Global warming is worsening heat waves that torture cities. In Louisville, Kentucky, the government is looking into large scale tree planting as a way to cut down on heat buildup. In the meantime, Los Angeles is using another tool against urban warming. The city is painting some of its streets white. By covering blacktop asphalt with a more reflective “cool pavement” treatment, Los Angeles authorities claim they’ll reduce temperatures on a summer afternoon by ten degrees or more.