"largest effect is in southern California, where the urban heat islands blur together to form an “urban heat archipelago” with average temperatures up to 19° F higher in the Riverside-San Bernardino region at the eastern end of the basin."

If it is really 19° F higher in Riverside due to urbanization(hard to believe that really), I can see why correcting historical temps for urban heat island effects is so necessary. How to do that accurately must be fiendishly difficult.