Long-time readers will recognize that the chart is rather similar to that for October mean temperatures, which have similarly become radically less variable; Rick pointed this out some years ago. Here's an updated chart for October mean temperatures.
It's obvious, I think, that the reduction in annual temperature variance partly reflects the October change, but curiosity led me to examine all months of the year to see which other months might have contributed to the annual change. The chart below shows the standard deviation (SD) of monthly mean temperatures for each month, both for the 1930-1990 period and for the past 10 years. October stands out as having the greatest drop in SD, but January and February have also become much less variable.
The interesting part of this analysis is that the average of the SD decrease in all 12 months is only about 24%, whereas the annual SD has dropped by 64%. At first glance this doesn't make sense - how can the annual variance drop so much when the individual months are, on average, only modestly less variable in the modern climate? Part of the answer is presumably that the winter temperature variance contributes more to the annual variance, and the variance has dropped much more in winter than in summer; but it's still puzzling, as no other month besides October comes close to the percentage variance reduction that has occurred on an annual basis.
To explore this question in more detail, I did some simulations of monthly mean temperature variability and combined the months into annual temperature values to see what we would expect from random chance. Specifically, for each year in the history I calculated the observed mean and variance of monthly mean temperatures within a