I am not a scientist but I've always had a sneaking suspicion that blacktop converting light from the sun into heat had something to do with it. I'd actually LOVE to hear what a scientist thought of this hypothesis.

I also wonder if all the coal we material we burned for heat (coal and such) didn't have a cooling effect - particularly in the cities where a lot of the accurate temperature measures were taken. Again, not a scientist - truly curiously.