Summer heat islands, urban trees, and white surfaces
Temperature trends for the last 100 years in several US cities were analyzed. Since approximately 1940 there has been a steady overall increase in urban temperatures. Summer monthly averages have increased by 0.25-1°F per decade (approximately 1°F for larger cities like Los Angeles and 0.25°F for smaller cities). There is no evidence that this rise is moderating, and of course global greenhouse warming will add a comparable rise. Typical electric demand of cities increases by 1% to 2% of the peak for each °F and most major cities are now approximately 5°F warmer than they were in the early 1900s. Hence, we estimate that about 5% to 10% of the current urban electric demand is spent to cool buildings just to compensate for the heat island effect. For example, downtown Los Angeles is now 5°F hotter than in 1940 and so the L.A. basin demand is up by 1500 MW, worth $150,000 per hour on a hot afternoon (the equivalent national bill is approximately$1M/hour). In major cities, smog episodes are absent below about 70°F, but they become unacceptable by 90°F, so a rise of 10°F because of past and future heat island effects is very significant. There are some strategies that can alleviate the heat island effect. Computer simulations and field studies have quantified the potential of trees and lighter surfaces for reducing summer heat islands. Results indicate that the cost of saved energy and avoided CO2, through greening and whitening of urban areas, is less than 1 cents kWh and 2 cents/kg of carbon, respectively.