Urban Heat Island Effect

Many US cities are "urban heat islands", meaning that on most summer days and nights temperatures are higher in the city than in the surrounding suburbs. A heat island develops when buildings, roads, sidewalks, and rooftops replace natural vegetation. Unlike natural surfaces that moderate temperature through evaporation from soils, transpiration from plants, and shading, the built surfaces are made of materials like brick, asphalt, and concrete which are impervious to water. These materials also have low albedos and high heat capacities, meaning that they are good at absorbing and retaining the sun's radiation during the day.

The impacts of the urban heat island effect include an increase in energy demand for air conditioning, the production of photochemical pollutants such as smog-creating ozone, and respiratory and heat stress-related illness.

By creating a vegetated surface, green roofs reduce outdoor air temperature and the urban heat island effect through transpiration from plants, shading, and increased reflectivity (albedo) of the roof. Additionally, because a vegetated roof is significantly cooler than a non-vegetated roof on hot days, less heat is transferred through the ceiling into the room below, reducing indoor air temperature and thus the demand for air conditioning. Therefore, green roofs can help reduce fossil fuel consumption, and thus greenhouse gas emissions — the major cause of global warming.