Here is an animation from NASA showing the distribution of summer temperatures in the Northern Hemisphere. The initial smooth distribution is based on data from 1950-1980. The video advances in yearly increments showing data for 10 year periods from 1950-1960 through to 2001-2011. As the data advances one can see a pronounced shift in the curve to the right and also, a little less clearly, the curve gets shorter and fatter. Thus, not only are we seeing an increase in the mean temperature but also a greater possibility for extremes in temperature around the increased mean.

The visualization is an extension of ideas from a paper by Hansen et al. which includes additional, global data.

Addendum: Here are graphs using 1930-1980 as the baseline.