When I lived in San Francisco, I used to ride my bike across the Golden Gate bridge to Marin County. I would ride in the hills for 20-30 miles and then return. I could always feel the temperature start to rise when I made it back to the city, especially on hot days. Although I didn't know it, I was experiencing what is called the "heat island" effect, which is basically caused by buildings and pavements taking the place of trees and other vegetation. The buildings and pavement absorb more heat and cause cities to be hotter than rural areas by as much as 3 to 8 degrees. Trees and vegetation provide shade and also give off water that evaporates and cools their surfaces and surrounding air. Thus the gaining popularity of "green roofs." Although aestically pleasing to surrounding buildings and to people that have access to their premises, green roofs also provide a positive environmental impact.
A green roof can be a huge asset to any building. With a green roof, the building will absorb less heat, which means less energy is needed to cool the building...which also reduces the overall HVAC size requirements of a building. In addition to energy, a green roof will absorb more rain water, reducing the need for complex stormwater management techniques. Other advantages of a green roof include: durability, accoustical comfort, energy efficiency, heat island reduction, human well-being, outdoor air quality, water efficiency.
If you don't use a green roof, using concrete or materials that have a higher solar reflectance, or albedo, will help reduce the heat island effect, saving energy by reducing the demand for air conditioning, and improve air quality.