From Newsweek this week:
http://www.msnbc.msn.com/id/17996836/site/newsweek/page/3/
"The greening of city centers isn't just metaphorical. In Chicago, city officials have mapped the "heat islands," where asphalt and black roofs absorb heat and raise the city's overall temperature. These areas are then planted with trees—”400,000 to date. "Trees are like big air conditioners," says Environment Commissioner Sadhu Johnston. They lower the temperature, filter air, remove carbon dioxide, absorb storm water and provide shade and beauty. (They even boost the economy. "Research has shown that people are willing to spend up to 12 percent more on a product if they're shopping in a district with mature trees," thanks to the pleasant ambience of the neighborhood, says Johnston.)"