During the last few decades many cities in the south, southwest, and west experienced dramatic population growth. Atlanta, Georgia, was among the fastest growing cities in the United States during the 1990s. The population of the city of Atlanta dropped from 495,039 in 1970 to 394,017 in 1990, but the suburban population in the surrounding 20 counties skyrocketed from 1.8 million to 3 million. The population of the city of Atlanta was 416,474 in 2000. The metropolitan area expanded so much that it is considered to have four central business districts rather than one. Rapid growth that spreads far from the center of a city and uses a lot of land is usually called urban sprawl. When heat builds up in a city it creates a hot spot within an area of cooler countryside. This hot spot is called an urban heat island.
Urban areas display higher temperatures than the more rural surrounding areas. The sun's heat is absorbed by buildings and pavement, causing surface temperatures to rise. The gradual loss of vegetation and the increase of built-up and paved areas cause the formation of hot spots and urban heat islands. Stored-up heat is released at night and causes a dome of high temperature over the city compared to the cooler countryside. Heat islands or hot spots may be found even on the outskirts of an urban area such as at airports that have large areas of pavement.
NASA scientists are studying the relationship between tree removal and urban heat islands in Atlanta, Georgia. Scientists estimate that Atlanta's vegetation and tree cover have declined by 65 percent between 1973 and 1993. The area of tree loss equals 153,781 hectares. The trees were removed to make way for new urban growth.
Rapid population growth in the last 25 years (27 percent between 1970-80, and 33 percent in 1980-90) has made Atlanta one of the fastest growing metropolitan areas in the United States. This rapid growth has translated into an approximately 17 percent decline in forest land in the Atlanta metropolitan region between 1973 and 1992. Atlanta is also a model for what is in store for other cities, especially in developing nations where the industrial revolution is taking off and threatens large areas of forest or jungle. This enormous transformation of land from forest and agriculture to an urban landscape causes changes in land-atmosphere energy balance relationships.
The amount and location of urban forest are very important to how cool or how hot temperatures become in different parts of the city. Trees contain a lot of water and release that water into the atmosphere to keep themselves cool. Water absorbs a large amount of heat before showing significant rises in temperature. Also, trees absorb a lot of heat and release it slowly, which moderates temperatures of urban heat islands.
What can be done to reduce the negative consequences of an urban heat island? One step to reduce heat buildup is to increase all forms of vegetation in the city. Some scientists believe that a city should contain 40 percent park land, forest, and green spaces. (Atlanta has only 27 percent). This can be accomplished by limiting the number of trees cut when building, and planting trees wherever possible, planting shrubs and grasses in bare ground areas, and planting trees in median strips along roads and in parking lots. Other specific steps include planting more trees to shade sidewalks, parking lots, and buildings and replacing old roofs with new reflective roofing material.
Modified from NASA's Mission Geography curriculum with permission