Tree leaves block sunlight as well, cooling islands of heat generated by hard city surfaces. The temperature of asphalt or concrete under a shade tree can be as much as 36°F (20°C) cooler than a patch of pavement in full summer sun; the air up under the canopy of mature trees may be five to ten degrees cooler.

Parks and gardens are also essential to human social and psychological well-being. Without access to grass and trees, says Frances Kuo, we humans are very different creatures. For the past decade, Kuo and her colleagues at the Landscape and Human Health Laboratory of the University of Illinois have researched the effects of green space on city dwellers. The team carries out many of its studies in Chicago's public housing neighborhoods, where barren expanses of hardscape reflect the old view that vegetation is an extravagance the city can't afford.

One sequence of studies focused on residents of the Robert Taylor Homes, a cluster of 28 identical high-rise buildings, now mostly torn down, that formed the nation's largest public housing development. Some of the buildings were surrounded by grass and trees, others by concrete and asphalt. Kuo and her team discovered that people living in buildings near green areas had a stronger sense of community and coped better with everyday stress and hardship. They were less aggressive and less violent, they performed better on tests of concentration, they managed their problems more effectively.

They also felt safer—and with good reason. In one of its more startling findings, the team upended the common belief that barren spaces are safer than green ones. A study of violent crime in a housing project of 98 apartment buildings showed that in and around buildings near vegetation that didn't hamper visibility there were only half as many crimes as in areas near no vegetation. The greener the surroundings, says Kuo, the lower the crime rate against people and property. The team also found less litter and graffiti in natural landscapes.

In their most recent research, a national study of 450 children ages five to eighteen, the scientists discovered that children with attention deficit disorders showed reduced symptoms when they were exposed to natural environments. After play in verdant settings, parents reported that the children's ability to concentrate, complete tasks, and follow directions improved dramatically—in all age groups, in all parts of the country.