I guarantee there have been times the uv index at your latitude has exceeded 13 near the summer solstice. There are archives to this , look though them.
UV radiation is what is killing the trees. Their leaves are thin and completely penetrated by it, and not designed or adapted for the 90% increase in uv radiation in the past 40 years. The conifers do between because of the structure of their leaves and the ability to shield themselves, they seem better adapted to it that flat leaved trees. Half of the trees in my yard are dead and 20 years ago they were all strong and healthy, My neighbor has lost three oaks known to be over over two hundred and fifty years old.