There are many socio-economic impacts related to the increase of urban temperature exacerbated by the increase in the global warming. The implications on the urban environment are mainly: local air quality, heat stress, morbidity, mortality, energy demand and effects on ecosystems. In climate science, local and global are strictly connected issues (Corburn, 2009). A mutual relationship between local and global scale exists. The control of global warming through international policy – such as the Kyoto Protocol – involves not only policy-makers but also local politics and planners who can play the key role in the enhancement of the local and global climate. The role of cities is also important in a global perspective, indeed cities are responsible of about 97% of the CO2 anthropogenic emissions (Svirejeva-Hopkins, Schellnhuber, & Pomaz, 2004), but at local scale the effects of urbanization can give rise to an increase of temperature variable in time and space that on average is of about 1-3°C and in more extreme conditions urban contexts are warmer than the rural surroundings of 10°C (Grimmond, 2007).
The increase in urban temperature affects primarily human heath, especially during summer, increasing the heat strokes and decreasing the air quality for the formation of photochemical ozone. About 1000 people die every year in the United States for extreme temperatures (Changnon, Kunkel, & Reinke, 1996). The UHI influences the heat waves exacerbating their magnitude and duration. In addition, also the secondary effects such as the increase in ground-ozone and pollutants affects human health generating mainly respiratory diseases. In this last case, children and old people are exposed at particular risks (EPA, 2009).
The local warming causes the increase in the use of energy for cooling and a reduction during winter for heating. Previous studies demonstrate the effects of the increase in the urban temperature on the use of energy. For example the Heat Island Group has estimated an increase of the use of energy in Los Angeles correspondent to 100 million of US dollars (Chang, 2000). Besides, Sailor shows that for each degree of temperature above 27°C in New Orleans there is an increase of energy use of about 37 kWh (Sailor, 2002). The Environmental Protection Agency (EPA) suggests that in the US cities, the increase in the peak of urban electric demand increases 1.5 to 2 percent for every 0.6°C increase in summertime temperature. This means that 5–10% of community-wide demand for electricity is necessary to compensate the heat island effect (EPA, 2009). Santamouris et al. (2001) found for the UHI in Athens a doubling of the energy use for cooling and the triple of the peak of energy use during the warmest days. The increase in the energy demand, especially in summer, causes the increase in the probability of blackouts provoking discomforts to the population and economic backlashes.
The augmentation in the energy demand produces an increase in primary pollutants and greenhouse gases associated with the energy production. The pollutants produced are mainly sulfur dioxide, nitrogen oxides, particulate and carbon monoxide. These pollutants are harmful to the human health and contribute to the formation of the ozone and to the acid rains. Besides, the carbon dioxide affects mainly the global warming.
The increase in urban temperatures and in particular in urban surface ones provokes the increase in the temperature of storm-water runoff. The storm-water drains into storm sewers raising the temperature of rivers, ponds or lakes in which it is released. The increase in water temperature affects many aquatic species acting on their metabolism and their reproduction (EPA, 2009).
Since the UHI is caused by a modification in the natural heat balance at the surface, the mitigation strategies should modify or enhance the contribution of some of the factors in the equation 14 and equation 15. First of all it is possible to increase the amount of reflected radiation increasing the mean urban albedo and the emissivity in order to reduce the outgoing long-waves. Then, it is possible to enhance the latent heat flux converting residual urban spaces into gardens, lawns or planting trees (Shahmohamadi, Che-Ani, Ramly, Maulud, & Mohd-Nor, 2010; EPA, 2009) or increasing the water bodies (Akbari, 2009). Other strategies can be related to the modification of the urban morphology, such as the amelioration of the natural ventilation, even though that is difficult to realize when the urban pattern is already defined.
Future policies can attempt to design cities in which efforts can be conducted to decrease the UHI phenomenon and all its impact on the environment and population.
Nowadays, most of time is not easy to modify the urban morphology because not residual spaces are available to convert into green spaces. It is easier to replace dark surfaces with high-albedo ones.
Tree planting can positively affect the UHI in two different ways. Trees reduce urban surface temperature by their shades blocking the incoming radiation and reducing the incoming energy reaching the soil (Rosenfeld, et al., 1995). Moreover, trees can decrease the air temperature through evapo-transpiration (Akbari, 2009) producing the so-called ‘oasis phenomenon’ (Santamouris, 2006) and as a consequence the use of energy for cooling (Simpson & McPherson, 1998). Other benefits in terms of urban air quality connected to trees planting are the increase in the amount of pollutants uptake (Akbari, Pomerantz, & Taha, 2001; Cardelino & Chameides, 1990), reduction of noise (Akbari, Pomerantz, & Taha, 2001), beautification, increase in biodiversity. Moreover, trees planting has positive effects also on global warming decreasing the amount of carbon dioxide.
The role of green areas in the mitigation of the UHI, has been also investigated by Petralli et al. (2006), who analyzed temperatures in gardens and courtyards. The temperatures recorded in the two green spaces had the same trend, but different values. This behavior is justified by the two green spaces having the same thermal characteristics, but different geometric characters. The temperature in courtyards is influenced by the canyon effect, caused by the surrounding walls resulting in the rise of temperature. Small green areas such as courtyards and gardens contribute to the mitigation of the UHI. A difference of approximately 1.5°C between streets and gardens, and 1°C between streets and courtyards, was recorded in the early morning. Moreover, urban parks can contribute to provide thermal comfort to people.
Typically, urban environments are characterized by low albedo surfaces. The replacement of natural surfaces with concrete, asphalt and tar lessens the urban albedo producing a reduction of the reflected radiation and as a consequence an increase of the surface temperature (Akbari, 2009). The quantity of the reflected radiation depends on the optic characteristics of the materials but also on latitude. At low latitude, such as at the equator, the amount of the incoming radiation is consistently higher than radiation at high latitude (Oleson, Bonan, & Feddema, 2010). This means that high-albedo surfaces are more effective at low latitudes rather than at high one (Lenton & Vaughan, 2009). As well as vegetation, increase in surface albedo can have positive effects both on the small and medium-scale. A high-albedo surface reaches a lower temperature than a dark one, but the urban-wide conversion of urban surfaces can accrue several benefits (Rosenfeld, et al., 1995); not only the decrease in surface temperature but also the decrease in air temperature and as a consequence the reduction in energy use for cooling and the enhancement of the air quality. As it will be discussed later the increase in world-wide surface albedo has positive effects also on climate change (Akbari, Menon, & Rosenfeld, 2009; Menon, Akbari, Mahanama, Sednev, & Levinson, 2010). The main problem in big and densely urbanized cities is that is not easy to convert dark impervious surfaces into light green areas. One solution could be to convert traditional black flat roofs into green roofs. Indeed, since roofs represent about 20-25% of the urban surface (Akbari, Rose, & Taha, 2003), their urban-wide conversion into green roofs can give rise to many benefits both on an urban scale – effects on UHI, air quality, storm-water management, biodiversity and urban amenities (Oberndorfen, et al., 2007); and on a building scale – increase in life span of the building materials underneath the soil, reduction of noise, and decrease in building energy use especially during summer (Saiz, Kennedy, Brass, & Pressnail, 2006).
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* Rearranged text from: Susca, T. (2011). Evaluation of the Surface Albedo in a LCA Multi-scale Approach. The Case Study of Green, White and Black Roofs in New York City. Ph.D. Thesis