Using cool roofs, urban shade trees, and high-albedo pavements to mitigate urban heat islands, can potentially reduce U.S. energy use for air conditioning by 20%, saving over $4 billion per year in energy use. When air quality improvements are accounted for, the total savings could be over $10 billion per year.
Akbari, Hashem, Mel Pomerantz, and Haider Taha. (2001). Cool surfaces and shade trees to reduce energy use and improve air quality in urban areas, Solar Energy, 70 (3), 295-310.