Indication of temperature changes (global warming?)

The file P02_56.xlsx contains monthly values of indexes that measure the amount of energy necessary to heat or cool buildings due to outside temperatures. (See the explanation in the Source sheet of the file.)
These are reported for each state in the United States and also for several regions, as listed in the Locations sheet, from 1931 to 2000.
Create summary measures and/or charts to see whether there is any indication of temperature changes (global warming?) through time, and report your findings