The economic, environmental, and human toll of urban heat islands is far-reaching and significant. These impacts can be minimized by both understanding the behavior of urban heat islands and by exploring new and innovative ways to design buildings and landscapes to mitigate the warming.
Urban heat islands have the most significant impact on the economy, human health, and the environment.
The Economics of Urban Heat Islands
Whether it is the additional costs consumers must bare for further cooling during the warm months or the higher rates utilities must pay when they exceed their capacity during periods of high demand (e.g., summer heat waves), urban heat islands can result in significant economic losses that total in the billions of dollars annually. For example, it has been estimated that mitigating the urban heat island effect in Los Angeles by planting trees, reroofing homes, and painting roads could save $170 million dollars annually in electricity costs and an additional $360 million dollars per year in smog-related health costs.
The relative importance of urban heat islands on energy demand is best expressed by an energy demand curve as depicted in Figure 4. The curve shows the power load of a typical American city as a function of temperature. The electrical load is smallest at a comfortable human temperature of approximately 20°C (70°F) as represented by the vertical dashed line. Any increase in temperature beyond the threshold value will result in increased electrical load as consumers activate air conditioners and fans. In this example, a moderate annual urban heat island of 5.5°C (10°F) translates into an increase of 200 megawatt-hours of electricity demand, which is equivalent to over $200 million dollars per year assuming a electrical rate of 12¢/kWh. In fact, because most utility rates are tiered (i.e., excessive use is charged at a higher rate), the cost may be even higher.
Developing mitigation strategies to combat urban warming are compounded by the fact that urban areas that experience harsh winters, such as the Twin Cities, do not necessarily benefit when already low winter temperatures are made even lower. A challenge in changing building properties and the landscape is to reduce urban warming in summer, but not necessarily winter. This project is addressing these concerns with building and landscape professionals in an attempt to provide a year-round solution to urban heat islands.
Urban Islands and Human Health
In July of 2003 most of Europe was in the grip of one of the warmest summers on record. In France alone, temperatures exceeded 40°C (104°F) for seven straight days, with little relief at night as low temperature records were also shattered. In the end, over 40,000 Europeans died as a direct result of the heat wave, most from hyperthermia (heat stroke) in dense urban centers. In terms of heat related mortality, Europe is not alone. In July 1995, an intense heat wave plagued Chicago where record temperatures and humidity contributed to heat index values in excess of 49°C (120°F) over five days. Over 700 people died, most of them elderly residents living in the heart of the city without air conditioning.
According to the Centers for Disease Control, the majority of heat-related deaths occur in urban and suburban areas. Couple this with the fact that more than half the global population lives in cities, and there is urgency to understand and mitigate urban warming, which is putting a large portion of the population at risk. In fact, the death toll from heat-related events exceeds all other weather disasters. Even more alarming is research indicating that the frequency of heat waves is projected to increase with climate change through the twenty-first century.
Urban heat islands, especially when coupled with extremes in temperature, have the potential to put a significant portion of an urban population at risk of health-related effects – especially heat stress. Populations residing in cities located in more northerly latitudes are at even greater risk because air conditioners are uncommon since they are not typically needed much during summer months.
Urban heat islands also contribute to high ground level ozone concentrations, a common pollutant in metropolitan regions, that contribute to numerous resperatory and cardiovascular problems in humans. Ground level ozone forms when nitrogen oxides, carbon monoxide, and volatile organic compounds (all called ozone precursors) react in the atmosphere in the presence of sunlight. All other things being equal, ground level ozone concentrations will be higher when it is warmer and more sunny.
Finally, if powerplants are located nearby large urban areas, the increase in electrical demand with urban warming can lead to greater air pollution and greenhouse gas concentrations if fossil-fuel power plants are the main type of energy production. Sulpher dioxide, carbon monoxide, and mercury are just some of the many pollutants that are emitted in greater concentrations during times of peak energy demand. An increase in their concentration can affect the populations that live around and downstream of these power plants.
Environmental Consequences of Urban Warming
Urban heat islands not only have significant economic and health impacts, they also affect urban ecosystems and the wellbeing of animal and plant life. For example, in the Twin Cities, urban heat islands degrade water quality – primarily by thermal pollution. With pavement, rooftops, and concrete surface experience temperatures that are 50-100ºF greater than the near-surface air temperature, rainfall gets warmed by these surfaces and then runs off into urban ponds, lakes, streams, and rivers. Temperatures in these water bodies can warm by more than 20-30ºF. A warming of this magnitude affects a wide range of aquatic life – especially the reproduction of many species. Fish, in particular, experience thermal stress and shock when water temperatures change too rapidly.
Urban heat islands can also contribute to a longer growing season for urban gardeners and fewer and less severe frosts. Urban invasive species may become more prevalent if the additional warming improves their success at taking root. Urban warming may also influence the health of climatically sensitive plant species, animals, and birds. In some urban areas around the United States, urban warming has been shown to affect the reproductive cycle and migratory patterns of certain bird species.