASA
You can see the steady shift in temperatures over time in the animation above.
NASA describes the dramatic warming of the temperature distribution like this:
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 [difference from average]. 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.
Essentially, the summer climate in the Northern Hemisphere has reached an entirely different state compared to 60 years ago.