Here is the next plot of Berkeley Earth Surface Temperature data to consider for the Contiguous United States. If the mean global temperature is meaningful to some people, then the mean surface temperature of the contiguous United States must be meaningful as well :).
So take a few minutes and study the plot below. I reached the limit of my software as to the number of years that could be displayed. This plot shows the earliest year for which data is available (1743) to 2004.