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.
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).
IMPACTS OF CLIMATE CHANGE ON CITIES
Given their concentration of people, industry, and infrastructure, cities and built environments are generally expected to face significant impacts from climate change. Some of the most important impacts will be associated with changes in the frequency and intensity of extreme weather. Hurricane Katrina in 2005 illustrated the potential for extreme events to cause catastrophic damage to human well-being as well as urban infrastructure; likewise, temperature extremes in cities increasingly cause severe human and environmental impacts, even in the developed world (see Box 12.1). The impacts of warming are amplified in large urban conglomerations because of the heat island effect and the interaction of other environmental stressors (Grimmond, 2007; Hayhoe et al., 2004; Rosenzweig et al., 2005; Solecki et al., 2005). For example, the urban heat island of Phoenix raises the minimum nighttime temperature in parts of the city by as much as 12.6°F (7°C), generating serious water, energy, and health consequences (Brazel et al., 2000). The growth of the southwestern U.S. “sunbelt” as well as that of megacities throughout other arid regions of the world increases the populations at risk from extreme heat as well as their demand for energy and water (Rosenzweig et al., 2005).
In addition, CO2, nitrogen oxides, volatile organic compounds, particulate matter, and other pollutants and pollutant precursors react in the urban airshed to produce high levels of surface ozone and other potential health hazards (see Chapter 11). In a warmer future world, stagnant air, coupled with higher temperatures and absolute humidity, will lead to worse air quality even if air pollution emissions remain the same (e.g., Cifuentes et al., 2001a,b In many cases, air pollution plumes extend well beyond the urban area per se, affecting people and agriculture over large areas, such as the Ganges Valley (e.g., Auffhammer et al., 2006). In the developing world, such decreases in outdoor air quality come on top of poor indoor air quality—for example, from wood fuel heating (Zhang and Smith, 2003).
As discussed in Chapter 11, certain groups (such as the elderly) are especially vulnerable to intensive heat waves in cities worldwide, especially in temperate climates. Groups with preexisting medical problems, without air-conditioned living quarters, who are socially isolated, or who live on top floors are particularly vulnerable (Naugh-ton et al., 2002; Patz et al., 2005; Semenza et al., 1996). The elderly, as well as portions of the population with asthma and related problems, are also susceptible to poor air quality (e.g., Hiltermann et al., 1998). The U.S. population over age 65 is expected to reach 50 million (20 percent of the total U.S. population) by 2030, with the overwhelming majority living in cities. Cities throughout the nation and the world are differentially prepared (CCSP, 2008a), as illustrated by the relative success of Marseille in the
Urban-Climate Interactions and Extreme Events
In the summer of 2003, a persistent anticyclone anchored above western Europe triggered temperatures in excess of 95°F-99°F (35°C-37°C) for as long as 9 days (see figure below). Temperatures were especially high in cities, where urban heat islands amplified the maximum temperatures (Beniston, 2004) and ground-level ozone concentrations climbed to 130 to 200 g/µm3 (equivalent to the Environmental Protection Agency’s code orange alert; Pirard et al., 2005). It is estimated that this heat wave and the associated poor air quality caused more than 50,000 excess deaths, mostly among elderly urbanites (Brüker, 2005). In France alone, where the housing infrastructure from Paris to Marseille commonly does not include air conditioning or insulation between roofs and rooms, more than 14,800 excess deaths occurred during that period, and the number of deaths is positively correlated with the number of consecutive hot days (Pirard et al., 2005). The rash of deaths, including over 2,200 excess deaths on a single day in August, overwhelmed emergency rooms and morgues.
2003 heat wave over France (Box 12.1; Pirard et al., 2005) versus the 700 excess deaths in Chicago’s 1995 heat wave (Semenza et al., 1996). As noted in Chapter 11 and consistent with the findings of the panel report Adapting to the Impacts of Climate Change (NRC, 2010a), research on health infrastructure and preparedness, especially in urban complexes, is needed to inform practice.
Other climate change impacts will also affect cities. Many of the 635 million people occupying coastal lands worldwide live less than 33 feet (10 meters) above sea level and are thus threatened by sea level rise (McGranahan et al., 2007; Wu et al., 2002, 2009; see Chapter 7). Existing tensions over water withdrawal between rapidly growing urban areas and agricultural sectors will be exacerbated by decreasing snowpack in the American West and other regions as a result of climate change and variability (NRC, 2007b). Water vulnerabilities in general are expected to pose major problems for cities in the developing world (Vorosmarty et al., 2000). Expected increases in the frequency of extreme events (Milly et al., 2002), such as intense and prolonged rain storms (see Chapter 8) that stress drainage and flood protection systems, also threaten aging urban infrastructure. Climate change impacts on the megalopolises will also stress regional ecosystem function, water withdrawal, and movement of biota, among other environmental issues (Folke et al., 1997; Grimm et al., 2008; IHDP, 2005).
Cities are centers of economic, cultural, educational, research, social, and political activity, and as such they experience a myriad of nonclimatic changes and stresses that affect their institutional, technological, and economic capacities, the social capital available within and among different population groups, and the relationships between urban centers and their surroundings. Climate change impacts cannot be fully appreciated and addressed without understanding the complex nature of multiple stressors and interacting climatic and nonclimatic factors that affect the vulnerability and adaptive capacity of cities (e.g., Campbell-Lendrum and Corvalán, 2007; Pelling, 2003).
SCIENCE TO SUPPORT LIMITING FUTURE CLIMATE CHANGE
Just as cities loom large in driving and being affected by climate change, they also have important roles to play in limiting the magnitude and ameliorating the impacts of climate change (Grove, 2009). The largest opportunities for reducing GHG emissions from urban centers lie in the transportation, construction, commercial, and industrial sectors, which typically lead in energy consumption and GHG emissions. Reducing industrial and transportation emissions provides a potential for multiple co-benefits to
cities in limiting future climate change, reducing the urban heat island effect, and also improving air quality (e.g., NRC, 2009e).
The design and geometry of cities and metropolitan areas afford various means for reducing emissions as well as surface reflectivity. The urban form of most cities has grown in an ad hoc way, through piecemeal planning, development, and control under multiple, independent decision-making units (Batty, 2008). Many, if not all, of these decision-making entities respond foremost to considerations other than climate change, and they rarely consider environmental spillovers beyond their area of control or concern. Yet, the development of cities has profound impacts on infrastructure, travel behavior, and energy consumption (e.g., Ewing and Rong, 2008; Filion, 2008; NRC, 2010g), all of which offer opportunities for interventions that could offset the role of cities in driving climate change. These interventions are only beginning to be explored and appreciated.
One potential response option is altering the reflectivity of surface structures by whitening roofs and road surfaces or employing green rooftop and landscaping options (Akbari et al., 2001; Betsill, 2001). Roofs and paved surfaces typically comprise about 25 and 35 percent, respectively, of dense urban areas (Akbari et al., 2009), so increasing the reflectivity of these surfaces offers the potential to offset some of the urban heat island effect and influence global climate (see Chapter 15). Green rooftops and land-scaping options not only reduce urban and regional heat islands but can also improve local and regional air quality (Taha et al., 1997) and provide recreational opportunities and other nonclimate benefits. Alternative city designs or configurations can also lower the heat island effect (Eliasson, 2000; Unger, 2004), although with varying impacts on water and energy consumption that introduce a new suite of trade-offs to consider. “Smart” or “green” redesigns of cities that foster less use of automobiles, among other factors, could reduce GHG emissions from urban areas (Ewing et al., 2007).
SCIENCE TO SUPPORT ADAPTING TO CLIMATE CHANGE
Options to adapt to the impacts of climate change in cities and built-up areas encompass a wide array of potential actions. To date, most of the options considered have fallen into the category of structural or engineering strategies such as protecting existing development and infrastructure from sea level rise (e.g., NYCDEP, 2008); improving water supply, drainage, and water treatment infrastructure; and reducing urban heat island effects. In some cases, local and regional entities sharing a common problem thought to be amplified by climate change, such as water in the American
West, have begun planning to address adaptation beyond infrastructure per se, including more efficient water markets. Although noninfrastructural strategies, such as improving emergency preparedness and response (above), have also been considered, in general there is insufficient concern with, or scientific understanding of, the underlying social-ecological vulnerabilities that cities and the people within them face (see Chapter 4). Many more ways to reduce vulnerability and enhance adaptive capacity may become available when the vulnerabilities of cities are better understood, particularly the vulnerability of subpopulations (e.g., the urban poor, minority groups, children, the elderly, or manual laborers; Campbell-Lendrum and Corvalán, 2007) and the differences between large and smaller urban areas in different regions (e.g., Bartlett, 2008; Hardoy and Pandiella, 2009; Hess et al., 2008; Porfiriev, 2009; Thomalla et al., 2006). Urban areas adjacent to ecological reserves or bordering on forested areas or wildlands may also have to take preventive and preparatory measures to reduce wildfire risks and find ways to protect urban ecology (Collins, 2005).
In general, urban areas face all the climate-related problems faced in other sectors described in this report, but focused on a particular spatial scale. While lessons and techniques on adaptation to climate change from one urban area may be transferrable to others, many will be location specific, and clusters of municipalities in close proximity will have to devise integrated responses across extended metropolitan areas. These considerations raise both institutional and economic opportunities and challenges for adaptation (see the companion report Adapting to the Impacts of Climate Change [NRC, 2010a]). They also open up the opportunity to develop sustainable solutions to climate change that integrate actions to limit the magnitude of climate change with those taken to adapt to its impacts—a challenge that some cities around the world are already exploring (e.g., Heinz Center, 2008b). Important scientific questions remain, however, about how to analyze these dual strategies in an integrated fashion (e.g., Hamin and Gurran, 2009; Wilbanks, 2005).
Because the majority of the U.S. and world population already lives in urban areas, and existing or new urban centers will continue to grow in size and economic importance, research on reducing the climate change and accompanying environmental impacts of urban areas is critical. This includes assessing the differential vulnerability of urban areas and populations to climate change impacts as well as the full range of options for limiting and adapting to climate change. Opportunities for integrated, multidisciplinary, and use-inspired research abound, but better connections are needed particularly to the applied science, engineering, and planning professions.
Characterizing and quantifying the contributions of urban areas to both local and global changes in climate. The role of large built environments and how they vary in terms of GHG emissions (including per capita emissions), aerosols, ground-level air pollution, and surface reflectivity need to be examined in a systematic and comparative way. Such research should include the extended effect of urban areas on surrounding areas (such as deposition of urban emissions on ocean and rural land surfaces) as well as interactions between urban and regional heat islands and urban vegetation-evapotranspiration feedbacks on climate. Examination of both local and supralocal institutions, markets, and policies will be required to understand the various ways urban centers drive climate change and identify leverage points for intervention.
Understanding the impacts of climate change on cities. Improving assessments of the impacts of extreme events (e.g., heat waves, drought, floods, and storms) and sea level rise on cities will require improved regional climate models, improved monitoring systems, and better understanding of how extreme events will change as climate change progresses. Evaluations of climate change impacts on urban heat islands and local-regional precipitation should extend to the analysis of their combined impacts on urban and periurban ecosystem and landscape function, ecosystem services, and demands on water and energy consumption.
Assessing the vulnerability of cities to climate change. Improved understanding is needed of who and what are threatened by climate change in the urban context, in both developed and developing countries. This includes human cohorts, neighborhoods, infrastructure, and coupled human-environment systems, as well as implications for food and water security. Most of the world’s largest cities are in developing nations and have difficulty achieving global standards for clean air and other healthy environmental qualities. At the same time, very few U.S. cities have received concerted attention from climate researchers. As a result, the relative vulnerability of different urban forms (e.g., design, geometry, and infrastructure) and urban configurations relative to other settlement forms is largely unknown and deserves further study. In addition, given the large population adjacent to coastlines, attention to the vulnerability of coastal cities to sea level rise deserves special attention.
Developing and testing methods and approaches for limiting and adapting to climate change in the urban context. Limiting the magnitude of climate change and adapting to its impacts in the urban context raises a wide range of issues, including the relationships among urban land use, heat islands, water and energy use, and air quality. Additional research is needed, for example, on the efficacy and sociological considerations involved in adoption and implementation of white and green roofs, landscape architecture, smart growth, and changing rural-urban socioeconomic and
political linkages. Additional questions include the following: What legacy or lock-in effects, including infrastructure and governance, serve as impediments to responses to climate change? What co-benefits can be gained in the reconfiguration of cities? Which adaptation strategies synergistically benefit the goal of limiting climate change, which potentially counteract it, and how can the trade-offs be adjudicated effectively?
Linking air quality and climate change. Research is needed to provide information for decision making about air quality in the face of climate change. This includes measurements, understanding, modeling, and analyses of socioeconomic benefits and trade-offs associated with different GHG emissions-reduction strategies, including those that simultaneously benefit both climate and air quality (see also Chapter 11) and those that could exacerbate one issue while monitoring the other.
Developing effective decision-support tools. What do we know about effective decision making under uncertainty, especially when multiple governance units may be involved? Much research is needed in comparing the results of city action plans for climate change and identifying similarities and differences between and among small and large cities. Questions that need answers include which qualities of these different plans break or create path dependencies (lock-in, e.g., through infrastructure design, tax policies, or other institutions), and which lead to more flexible, adaptive responses to the risks of climate change.