In the 1950s, 30% of the world's population lived in urban areas. By 2030, 60% of the world's population will live in cities. The phenomenon of rapid urbanization is resulting in the transition of native vegetation to manmade engineered paved surfaces for roads, parking areas, sidewalks, and building structures.
The urban environment—with its impervious paved surfaces, varied building geometries, and reduced vegetation—causes less of the incoming radiant energy from the sun to be reflected or re-released. This heat island effect in urban areas also reduces the conversion of energy to latent energy that is associated with evaporation or transpiration of moisture. Compounding this effect is the larger volume of paving and building materials, which gives urban areas a much higher thermal storage capacity than natural surfaces.
This can be observed by satellite images showing the impact of engineered materials on surface and ambient temperatures. Satellite (ASTER) imagery of the Phoenix region captured in October 2003 provides a coarse visual representation of the paved surfaces, including local roads, highways, and parking lots. The image shows that these areas contribute significantly to the Urban Heat Island (UHI) and exhibit variability of surface temperatures related to spatial patterns and pavement designs.
When the urban region's temperatures exceed a rural setting's temperature, the result is known as the UHI effect, which is generally thought of as a nocturnal phenomenon, though many regions are hotter both day and night. As surfaces throughout a community or city become more abundant and urban geometry from buildings traps energy and prevents it from re-radiating to the atmosphere, the overall ambient air temperatures increase in comparison to the surrounding rural region. In Phoenix, the difference has been documented to be as large as 12° F.
The UHI can adversely impact the sustainability of regions by increasing the dependence on mechanical cooling, which results in increased greenhouse gas emissions, consumption of water to make electricity, and increased cost of living for residents. The UHI also can increase the incidence and severity of heat-related illnesses. Summertime heat is known to have a greater impact on human health than any other form of severe weather in the United States. Heat waves claim more lives each year than floods, tornadoes, and hurricanes combined.
Extremes of heat-related impacts are exemplified by the European heat wave of August 2003 where thousands died, and the Chicago heat wave in July 1995, which is thought to have caused 465 deaths. Although the UHI was not the sole cause of the deaths, the elevated nighttime temperatures played a critical role.
The UHI also has serious financial impacts for local governments. According to a U.S. Environmental Protection Agency report, the heat island in Los Angeles raises ozone concentrations by 10% to 15%; the report indicates that heat-island reduction measures could lower the city's smog-related expenses by $360 million per year. The EPA also has recently promulgated guidance for incorporating UHI mitigation strategies as part of the State Implementation Program (SIP) process.
The federal Clean Air Act requires states with counties failing to meet national ambient air quality standards to produce an SIP. If a state fails to submit or implement an SIP, or if it submits an SIP that is unacceptable, the EPA has the power to impose sanctions or other penalties on that state. Typical sanctions include the threat of cutting off federal highway funds. A recent study by Arizona State University's Sustainable Materials and Renewable Technologies (SMART) Institute has shown a steady and increased annual peak electricity demand on residential electricity consumption due to the UHI in the Phoenix region. This took into consideration more efficient building design and HVAC systems.