Cool surfaces and shade trees to reduce energy use and improve air qualityin urban areas

Elevated summertime temperatures in urban 'heat islands' increase cooling-e nergy use and accelerate the formation of urban smog. Except in the city's core areas, summer heat islands are created mainly by the lack of vegetatio n and by rile high solar radiation absorptance by urban surfaces. Analysis of temperature trends for the last 100 years in several large U.S. cities i ndicate that, since similar to 1940, temperatures in urban areas have incre ased by about 0.5-3.0 degreesC. Typically, electricity demand in cities inc reases by 2-4% for each 1 degreesC increase in temperature. Hence, we estim ate that 5-10% of the current urban electricity demand is spent to cool bui ldings just to compensate for the increased 0.5-3.0 degreesC in urban tempe ratures. Downtown Los Angeles (L.A.), for example, is now 2.5 degreesC warm er than in 1920, leading to an increase in electricity demand of 1500 MW. I n L.A., smoggy episodes are absent below about 21 degreesC, but smog become s unacceptable by 32 degreesC. Because of the heat-island effects, a rise i n temperature can have significant impacts. Urban trees and high-albedo sur faces can offset or reverse the heat-island effect. Mitigation of urban hea t islands can potentially reduce national energy use in air conditioning by 20% and save over $10B per pear in energy use and improvement in urban air quality. The albedo of a city may be increased at minimal cost if hi,oh-al bedo surfaces are chosen to replace darker materials during routine mainten ance of roofs and roads. Incentive programs, product labeling, and standard s could promote the use of high-albedo materials for buildings and roads. S imilar incentive-based programs need to be developed for urban trees. Publi shed by Elsevier Science Ltd.