It must be recognized that ultimately all sustainability is limited by biophysical limits and finite resources at the global scale (e.g., Burger et al., 2012; Rees, 2012). A city or region cannot be sustainable if its principles and actions toward its own, local-level sustainability do not scale up to sustainability globally. Thus, localities that develop an island or walled-city perspective, where sustainability is defined as only activities within the city’s boundaries, are by definition not sustainable.
At its core, the concept of sustainable development is about reconciling “development” and “environment” (McGranahan and Satterthwaite, 2003). Development, i.e., the meeting of people’s needs, requires use of resources and implies generation of wastes. The environment has finite resources, which present limits to the capacity of ecosystems to absorb or break down wastes or render them harmless at local, regional, and global scales.
If development implies extending to all current and future populations the levels of resource use and waste generation that are the norm among middle-income groups in high-income nations, it is likely to conflict with local or global systems with finite resources and capacities to assimilate wastes. As described in Chapter 2, many indicators and metrics have been developed to measure sustainability, each of which has its own weaknesses and strengths as well as availability of data and ease of calculation. Some of the most prevailing indicators include footprinting (e.g., for water and land) and composite indices (e.g., well-being index and environmental sustainability index). It is beyond the scope of this report to examine all available measures, and readers are directed to any of the numerous reviews that discuss their relative merits (see, for example, Čuček et al., 2012; EPA, 2014a; Janetos et al., 2012; Wiedmann and Barrett, 2010; Wilson et al., 2007; The World Bank, 2016; Yale University, 2016). New sustainability indicators and metrics are continually being developed, in part because of the wide range of sustainability frameworks used as well as differences in spatial scales of interest and availability (or lack thereof) of data. In recent years, city-level sustainability indicators have become more popular in the literature (e.g., Mori and Christodoulou, 2012).
Here we use the concept of ecological footprint, which has been proposed as an analytic tool to estimate the “load” imposed on the ecosphere by any specified human population (Berkowitz and Rees, 2003). We choose it not because it is without controversy, but rather because it is one of the more commonly cited indicators that has been widely used in many different contexts around the world. The metric most often used is the total area of productive landscape and waterscape required to support that population (Rees, 1996; Wackernagel and Rees, 1996). Ecological footprint analysis has helped to reopen the controversial issue of human “carrying capacity.” The ecological footprint of a specified population is the area of land and water ecosystems required continuously
over time to produce the resources that the population consumes, and to assimilate the wastes that the population produces, wherever on Earth the relevant land and/or water is located.
Ecological footprint calculations show that the wealthy one-fifth of the human family appropriates the goods and life support services of 5 to 10 hectares (12.35 to 24.70 acres) of productive land and water per capita to support their consumer lifestyles using prevailing technology. Only about 2 hectares (4.94 acres) of such ecosystems are available, however, for each person on Earth (with no heed to the independent requirements of other consumer species). In discussing sustainability from a global perspective, Burger et al. (2012) argued that the laws of thermodynamics and biophysical constraints place limitations on what is possible for all systems, including human systems such as cities. Given the relevance and impact of these constraints to the discussion of various pathways to urban sustainability, a further examination of these issues and their associated challenges are described in Appendix C (as well as by Day et al., 2014; Seto and Ramankutty, 2016; UNEP, 2012).
Daly (2002) proposed three criteria that must be met for a resouce or process to be considered sustainable:
- For a renewable resource—soil, water, forest, fish—the sustainable rate of use can be no greater than the rate of regeneration of its source.
- For a nonrenewable resource—fossil fuel, high-grade mineral ores, fossil groundwater—the sustainable rate of use can be no greater than the rate at which a renewable resource, used sustainably, can be substituted for it.
- For a pollutant—the sustainable rate of emission can be no greater than the rate at which that pollutant can be recycled, absorbed, or rendered harmless in its sink.
Fiala (2008) pointed to two issues that can be raised regarding the ecological footprint method. One is that the ecological footprint is dominated by energy as over 50 percent of the footprint of most high- and middle-income nations is due to the amount of land necessary to sequester greenhouse gases (GHGs). The other is associated to the impact of technology intensity that is assumed for characterizing productivity in terms of the global hectare. The results do show that humans’ global ecological footprint is already well beyond the area of productive land and water ecosystems available on Earth and that it has been expanding in the recent decades.
Urban sustainability has been defined in various ways with different criteria and emphases, but its goal should be to promote and enable the long-term well-being of people and the planet, through efficient use of natural resources and production of wastes within a city region while simultaneously improving its livability, through social amenities, economic opportunity, and health, so that it can better fit within the capacities of local, regional, and global ecosystems, as discussed by Newman (1999).
Because an increasing percentage of the world’s population and economic activities are concentrated in urban areas, cities are highly relevant, if not central, to any discussion of sustainable development. While urban areas can be centers for social and economic mobility, they can also be places with significant inequality, debility, and environmental degradation: A large proportion of the world’s population with unmet needs lives in urban areas.
Although cities concentrate people and resources, and this concentration can contribute to their sustainability, it is also clear that cities themselves are not sustainable without the support of ecosystem services, including products from ecosystems such as raw materials and food, from nonurban areas. Indeed, it is unrealistic—and not necessarily desirable—to require cities to be solely supported by resources produced within their administrative boundaries. Thinking about cities as closed systems that require self-sustaining resource independence ignores the concepts of comparative advantage or the benefits of trade and economies of scale. Since materials and energy come from long distances around the world to support urban areas, it is critical for cities to recognize how activities and consumption within their boundaries affect places and people outside their boundaries. Here it is important to consider not only the impact on land-based resources but also water and energy that are embodied in products such as clothing and food.
Ultimately, all the resources that form the base on which urban populations subsist come from someplace on the planet, most often outside the cities themselves, and often outside of the countries where the cities exist. Indeed, often multiple cities rely on the same regions for resources. Thus, urban sustainability cannot be limited to what happens within a single place. The sustainability of a city cannot be considered in isolation from the planet’s finite resources, especially given the aggregate impact of all cities. Therefore, urban sustainability will require making explicit and addressing the interconnections and impacts on the planet.
Urban sustainability is therefore a multiscale and multidimensional issue that not only centers on but transcends urban jurisdictions and which can only be addressed by durable leadership, citizen involvement, and regional partnerships as well as vertical interactions among different governmental levels.
In this context, we offer four main principles to promote urban sustainability, each discussed in detail below:
Principle 1: The planet has biophysical limits.
Principle 2: Human and natural systems are tightly intertwined and come together in cities.
Principle 3: Urban inequality undermines sustainability efforts.
Principle 4: Cities are highly interconnected.
Principle 1: The Planet Has Biophysical Limits
Urban areas and the activities within them use resources and produce byproducts such as waste and pollution that drive many types of global change, such as resource depletion, land-use change, loss of biodiversity, and high levels of energy use and greenhouse gas emissions. Over the long term and at global scales, economic growth and development will be constrained by finite resources and the biophysical limits of the planet to provide the resources required for development, industrialization, and urbanization. Currently, many cities have sustainability strategies that do not explicitly account for the indirect, distant, or long-lived impacts of environmental consumption throughout the supply and product chains. Because urban systems connect distant places through the flows of people, economic goods and services, and resources, urban sustainability cannot be focused solely on cities themselves, but must also encompass places and land from which these resources originate (Seto et al., 2012). Consequently, what may appear to be sustainable locally, at the urban or metropolitan scale, belies the total planetary-level environmental or social consequences. Urban sustainability strategies and efforts must stay within planetary boundaries,1 particularly considering the urban metabolism, constituted by the material and energy flows that keep cities alive (see also Box 3-1) (Burger et al., 2012; Ferrão and Fernández, 2013). Without paying heed to finite resources, urban sustainability may be increasingly difficult to attain depending on the availability and cost of key natural resources and energy as the 21st century progresses (Day et al., 2014, 2016; McDonnell and MacGregor-Fors, 2016; Ramaswami et al., 2016). In practice cities could, for example, quantify their sustainability impacts using a number of measures such as per capita ecological footprint and, making use of economies of scale, make efforts to reduce it below global levels of sustainability. Together, cities can play important roles in the stewardship of the planet (Seitzinger et al., 2012).
In an increasingly urbanized and globalized world, the boundaries between urban and rural and urban and hinterland are often blurred. In an era that is characterized by global flows of commodities, capital, information, and people, the resources to support urban areas extend the impacts of urban activities along environmental, economic, and social dimensions at national and international levels, and become truly global; crossing these boundaries is a prerequisite for sustainable governance. Ultimately, given its U.S. focus and limited scope, this report does not fully address the notion of global flows. It nevertheless serves as an indicator for advancing thinking along those lines.
1 Planetary boundaries define, as it were, the boundaries of the “planetary playing field” for humanity if we want to be sure of avoiding major human-induced environmental change on a global scale (Rockström et al., 2009). The concept of planetary boundaries has been developed to outline a safe operating space for humanity that carries a low likelihood of harming the life support systems on Earth to such an extent that they no longer are able to support economic growth and human development . . . planetary boundaries do not place a cap on human development. Instead they provide a safe space for innovation, growth, and development in the pursuit of human prosperity in an increasingly populated and wealthy world (Rockström et al., 2013).
There are many policy options that can affect urban activities such that they become active and positive forces in sustainably managing the planet’s resources. In many ways, this is a tragedy of the commons issue, where individual cities act in their own self-interest at the peril of shared global resources. One challenge in the case of cities, however, is that many of these shared resources do not have definable boundaries such as land. Moreover, because most cities are geographically separated from their resource base, it is difficult to assess the threat of resource depletion or decline. Thus, some strategies to manage communal resources, such as community-based, bottom-up approaches examined by Ostrom (2009a), may be more difficult to obtain in urban settings. Another approach is for government intervention through regulation of activities or the resource base.
As one example, McGranahan and Satterthwaite (2003) suggested that adding concern for ecological sustainability onto existing development policies means setting limits on the rights of city enterprises or consumers to use scarce resources (wherever they come from) and to generate nonbiodegradable wastes. Such limits can be implemented through local authorities’ guidelines and regulations in planning and regulating the built environment, e.g., guidelines and regulations pertaining to building material production, construction, building design and performance, site and settlement planning, and efficiency standards for appliances and fixtures. Ultimately, the laws of thermodynamics limit the amount of useful recycling.
Goals relating to local or global ecological sustainability can be incorporated into the norms, codes, and regulations that influence the built environment. But city authorities need national guidelines and often national policies. In most political systems, national governments have the primary role in developing guidelines and supporting innovation allied to regional or global conventions or guidelines where international agreement is reached on setting such limits.
The effort of promoting sustainable development strategies requires a greater level of interaction between different systems and their boundaries as the impacts of urban-based consumption and pollution affect global resource management and, for example, global climate change problems; therefore, pursuing sustainability calls for unprecedented system boundaries extensions, which are increasingly determined by actions at the urban level. This is to say, the analysis of boundaries gives emphasis to the idea of “think globally, act locally.”
Principle 2: Human and Natural Systems Are Tightly Intertwined and Come Together in Cities
Healthy people-environment and human-environment interactions are necessary synergistic relationships that underpin the sustainability of cities. In order for urban places to be sustainable from economic, environmental, and equity perspectives, pathways to sustainability require a systemic approach around three considerations: scale, allocation, and distribution (Daly, 1992). Human well-being and health are the cornerstones of livable and thriving cities although bolstering these relationships with myopic goals that improve human prosperity while disregarding the health of natural urban and nonurban ecosystems will only serve to undermine both human and environmental
urban sustainability in the long run. The future of urban sustainability will therefore focus on win-win opportunities that improve both human and natural ecosystem health in cities. These win-win efficiencies will often take advantage of economies of scale and adhere to basic ideas of robust urbanism, such as proximity and access (to minimize the time and costs of obtaining resources), density and form (to optimize the use of land, buildings, and infrastructure), and connectedness (to increase opportunities for efficient and diverse interactions).
Local decision making must have a larger scope than the confines of the city or region. Discussions should generate targets and benchmarks but also well-researched choices that drive community decision making. Sustainability is a community concern, not an individual one (Pelletier, 2010). Healthy human and natural ecosystems require that a multidimensional set of a community’s interests be expressed and actions are intentional to mediate those interests (see also Box 3-2).
Principle 3: Urban Inequalities Undermine Sustainability Efforts
Reducing severe economic, political, class, and social inequalities is pivotal to achieving urban sustainability. Many of these class and cultural inequalities are the products of centuries of discrimination, including instances of officially sanctioned discrimination at the hands of residents and elected leaders (Fullilove and Wallance, 2011; Powell and Spencer, 2002). Extreme inequalities threaten public health, economic prosperity, and citizen engagement—all essential elements of urban sustainability. Although perfect class and economic equality is not possible, severe urban disparities should remain in check if cities are to realize their full potential and become appealing places of choice for multigenerational urban dwellers and new urban immigrants alike.
Discriminatory practices in the housing market over many decades have created racial segregation in central cities and suburbs. Restrictive housing covenants, exclusionary zoning, financing, and racism have placed minorities and low-income people in disadvantaged positions to seek housing and neighborhoods that promote health, economic prosperity, and human well-being (Denton, 2006; Rabin, 1989; Ritzdorf, 1997; Sampson, 2012; Tilley, 2006). Poor neighborhoods have felt the brunt of dumping, toxic waste, lack of services, and limited housing choices (Collin and Collin, 1997; Commission for Racial Justice, 1987). There is evidence that the spatial distribution of people of color and low-income people is highly correlated with the distribution of air pollution, landfills, lead poisoning in children, abandoned toxic waste dumps, and contaminated fish consumption. Inequitable environmental protection undermines procedural, geographic, and social equities (Anthony, 1990; Bullard, 1995). These same patterns of inequality also exist between regions and states with poor but resource-rich areas bearing the cost of the “resource curse” (see also Box 3-3).
Principle 4: Cities Are Highly Interconnected
Cities are not islands. Urban systems are complex networks of interdependent subsystems, for which the degree and nature of the relationships are imperfectly known. The spatial and time scales of various subsystems are different, and the understanding of individual subsystems does not imply the global understanding of the full system. Meeting the challenges of planetary stewardship demands new governance solutions and systems that respond to the realities of interconnectedness. Currently, urban governance is largely focused on single issues such as water,
transportation, or waste. A multiscale governance system that explicitly addresses interconnected resource chains and interconnected places is necessary in order to transition toward urban sustainability (Box 3-4).
Urban sustainability requires the involvement of citizens, private entities, and public authorities, ensuring that all resources are mobilized and working toward a set of clearly articulated goals. This is particularly relevant as places undergo different stages of urbanization and a consequent redrawing of borders and spheres of economic influence. Sustainable solutions are to be customized to each of the urban development stages balancing local constraints and opportunities, but all urban places should strive to articulate a multiscale and multipronged vision for improving human well-being. An important example is provided by climate change issues, as highlighted by Wilbanks and Kates (1999): Although climate change mainly takes place on the regional to global scale, the causes, impacts, and policy responses (mitigation and adaptation) tend to be local.
As discussed by Bai (2007), the fundamental point in the scale argument is that global environmental issues are simply beyond the reach and concern of city government, and therefore it is difficult to tackle these issues at the local level. As simple and straightforward as this may sound, the scale argument encompasses more than spatial scale—it is composed of multiple dimensions and elements. Bai (2007) points to three—the spatial, temporal, and institutional dimensions—and in each of these dimensions, three elements exist: scale of issues, scale of concerns, and scale of actions and responses. Understanding these interconnections within system boundaries, from urban to global, is essential to promote sustainability. In particular, the institutional dimension plays an important role in how global issues are addressed, as discussed by Gurr and King (1987), who identified the need to coordinate two levels of action: the first relates to “vertical autonomy”—the city’s relationship with federal administration—and the second relates to the “horizontal autonomy”—a function of the city’s relationship with local economic and social groups that the city depends on for its financial and political support.
Designing a successful strategy for urban sustainability requires developing a holistic perspective on the interactions among urban and global systems, and strong governance. This lens is needed to undergird and encourage collaborations across many organizations that will enable meaningful pathways to urban sustainability. In order to facilitate the transition toward sustainable cities, we suggest a decision framework that identifies a structured but flexible process that includes several critical elements (Figure 3-1).
The roadmap is organized in three phases: (1) creating the basis for a sustainability roadmap, (2) design and implementation, and (3) outcomes and reassessment. A description of each of these phases is given below.
Phase 1: Creating the Basis for a Sustainability Roadmap
Adopt Urban Sustainability Principles
This is the first step to establish an urban sustainability framework consistent with the sustainability principles described before, which provide the fundamental elements to identify opportunities and constraints for different contexts found in a diversity of urban areas.
Identify Opportunities and Constraints
Any urban sustainability strategy is rooted in place and based on a sense of place, as identified by citizens, private entities, and public authorities. In this step it is critical to engage community members and other stakeholders in identifying local constraints and opportunities that promote or deter sustainable solutions at different urban development stages. Community engagement will help inform a multiscale vision and strategy for improving human well-being through an environmental, economic, and social equity lens. Often a constraint may result in opportunities in other dimensions, with an example provided by Chay and Greenstone (2003) on the impact of the Clean Air Act amendments on polluting plants from 1972 and 1987. They found that while those companies lost almost 600,000 jobs compared with what would have happened without the regulations, there were positive gains in health outcomes. Complementary research showed that clean air regulations have reduced infant mortality and increased housing prices (Chay and Greenstone, 2005; EPA, 1999). This study provides direct and easily interpreted estimates of the air quality and infant health benefits of the 1970 Act. The results imply that poor air quality had substantial effects on infant health at concentrations near the U.S. Environmental Protection Agency–mandated air quality standard and that roughly 1,300 fewer infants died in 1972 than would have in the absence of the Act. Furthermore, this study’s findings cross-validate the findings of earlier work examining the recession-induced pollution reductions of the early 1980s.
Prioritize and Identify Co-net Benefits
Decision making at such a complex and multiscale dimension requires prioritization of the key urban issues and an assessment of the co-net benefits associated with any action in one of these dimensions. Where possible, activities that offer co-occurring, reasonably sized benefits in multiple dimensions of sustainability should be closely considered and pursued as primary choices while managing tradeoffs. Activities that provide co-benefits that are small in magnitude, despite being efficient and co-occurring, should be eschewed unless they come at relatively small costs to the system.
Phase 2: Design and Implementation
Catalyze and Engage Partnerships with Major Stakeholders and the Public
Urban sustainability requires durable, consistent leadership, citizen involvement, and regional partnerships as well as vertical interactions among different governmental levels, as discussed before. Furthermore, the governance of urban activities does not always lie solely with municipal or local authorities or with other levels of government. Nongovernmental organizations and private actors such as individuals and the private sector play important roles in shaping urban activities and public perception.
Establish Goals, Targets, and Indicators
Commitment to sustainable development by city or municipal authorities means adding new goals to those that are their traditional concerns (McGranahan and Satterthwaite, 2003). Meeting development goals has long been among the main responsibilities of urban leaders. These goals generally include attracting new investment, improving social conditions (and reducing social problems), ensuring basic services and adequate housing, and (more recently) raising environmental standards within their jurisdiction. These goals do not imply that city and municipal authorities need be major providers of housing and basic services, but they can act as supervisors and/or supporters of private or community provision. A concern for sustainable development retains these conventional concerns and adds two more. The first is to consider the environmental impacts of urban-based production and consumption on the needs of all people, not just those within their jurisdiction. The second is an understanding of the finite nature of many natural resources (or the ecosystems from which they are drawn) and of the capacities of natural systems in the wider regional, national, and international context to absorb or break down wastes.
Develop Urban Sustainability Strategies
Urban sustainability goals often require behavior change, and the exact strategies for facilitating that change, whether through regulation or economic policies, require careful thought. Specific strategies can then be developed to achieve the goals and targets identified. These strategies should not be developed in isolation, but rather in collaboration with, or ideally, developed by, the practitioners responsible for achieving the goals and targets. This helps to facilitate the engagement, buy-in, and support needed to implement these strategies.
The strategies employed should match the context. Specifically, market transformation can traditionally be accomplished by first supporting early adopters through incentives; next encouraging the majority to take action through market-based approaches, behavior change programs, and social norming; and, finally, regulating to prompt action from laggards. This common approach can be illustrated in the case of urban food scraps collection where many cities first provided in-kind support to individuals and community groups offering collection infrastructure and services, then rolled out programs to support social norming in communities (e.g., physical, visible, green bins for residents to be put out at the curb), and finally banned organics from landfills, providing a regulatory mechanism to require laggards to act.
Identify Data Availability and Gaps and Establish an Indicator Framework
The development of analysis to improve the sustainability of urbanization patterns, processes, and trends has been hindered by the lack of consistent data to enable the comparison of the evolution of different urban systems, their dynamics, and benchmarks. Providing the data necessary to analyze urban systems requires the integration of different economic, environmental, and social tools. These tools should provide a set of indicators whose political relevance refers both to its usefulness for securing the fulfillment of the vision established for the urban system and for providing a basis for national and international comparisons, and the metrics and indicators should be policy relevant and actionable. Furthermore, the development of indicators should be supported with research that expresses the impact of the indicator. Every indicator should be connected to both an implementation and an impact statement to garner more support, to engage the public in the process, and to ensure the efficiency and impact of the indicator once realized. Understanding indicators and making use of them to improve urban sustainability could benefit from the adoption of a DPSIR framework, as discussed by Ferrão and Fernández (2013). The DPSIR framework describes the interactions between society and the environment, the key components of which are driving forces (D), pressures (P) on the environment and, as a result, the states (S) of environmental changes, their impacts (I) on ecosystems, human health, and other factors, and societal responses (R) to the driving forces, or directly to the pressure, state, or impacts through preventive, adaptive, or curative solutions. Such a framework of indicators constitutes a practical tool for policy making, as it provides actionable information that facilitates the understanding and the public perception of complex interactions between drivers, their actions and impacts, and the responses that may improve the urban sustainability, considering a global perspective.
Practitioners starting out in the field would be well served by adopting one or more of the best practice standards (e.g., United Nations Sustainable Development Goals, Urban Sustainability Directors Network Sustainability Tools for Assessing and Rating Communities, and International Organization for Standardization Sustainability Standards) rather than endeavoring to develop their own unique suite of metrics as their data would be more comparable between cities and would have some degree of external validity built in. A practitioner could complement the adopted standard(s) with additional indicators unique to the city’s context as necessary.
Institutional scale plays an important role in how global issues can be addressed. For example, as discussed by Bai (2007), at least two important institutional factors arise in addressing GHG emission in cities: The first is the vertical jurisdictional divide between different governmental levels; the second is the relations between the local government and key industries and other stakeholders. According to the definition by Gurr and King (1987), the first relates to vertical autonomy, which is a function of the city’s relationship with senior-level government,
and the second relates to horizontal autonomy, which is a function of the city’s relationship with local economic and social groups that the city depends on for its financial and political support. The implementation of long-term institutional governance measures will further support urban sustainability strategies and initiatives.
Phase 3: Outcomes and Reassessment
Assessing Impacts from Local to Global
Conceptually, the idea that there is an ecological footprint, and that sustainable cities are places that seek to minimize this footprint, makes great sense (Portney, 2002). Assessing a city’s environmental impacts at varying scales is extremely difficult. In practice, simply trying to pin down the size of any specific city’s ecological footprint—in particular, the ecological footprint per capita—may contribute to the recognition of its relative impacts at a global scale.
How many goods are imported into and exported from a city is not known in practically any U.S. city. Getting an accurate picture of the environmental impacts of all human activity, including that of people working in the private sector, is almost impossible. However, some cities are making a much more concerted effort to understand the full range of the negative environmental impacts they produce, and working toward reducing those impacts even when impacts are external to the city itself. Cities that are serious about sustainability will seek to minimize their negative environmental impacts across all scales from local to global.
Develop Ground Truthing and Public Buy-in
As discussed by Bai (2007), although there are factors beyond local control, the main obstacles to bringing the global concerns onto the local level are the reflection of contradictory perceptions, concerns, interests, and priorities, rather than the scale of the issue. Therefore, the elimination of these obstacles must start by clarifying the nature of the issue, identifying which among the obstacles are real and which can be handled by changing perceptions, concerns, and priorities at the city level. For instance, over the past 50 years, many U.S. cities experienced unprecedented reductions in population, prominently driven by highly publicized perceptions that city environments are somehow innately unsafe. However, recent scientific analyses have shown that major cities are actually the safest areas in the United States, significantly more so than their suburban and rural counterparts, when considering that safety involves more than simply violent crime risks but also traffic risks and other threats to safety (Myers et al., 2013). It is crucial for city leaders to be aware of such perceptions, both true and artificial, and the many opportunities that may arise in directly addressing public concerns, as well as the risks and consequences of not doing so.
Some of the major advantages of cities as identified by Rees (1996) include (1) lower costs per capita of providing piped treated water, sewer systems, waste collection, and most other forms of infrastructure and public amenities; (2) greater possibilities for, and a greater range of options for, material recycling, reuse, remanufacturing, and the specialized skills and enterprises needed to make these things happen; (3) high population density, which reduces the per capita demand for occupied land; (4) great potential through economies of scale, co-generation, and the use of waste process heat from industry or power plants, to reduce the per capita use of fossil fuel for space heating; and (5) great potential for reducing (mostly fossil) energy consumption by motor vehicles through walking. There is the issue, however, that economic and energy savings from these activities may suffer from Jevon’s Paradox in that money and energy saved in the ways mentioned above will be spent elsewhere, offsetting local efficiencies (Brown et al., 2011; Hall and Klitgaard, 2011).
Learning from Outcomes and Ongoing Reassessment of Goals and Priorities
The continuous reassessment of the impact of the strategy implemented requires the use of metrics, and a DPSIR framework will be particularly useful to assess the progress of urban sustainability. Here we advocate a DPSIR conceptual model based on indicators used in the assessment of urban activities (transportation, industry,
tourism, etc.), as discussed in Chapter 2. Classifying these indicators as characterizing a driver, a pressure, the state, the impact, or a response may allow for a detailed approach to be used even in the absence of a comprehensive theory of the phenomena to be analyzed. The use of a DPSIR model posits an explicit causality effect between different actors and consequences and ensures exhaustive coverage of the phenomena contained in the model (Ferrão and Fernandez, 2013).
Developing new signals of urban performance is a crucial step to help cities maintain Earth’s natural capital in the long term (Alberti, 1996). The task is, however, not simple. The challenge is to develop a new understanding of how urban systems work and how they interact with environmental systems on both the local and global scale. Three elements are part of this framework:
- Key variables to describe urban and environmental systems and their interrelationships;
- Measurable objectives and criteria that enable the assessment of these interrelationships; and
- Feedback mechanisms that enable the signals of system performance to generate behavioral responses from the urban community at both the individual and institutional levels.
A DPSIR framework is intended to respond to these challenges and to help developing urban sustainability policies and enact long-term institutional governance to enable progress toward urban sustainability.
Urban sustainability is a large and multifaceted topic. The following discussion of research and development needs highlights just a few ways that science can contribute to urban sustainability. This discussion focuses on promoting a systems approach—connections, processes, and linkages—that requires data, benchmarks, and guidance on what variables are relevant and what processes are most critical to understanding the relationships among the parts of the system. As such, there are many important opportunities for further research. These opportunities can be loosely placed in three categories: first, filling quantitative data gaps; second, mapping qualitative factors and processes; and third, identifying and scaling successful financing models to ensure rapid adoption.
First, large data gaps exist. Efforts have been made by researchers and practitioners alike to create sets of indicators to assist in measuring and comparing the sustainability of municipalities, but few thresholds exist, and those that do often seem unattainable to municipal leaders. For example, in order to ensure that global warming remains below two degrees Celsius, the theoretical “safe limit” of planetary warming beyond which irreversible feedback loops begin that threaten human health and habitat, most U.S. cities will need to reduce GHG emissions 80 percent by 2050. This is a target that leading cities have begun to adopt, but one that no U.S. city has developed a sound strategy to attain. Second, cities exist as part of integrated regional and global systems that are not fully understood. Further mapping of these processes, networks, and linkages is important in order to more fully understand the change required at the municipal level to support global sustainability. Some promising models exist, such as MIT’s Urban Metabolism framework, that warrant further development (Ferrão and Fernández, 2013). Third, the critical task of developing finance models to support urban sustainability action requires urgent attention. Successful models exist elsewhere (such as British Columbia, Canada’s, carbon tax), which can be adapted and scaled to support urban sustainability action across America.
A summary of major research and development needs is as follows.
Urban metabolism2 may be defined as the sum of the technical and socioeconomic processes that occur in cities, resulting in growth, production of energy, and elimination of waste (Kennedy et al., 2007).
2Abel Wolman (1965) developed the urban metabolism concept as a method of analyzing cities and communities through the quantification of inputs—water, food, and fuel—and outputs—sewage, solid refuse, and air pollutants—and tracking their respective transformations and flows. See also Holmes and Pincetl (2012).
Characterizing the urban metabolism constitutes a priority research agenda and includes quantification of the inputs, outputs, and storage of energy, water, nutrients, products, and wastes, at an urban scale. This task is complex and requires further methodological developments making use of harmonized data, which may correlate material and energy consumption with their socioeconomic drivers, as attempted by Niza et al. (2009), NRC (2004), Pina et al. (2015), and Rosado et al. (2014).
Once established, urban metabolism models supported by adequate tools and metrics enable a research stream to explore the optimization of resource productivity and the degree of circularity of resource streams that may be helpful in identifying critical processes for the sustainability of the urban system and opportunities for improvement.
Flows Between Places, Linkages, and Network Characterization
Much of the current information on urban areas is about “stocks” or snapshots of current conditions of a single place or location. However, what is needed is information on flows between places, which allows the characterization of networks, linkages, and interconnections across places. This type of information is critically important to develop new analyses to characterize and monitor urban sustainability, especially given the links between urban places with global hinterlands.
Research Aimed at Detecting Thresholds
To improve the threshold knowledge of sustainability indicators and their utility in defining an action strategy, it is necessary to have empirical tests of the performance and redundancy of these indicators and indicator systems.3 This is of increasing importance to policy makers and the public as human production and consumption put increased stress on environmental, economic, and social systems. In each parameter of sustainability, disruptions can only be withstood to a certain level without possible irreversible consequences. To avoid negative consequences, it is important to identify the threshold that is available and then determine the actual threshold values. The scientific study of environmental thresholds, their understanding, modeling, and prediction should also be integrated into early warning systems to enable policy makers to understand the challenges and impacts and respond effectively (Srebotnjak et al., 2010).
Understanding Different Types of Data
There is a need to go beyond conventional modes of data observation and collection and utilize information contributed by users (e.g., through social media) and in combination with Earth observation systems. The key here is to be able to provide information on processes across multiple scales, from individuals and households to blocks and neighborhoods to cities and regions.
Decision-Making Processes That Link Across Scales
Very little information on the phases of urban processes exists, be it problem identification or decision making. Information is needed on how the processes operate, including by whom and where outcomes and inputs are determined as well as tipping points in the system.
All of the above research needs derive from the application of a complex system perspective to urban sustainability. In other words, the needs call for the study of cities as complex systems, including the processes at different scales, determining factors, and tipping points to avoid adverse consequence. Ultimately, the goal of urban sustainability is to promote and enable the long-term well-being of people and the planet, yet doing so requires recognition of the biophysical constraints on all human and natural systems, as well as the acknowledgment that urban sustainability is multiscale and multidimensional, both encompassing and transcending urban jurisdictions.
3 Clark, C. M. 2015. Statement at NAS Exploratory Meeting, Washington, DC. October 15, 2015.
Urban sustainability therefore requires horizontal and vertical integration across multiple levels of governance, guided by four principles: the planet has biophysical limits, human and natural systems are tightly intertwined and come together in cities, urban inequality undermines sustainability efforts, and cities are highly interconnected. A comprehensive strategy in the form of a roadmap, which incorporates these principles while focusing on the interactions among urban and global systems, can provide a framework for all stakeholders engaged in metropolitan areas, including local and regional governments, the private sector, and nongovernmental organizations, to enable meaningful pathways to urban sustainability. Science can also contribute to these pathways by further research and development of several key facets of urban areas including urban metabolism, threshold detection of indicators, comprehension of different data sets, and further exploration of decision-making processes linked across scales.