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published in the 2002 State of the Nation’s Ecosystems Report as well as the 2003 and 2005 Web-Only Updates have been superseded by the 2008 Report and thus should be used with caution.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.

The Indicator

Cities have modified climates based on factors such as building density and type and energy use, as well as local topography and regional weather patterns. The “urban heat island” represents the difference between urban and nearby rural air temperatures and is directly related to urban land cover and human energy use. For most cities, this difference often is negligible in the daytime but develops rapidly after sunset. Maximum difference occurs 2–3 hours after sunset and may be as great as 18°F. In general, as the population density of a city increases, the difference in minimum temperature between the urban core and rural site increases nonlinearly. Urban heat island effect for a city is calculated by comparing the temperature of a monitoring station in the urban core with a monitoring station from a neighboring rural location. This difference might be reported as the average monthly difference between urban and rural sites. Nationally, the indicator might report the number of cities with various levels of difference between urban and rural sites: 0–6°F, 6 to less than 13°F, or more than 13°F.

As constructed surfaces replace natural vegetation, an area’s ability to absorb and store heat increases; the natural cooling effect mediated by trees and other vegetation is reduced (water moves from the soil into a plant via its roots, exiting ultimately by evaporation through pores in the leaves in a process called evapotranspiration —a cooling process much like when sweat evaporates from our skin). The urban heat island represents a change in the diurnal pattern of ambient temperature. Because many biological processes are temperature dependent, changes in the temperature regime may have profound effects on species and ecological processes. In fact, many of the proposed effects from elevated global temperatures occur in urban areas because of the urban heat island effect.

It is reported by the Centers for Disease Control’s National Center for Environmental Health that extreme heat events, some of which may be directly attributable to the heat island effect, are responsible for greater loss of human life in the United States than hurricanes, lightning, tornadoes, floods, and earthquakes combined (http://www.cdc.gov/nceh/hsb/extremeheat/). Other effects may include physiological stress in some species, altered species composition and structure in ecological communities, modified nutrient and carbon availability, and altered home range of pathogens. For example, physiological stress results from altered phenology and modified moisture nutrient availability. The urban heat island also modifies energy use for heating and cooling buildings and vehicles.

The Data Gap

National Weather Service temperature data are available for a large number of locations in the United States and could be used to determine urban heat island effect and how this temperature differential has changed over time. Analyzing historical data would require a significant amount of time, energy, and funding to retrieve archival information, to conduct quality assurance and quality control on data, and to perform the analysis. Data problems include obtaining long-term data records for both urban and adjacent rural sites and accounting for changes in monitoring locations or instrumentation and for changes in population densities and human activities around monitoring sites. Another problem occurs for desert cities where the maximum temperature difference between urban and rural monitoring locations may occur during midday rather at sunset. Although a temperature differential exists 2–3 hours after sunset, the evaporative cooling from vegetation within the city may create cooler temperatures during the day than adjacent desert temperature.

Remote-sensing data have been used to examine temperature differences between urban and rural sites; however, these measurements record surface temperatures rather than ambient temperatures.