Note that the data published in the 2002 State of the Nation’s Ecosystems Report as well as the 2003 and 2005 Web-Only Updates have been superseded by the 2008 Report and thus should be used with caution. For the most recent data, purchase the 2008 Report from Island Press.

What Is This Indicator, and Why Is It Important? This indicator will describe the difference between urban and rural air temperatures for major U.S. metropolitan areas. Temperatures within urban areas will be compared to those in less-developed surrounding areas.

Extremely hot weather is responsible for greater loss of human life in the United States than hurricanes, lightning, tornadoes, floods, and earthquakes combined. Building density and type, amount of road surface, and energy use, as well as local topography and regional weather patterns, all work together to modify a city’s climate. The urban heat island effect is often noticed most at night when buildings and other constructed surfaces radiate the heat they have accumulated during the day. Beyond posing a threat to human health (through heat stroke, for example) and raising air conditioning costs, the heat island effect can cause physiological stress in other animals, change the mix of plants and animals that live in the area, and even lead to changes in the distribution of pathogens. Elevated temperatures also accelerate the formation of ground-level ozone (see Air Quality) and other air pollutants that adversely affect human health.

Why Can't This Indicator Be Reported at This Time? There is no single metric that has been adopted by the scientific community as an indicator of the heat island effect. One possible presentation would report the percentage of all U.S. metropolitan areas where the average annual difference between urban and rural air temperatures is relatively small (less than 7ºF), moderate (7º to less than 13ºF), or large (more than 13ºF).

National Weather Service data could be used to determine current and historic heat island effects in many locations. However, there is no program in place either to retrieve and analyze historic information or to identify appropriate pairs of urban and rural sites necessary to make calculations of the heat island effect.