Cities and the Built Environment

The world is rapidly urbanizing. Cities now house slightly more than half the world's population, and 70 percent of the global population will live in urban areas by 2050 (UN, 2007). An unprecedented reorganization is occurring in where people live and how they are restructuring their physical environment. Such growth has led to the emergence of urban conglomerations, or "megalopolises," in which one built environment stretches to another (urban to suburban to "exurban" infrastructure and design), covering entire ecosystems, landscapes, and watersheds (Figure 12.1). The majority of growth in global population over the next several decades is projected to take place in the cities of the developing world (Cohen, 2006), with much of it focusing on emerging urban conglomerations. Given these factors, cities and the built environment are becoming a major focus area for understanding and responding to climate change.

Questions decision makers are asking, or will be asking, about cities, the built environment, and climate change include the following:

• What is the potential for cities to contribute to limiting the magnitude of climate change in ways that also improve air quality and reduce overall environmental impact?

• Which cities and urban conglomerations are most vulnerable to the negative impacts of climate change, including sea level rise, water supply changes, heat waves, and extreme precipitation events?

• What are the most feasible and efficient adaptation actions that cities can take to reduce the stresses associated with climate change?

• How can cities enhance ecosystem services and human well-being in the face of climate change and other environmental stresses?

This chapter summarizes research on how the concentration of people, industry, and infrastructure in cities and built environments plays a major role in driving climate change. It also outlines current scientific knowledge regarding the impacts of climate change on cities, adaptation options, and the potential of cities to limit the magnitude of future climate change. Finally, it details some of the research needed to address the impacts, adaptation, and special vulnerabilities of urban environments with respect to climate change.

CHAPTER TWELVE

FIGURE 12.1 Lights of North America at night. Note the continuous lighting of extended concentrations of large cities (urban conglomerations), such as Washington to Boston, San Diego to Santa Barbara, and southwestern Lake Michigan. SOURCE: NASA (2001).

ROLE OF CITIES IN DRIVING CLIMATE CHANGE

Urbanized areas play an increasingly important role in driving climate change. For example, energy production and use generate about 87 percent of U.S. greenhouse gas (GHG) emissions; of this amount, the majority is associated with electricity, heat, industrial production, transportation, and waste located in cities and other built-up areas (Folke et al., 1997). The concentration of emissions from urban areas also commonly generates major problems for urban air quality (e.g., Mage et al., 1996). The economies of scale associated with concentrating people in cities generally result in lower per capita emissions relative to nonurban settlements (Dodman, 2009; Satterthwaite, 2008). However, especially in developing economies, the shift to an urban economy and lifestyle increases expectations of consumption and triggers rapid urban expansion (Angel et al., 2005; Guneralp and Seto, 2008), thus enlarging the urban ecological footprint (Rees and Wackernagel, 2008). This footprint involves land use changes in, and resource extraction from, not only the immediate city hinterland but also in distant areas as a result of globalization (DeFries et al., 2010). Thus, energy consumption, indirect land use change (e.g., deforestation), and ecosystem impacts (e.g., ground-level air pollution) beyond the city's boundaries play important roles in climate change (e.g., Auffhammer et al., 2006).

Urbanized or built-up areas directly change reflectivity (Sailor and Fan, 2002), especially through the concentration of roads and other dark surfaces, and so can affect global radiative forcing even though they cover only 1 to 2 percent of the land surface of the Earth (Akbari et al., 2009). The urban heat island effect is relatively well understood (see Figure 12.2) and also has consequences for regional and global climate (e.g., Jin et al., 2005; Lin et al., 2008); for example it may have amplified the effects of the 2003 heat wave in western Europe (Stott et al., 2004). Sustained research demonstrates that urbanization also affects precipitation, including its variability and intensity over and on the leeward side of cities (e.g., Changnon, 1969; Jauregui and Romales, 1996; Shem and Shepherd, 2009). In addition, large built-up areas affect the global carbon balance via their configuration, which affects vegetation and soils (Pickett et al., 2008), and their almost inevitable spread over prime croplands (Angel et al., 2005; Seto and Shepherd, 2009).

FIGURE 12.2 Schematic representation of an urban heat island, showing how urbanized areas can be several degrees warmer than the surrounding rural areas. The effect can be especially strong on warm summer days. SOURCE: Heat Island Group, Lawrence Berkeley National Laboratory (http://heatisland.lbl. gov/High Temps/).

FIGURE 12.2 Schematic representation of an urban heat island, showing how urbanized areas can be several degrees warmer than the surrounding rural areas. The effect can be especially strong on warm summer days. SOURCE: Heat Island Group, Lawrence Berkeley National Laboratory (http://heatisland.lbl. gov/High Temps/).

0 0

Post a comment