32  Sustainability and cities

Meeting the grand challenge for the twenty-first century

Peter J. Marcotullio and William D. Solecki

Introduction

Urbanization in the twenty-first century is approaching a tipping point. The scale of increase in urban population and the rate of urbanization are unprecedented, and we are in the midst of a dramatic transition that will impact economic, social and environmental conditions at all scales (UNFPA, 2007; United Nations 2010). All the promises and challenges associated with human development are now increasingly concentrated in cities.

The scope, extent, speed, and timing of contemporary urbanization, combined with the demands for resource efficiency, pollution reduction, social equity and cohesion, and economic growth, create grand challenges for human security. By definition these are interrelated challenges, and they require urgent attention. As we expand current cities and construct new ones, we put into place forces that will constrain future choices. At the same time, there are fundamental questions about the limits to urban size, density, and environmental impact. Moreover, there is no robust theoretical understanding of what urban forms and processes are most economically efficient, socially beneficial, and environmentally benign. Finally, there is no evidence that the planet will be able to maintain an urban population of over 6 billion without severe ecological and/or global system collapse.

As the urbanization process continues, theory building and research into these challenges has become critical. Unfortunately, the trajectory of the knowledge development through the sustainable cities literature is not meeting the challenge. The details of this transition and the effects of increases in human density are not well understood. There remain myths about urbanization that have cast their shadows on policies (Satterthwaite, 2007b; UNFPA, 2007). This, in part, is due to the fragmentation of the sustainable cities literature, with researchers working within distinct frameworks representing different perspectives and understandings of sustainability (Haughton, 1999; McGranahan and Satterthwaite, 2000). A new holistic perspective on urban research is needed, and such a body of knowledge is beginning to emerge in response to policy demands (Bettencourt and West, 2010).

This chapter introduces the wider issues associated with contemporary urban transitions. We first briefly describe the current state and trends in global urbanization and the dynamics underpinning contemporary patterns of urban development. Next we provide a description and analysis of the sustainable cities literature. We discuss the trajectory of the literature and identify how it has splintered. We argue that although a new perspective on urbanization and sustainable development is developing, greater theory building efforts and critical reflection of our current understanding of urbanization processes are necessary before a holistic “urbanization science” will emerge.

State and trends of global urbanization

Traditionally, urbanization has been defined both by the concentration of population in urban areas and the increase in numbers of these centers (Tisdale, 1942). Over time, the average population of large cities around the world has increased. For example, Satterthwaite (2007b) notes that the average size of the world’s 100 largest cities increased from 200,000 in 1800 to well over 6 million by 2000. In 1800, there were only two cities in the world with populations of over 1 million, but by 2010 this figure increased to 452 cities (Chandler, 1987; United Nations, 2010). As with the global urban population, much of the growth in both the size and number of large cities occurred after 1950. Up until 1975, there were just three megacities (those with 10 million or more) in the world: New York, Tokyo and Mexico City¹ (United Nations, 2010). Since then, their number has increased markedly such that one of the features of contemporary urbanization is the rise in the number of large cities around the world (Dogan and Kasarda, 1988a, 1988b; Lo and Yeung, 1998; Fuchs et al., 1994; Dogan, 2004).

Global urbanization has been an uneven process; both temporally and spatially. Temporally, three major “waves” of global urbanization can be identified. The first and longest wave began 7,000 years ago, and was defined by slow and sporadic growth. With the emergence of cities, the development of global urbanization waxed and waned for approximately 6,750 years. The second wave occurred over the period between 1750 and 1950, and involved nations in Europe, North America and Japan. These nations experienced an increase in urban population share from 10 to 52 percent, representing an increase in urban population from 15 to 423 million residents (UNFPA, 2007). We are now in the midst of the third wave: 1950–2050. This wave is largely occurring in the developing world and represents an increase in urbanization level from 18 to 66 percent, corresponding to an increase from 302 million to 5.2 billion urban residents.

Spatially, urbanization processes have been highly uneven; during the first wave, urban centers were largely located in Western, Central and South Asia and in Central America. During the second wave, urbanization occurred most intensely in Europe and North America. By the middle of the twentieth century, urbanization shifted to regions in the developing world. By 1970, urbanization in Asia, Latin America and Africa reached 27 percent. As Table 32.1 demonstrates, while the general trend of rapid urbanization was experienced in all developing areas, there were important intra-regional distinctions.² Latin America, for example, has always had high urbanization levels due to the particularities of colonial rule. In 1970s, South American countries as a group had already reached 57 percent urbanization. Africa as a whole had reached urbanization levels of approximately 25 percent and in Asia the urban share was 23 percent. It has only been recently that China and India have attended significant urban population shares; by 2010, China reached 50 percent urban population and India 30 percent.

Given the large populations within the developing world, increasing levels of urbanization translates into a dramatic shift of the world’s urban population. Between the 1970s and 2010, the less developed countries increased their share of the total global urban population from approximately 51 percent to 73 percent. Asia, which has been experiencing an rapid economic development in the past decades, now has half the world’s urban population, followed by Latin America and Africa. Africa’s urban population is now larger than that of North America. It is interesting to note that Asia has always been the location of the majority of the world’s largest cities. As of 2010, 16 of the world’s largest cities were located in Asia, and only six were located in either North America or Europe (Table 32.2). By 2025, when the number of megacities is expected to reach 29, Asia would have gained another five (UN, 2010).

Given the large rural populations in developing countries and given that much of the developed world is already urbanized, the focus of urbanization in the future will continue to center on the developing world. Global urban population is expected to increase by 84 percent by 2050, from 3.4 billion in 2009 to 6.3 billion in 2050, and virtually all of the expected growth will be concentrated in the urban areas of the less developed regions, whose population is projected to increase from 2.5 billion in 2009 to 5.2 billion in 2050 (UN, 2010). It is in these spaces where vulnerabilities to global environmental change may be most significant, as the populations will be largest, most concentrated and have the least resources with which to adapt.

Perspectives on sustainable cities

Sustainable development was popularized by the Brundtland Commissions report, Our Common Future (1987), which included a chapter on urban issues that provided acknowledgment that cities were important to global development. The chapter primarily addressed urban service deficiencies in fresh water supply, sanitation, housing, waste disposal and transport. Within five years, however, the UN “Earth Summit” in Rio de Janeiro helped to redirect the focus to global issues associated with natural resources, climate change, ozone depletion, trans-boundary pollution, deforestation, soil loss, etc. Urbanization was left out, and poverty and health were termed “cross-sectoral issues.”

The literature on sustainable cities grew based upon an attempt to follow the basic definition of the Brundtland Commission. As Campbell (1996: 304) defined it, urban sustainable development is “the long-term ability of a system to reproduce.” The central focus was on long-term sustainable development of cities. A variety of books emerged that attempted to bring coherence to the growing literature (Haughton and Hunter, 1994; Leitmann, 1999; Satterthwaite, 1999; Wheeler and Beatley, 2004). Many simply combined a diverse set of writings to point out key issues.

What emerged in the urban literature were two main streams of thought differentiated by their focus on overcoming different sets of tensions. First, there are literatures based upon environmental degradation (“green” agenda issues) versus environmental hazards (“brown” agenda issues) (Hardoy et al., 2001). Second, there are literatures based upon ideological viewpoints about the inherent factors that could create sustainable cities (Haughton and Hunter 1994; Haughton, 1999).

Green and brown agendas

Green agenda

The “green” agenda dominated early studies, as it was compatible with the growing interests in global resource conservation and management and cities were seen as major sources of degradation (Haughton and Hunter, 1994). This interest was underpinned by a long history of urban biophysical studies dating back to before the 1800s, but increasing after the Second World War (Sukopp, 2008). As information on the urban environment generated from individual studies increased, entire book-length treatments emerged on individual cities, such as New York (Kieran, 1972) or London (Fitter, 1945), or on a set of environmental issues associated with the urban development (Detwyler and Marcus, 1972; Gill and Bonnett 1973; Hough 1984; Gilbert, 1989; Platt et al., 1994). These studies examined the “natural” environments of cities focusing on the provision of green space or the ecology of these spaces. Much emphasis was placed on how humans impacted urban biodiversity, hydrology, soils, and atmospheric conditions. The rationale driving these studies was that identifying the processes by which cities impacted the environment would help to inform policies on how to reduce this impact and hence create sustainable development. Few of these studies attempted to integrate the interactions between humans and environmental factors (exceptions are Havlick 1974; Douglas 1981, 1983), but rather saw urban residents as solely driving change. Moreover, environmental hazards associated with urban development were addressed simply as an infrastructure problem.

Brown agenda

The “brown” agenda focused on cities as centers of inequity and injustice (Drakakis-Smith, 1995, 1996, 1997; Hardoy et al., 2001). These studies started by examining the environmental conditions and hazards within poor cities, or within the poorest locations within cities. The poor quality of basic infrastructure and services has been a major theme in the urban literature on development (see for example, Hardoy et al., 1990; Hardoy and Satterthwaite, 1989). Research interest emerged with the growth of squatter settlements starting in the 1960s (Abrams, 1964). These studies pointed out that increasingly larger shares of urban residents of the developing world were living in slum settlements. They emphasized the lack of infrastructure, particularly housing, water supply and sanitation, as the source of health risks. Furthermore, these researchers positioned solutions as embedded in governance, as much as in technical fixes and improved ecological conditions. At the same time, governments were known for associating urbanization with poverty, international development agencies that perceived agricultural development as the first step to development and academic attention on urban policy as preventing nations from developing help to divert attention away from the growing crisis (Satterthwaite, 2007a). As a result, urban poverty has not been addressed and “brown agenda” issues remain an important concern for most cities in the developing world.

Different ideological viewpoints

The literature on sustainable cities also splintered into several different variants based upon ideology. Haughton (1999) and McCarney (1996), for example, have helpfully outlined the different perspectives of the sustainable city. At least four models of sustainable cities emerged within the literature: the free market model, the design model, the self-reliant city model, and the fair share model (Haughton, 1999). Each model has its own set of criteria for evaluating urban sustainable development. Together they form competing visions of the sustainable city and each embodies a different analysis of the problem and alternative attempts to re-order socio-technical relations. The results of each model chart the same city along very different pathways. Moreover, there is little leeway for synthesizing these models into one larger perspective. This is because the models prioritize different fundamental drivers of change, understand feedbacks and internal processes in cities differently, and use different criteria for sustainability. While each provides part of the vision and analysis needed to understand urban sustainable challenges, none alone provide a holistic and integrated (social, environmental and economic) perspective.

Free market perspectives

The externally dependent cities (free-market model) center on the largely non-spatial views of economists seeking to address urban environmental problems through altering market mechanisms. Hence, cities are the sources of economic growth and therefore sustainable development should be based upon their continued and improved economic performance. Those using this model focus on productivity, urban competitiveness and economic growth. Proponents espouse free trade, de-regulation and generally what has been called the “neo-liberal” agenda. This model interprets the sustainable problems of cities, be it poverty or degraded environments, as being largely created by market and regulatory failures (World Bank, 1991; for a critique see Burgess et al., 1997).

Design perspectives

The second model has been termed the “redesigning the city model” and it centers on the physical fabric of the existing city. Researchers examine how changes to the layout and structure of the city can encourage greater resource efficiency, reduced environmental impact, social safety, security and protection, conducive social environments, creativity, prestige, and livability (Frey, 1999). Those examining the city in this light focus on how cities are planned and specifically how this planning has impacted energy flows, usage and conservation (Lyle, 1994). This form of analysis interprets the problems of cities as largely based upon the inappropriate use of materials and lack of (e.g., sprawl, energy inefficient buildings) or misguided (e.g., segregated land use, lack of open space) planning (Calthorpe, 1993; Jenks and Burgess, 2000).

Self-reliant perspectives

The “self-reliant city model” centers on combining a more sensitive approach to nature with a decentralized, community-based, grass-roots politics. This model embraces the deep ecology approach to sustainable development, meaning that all healing activities start with the assumption that living things have an inherent worth aside from their utility to humans. The model emphasizes preservation of natural assets and a philosophical ethics that focuses on reducing human impact to natural systems. Importantly, the model sees the changes needed for urban sustainability as driven by bottom-up forces and that social diversity, like bio-diversity is a “good.” Researchers examine how small, autonomous communities reduce ecological footprints, and the use of “appropriate” technologies associated with small-scale economic production. The goal is to create a city that does not need external resources. Urban problems are understood as resulting from inharmonious growth and therefore balance should be achieved of human and natural structures and processes (Walter et al., 1992; Roseland, 1997; Register, 2006).

Fair-share perspectives

The “fair shares model” centers on greater attention to the conditions of trading between a city-region and its hinterland, based on resource consumption and waste-emission trends. This model is best understood as an aggregate approach that draws on many of the features of redesigning the city and the self-reliant city models, such as increase regional autarky, greater urban compaction and improved use of market tools, and adding tools for engaging in more equitable trading relationships with other urban areas. It also emphasizes the importance of domestic and increasingly international trade and investment relationships. Urban problems generate from unequal trade relationships or the dominance of foreign investments (Ravetz, 2000; Marcotullio, 2001).

Reconciling different perspectives

Attempts to reconcile the differences in these streams have made important contributions to the urban sustainable development literature. One major attempt put forth a model that suggested a transition of types of environmental issues with growing wealth, called the urban environmental transition (McGranahan et al., 2001). This model suggests three agendas: “brown,” “gray,” and “green” (Smith and Ezzati, 2005) and postulates that as cities grow in affluence, urban environmental burdens tend to shift from local, health threatening challenges that have immediate impacts, to global ecosystem threatening, with delayed impacts. Importantly, the model is based upon the notion that the type and scale of the environmental impact change with wealth. Accordingly, as a poor city moves beyond the “brown” agenda, which prioritizes concerns such as inadequate water supply and sanitation, indoor air quality, drainage and solid waste disposal, the environmental impacts of urban activities shift from the household and neighborhood scale to a citywide regional scale. At the metropolitan scale the dominant issues that come into focus include water and air pollution, sometimes called the “gray agenda.” As cities continue to grow in wealth, urban environmental burdens shift from metropolitan scale challenges to those most easily observed at the regional and global scale. Global scale challenges are associated with the “green” agenda and include acid rain, water scarcity, greenhouse gas and ozone depleting substance emissions, for example (Marcotullio, 2005).

In this way the impact of cities on the environment is seen along a continuum of increasing wealth with changing but ever present environmental challenges. Moreover, environmental problems are never completely resolved but rather outcomes are displaced across time and space. Historical urban research that examining urban growth and environmental impacts suggests that in the past, urban environmental challenges associated with water supply, sanitation and solid waste management were addressed by simply dispersing the harms to greater scales (Tarr, 1996; Melosi, 2000). Historians have identified changes in type, geographic, and temporal aspects of environmental burdens that are comparable to urban environmental transition theory (Marcotullio, 2005).

Another way of resolving the conflicts between the “brown” and “green” agenda is to include both challenges in a systems approach that allows policy makers to choose the most appropriate solution (McGranahan and Satterthwaite, 2000). When all challenges are considered, priorities and potential alternatives that address both can be examined. First, reconciling these agendas means that different cities or even different neighborhoods within a single city may have different priorities. Second, some agendas can be addressed through carefully crafted policies. For example, air pollution from automobiles that cause health problems which is also related to trans-boundary pollution and global greenhouse gas emissions, can be addressed through transit, traffic and mobility policies. Solid waste problems that create pest infestation and health impacts through oral-fecal material contact can be addressed through sanitation infrastructure, materials storage, recycling and energy policies (McGranahan and Satterthwaite, 2003).

While the urban environmental transition theory is powerful, the focus is not inherently on the urban. Indeed, one could substitute individuals, households, neighborhoods, states, or nations for cities and the theory would generally hold. The reality that the theory represents is change in environmental conditions with increasing wealth, not what is inherently different or unique about cities in this process.

There has been less success in integrating different normative approaches within the sustainable cities literature. Care is required in interpreting attempts to reconcile sustainable city models. Some research that portends universal findings remains biased due to implicit ideological perspectives. Lists of principles for creating sustainable cities, for example, provide a general blueprint to facilitate conforming to one particular agenda (Roseland 1997; Kenworthy, 2006). However, detailed studies of how to manage urban sectors in a sustainable fashion, such as transportation or urban agriculture, typically do not consider cross-sector and cross-scale interactions. Researchers have continued to search for ways to integrate agendas in pursuit of a knowledge base on urban sustainable development (Seto and Satterthwaite, 2010). As Grubler et al. (2012) point out, however, there is a lack of understanding of how higher-order organizations of urban systems (i.e., urban functions, urban economy, urban infrastructural setup) shape the scale and intensity of urban activities and hence environmental impact. The problem with much of the current literature is not that it is wrong, but that is remains partial (Holling, 2001; Gunderson and Holling, 2002).

Integrating perspectives on urban sustainable development

What is needed is a more holistic and comprehensive approach; a new science of urbanization. The focus of this science would be on the fundamental laws of the urbanization process including the origin, development, organization, emergent properties and connection to other social and biophysical systems. We see the formation of this science in recent research that attempts to bring together both the human and environmental aspects of the sustainable development agenda. These studies are converging from research fields such as urban ecosystems, coupled human-environment interactions and urban socio-ecological system analysis. Some of the important components of this new approach include a systems approach which attempts to integrate social and biophysical elements, an emphasis on scale and a focus on the higher-order aspects of urbanization. We believe that this literature forms the basis of new scientific understandings of cities and can lead to policies that enhance the promotion of sustainable development both within urban areas and in the distal areas associated with and impacted by urban activities. In this last sub-section we describe these elements and point out types of research that are helping to create this new perspective.

Systems approaches developed during the 1950s and 1960s have been used to study cities in the past and to provide the basis of urban plans (McLoughlin, 1969; Chadwick, 1971). For example, as early as the 1960s, Berry (1964), explained that cities were systems and operated within systems. The contemporary systems approaches are more inclusive of components of the urban system. Urban system thinking now includes economic, social and institutional aspects, as in the past, but now these studies also include biophysical components (Grimm et al., 2000). Cities are perceived as complex systems with important biophysical elements that affect and are affected by the social elements associated with urban life. In this regard, previous urban system models have been under-theorized. These new studies include accounting techniques such as material flows analysis, ecological foot-printing and human appropriation of net primary productivity, and identify the flows and linkages between social and biophysical change. They also allow for a quantification and analysis of the impact of socio-economic activities on the environment as well as what changes in the environment impact human activities. At the same time, they emphasize processes and move away from a focus on places.

Recent studies highlight the lack of consensus on the basic components of urbanization (Seto et al., 2012). Strongly held axiomatic beliefs on the relationship between the population share in urban centers and income generation, or the space for time substitution models associating urban land use and ecosystem service degradation are coming into question. What is desperately needed is an exploration of the most fundamental aspects of urbanization, across space, place, time and cultures. A proactive approach to urbanization and the complex set of socio-ecological systems interactions within and across urban areas demands an understanding of all system components and their interactions (Solecki et al., 2012).

Urban science approaches emphasize the importance of scale. Scale, meaning extent, is perceived as crucial in understanding urban activities, their effects and influence (Sanchez-Rodriguez et al., 2005). The notion of emergent phenomena is central to all complex systems. Emergence is defined by the patterns that arise from a multiplicity of relatively simple interactions (Alberti, 2008). Emergent phenomena may not be visible at a specific scale (i.e., sprawl cannot be understood by examining a single city block). Given the importance of emergence in urban systems, it is necessary to examine urban activities at a variety of scales, for example, the various environmental challenges associated with urbanization at different scales (Satterthwaite, 1997; McGranahan et al., 2001).

Currently, some researchers are examining urbanization at the global level looking for universal patterns. These types of studies were also performed in the past, exemplified by some basic urban laws, such as the rank-size rule (Zipf, 1946, 1947) that states that within a country or region there is a log–linear relationship between the rank of the city and its population size such that the largest city is twice the size of the next largest and three times the size of the third largest and so on. This finding spanned a set of studies that defined national urban systems and subsequent debates over the meaning of different patterns. Exactly why this rule works is still debated (see Fonseca, 1988). Rather than being based upon socially explanatory variables scholars studying this phenomenon in the past used a normative set of underlying assumptions to drive research. Terms such as “parasitic,” “over urbanization” (Davis and Golden, 1954; Hoselitz, 1954, 1957), “urban bias” (Lipton, 1977; Todaro, 1981), and “urban primacy” (Jefferson, 1939), were used to describe dysfunctional national urban development patterns because they failed to follow the rank-size rule. Rather than privileging any particular pattern of urban development, today’s research critically examines the empirical evidence that presents the urbanization system in-and-of-itself as unique amongst systems (at any scale) and asks how the associated elements and processes influence other systems. Scholars are attempting to identify a theory of urbanization with fundamental and unique components that can withstand scientific scrutiny and produce valuable universal laws and theories (Bettencourt et al., 2007; Bettencourt and West, 2010, 2011).

As the amount of data on cities and urbanization processes at the national level increased, some researchers have re-examined global-scale, urban-related relationships between urban activities and socio-economic variables (Bettencourt et al., 2007). These studies are providing evidence for a theoretical understanding of scaling laws associated with global urbanization. Population size of cities correlates to, for example, rates of innovation, wealth creation, energy consumption, and crime per capita. Some of the relationships are linear, others sublinear, meaning that economies of scale exist (larger cities are more efficient) and other relationships are supralinear, meaning increasing returns with population size (as population increases, the variables increases at a greater rate). This type of research, given its comprehensiveness, can help to identify the relationships between urbanization and other aspects of the Earth system.

Urbanization science focuses on the need to understand how the urbanization process unfolds so that we can develop generalities to larger scales, and with that, understand how this process interacts with local and global environments. For example, while there have been studies of how biodiversity changes in urban to rural gradients and statements on the increasing homogeneity of urban biota, there has not been a study that examines whether and how urbanization has affected, for example, the latitudinal gradient in biodiversity or any of the eco-geographic rules of biodiversity. The answers to this and other questions will lead to a better understanding of how, when, where and at what scale urbanization connects to laws and principles in other sciences.

Other research has examined the differences in socio-ecological systems of cities at the regional scale, examining differences among different groups of cities. Again, urban studies scholars have long noted the differences between cities, urbanization processes and urban activities of the developed and developing worlds (see for example Weber, 1899). Given the dynamics associated with contemporary urbanization, current studies, using databases over time and across space, emphasize comparative studies of historical transition experiences. While not all urban related experiences are universal, there are seemingly some principles that define differences between those of the developed and the developing worlds. For example, when it comes to urban environmental dynamics, urbanization is related to environmental harms sooner (at lower economic levels), which change faster (over time) and emerge in a more simultaneous manner (Marcotullio et al., 2005). Others have identified different types of environmental dynamics, such as levels of air pollution with different levels of income (Sarzynski, 2012).

Furthermore, there is an increasing interest in urban resilience and vulnerability. Recent events, such as Tropical Storm Sandy that hit the Northeast USA in October, 2012 causing more than $33 billion of damages, demand that a scientific basis for assessing urban climate change adaptation strategies is advanced (Rosenzweig et al., 2011b). Moreover, these types of studies allow for scenario development that facilitates both popular understanding of environmental change and furthers civil society discussion of potential future alternatives.

The goal of urbanization science is to develop relational models (as opposed to end state physical models such as “compact cities”) that can help to uncover patterns associated with urbanization including the effect of wider political and commercial processes and the variety (both good and bad) of the potential impacts of new technologies. The outcomes of the analyses provide not only the description of the physical patterns that emerge, but also the patterns of relationships between socio-economic and biophysical variables and processes. Because urbanization unfolds on multiple scales it needs to be addressed at these scales. While there are all too few examples of regional and national urban programs that address urbanization, there is nothing at the global scale. Urbanization science is an effort to fill this gap and therefore support policy development at these larger scales (Rosenzweig et al., 2011a; Grubler et al., 2012). Stakeholders can positively influence urbanization, but only if they have the scientific evidence and understanding of how the system works; how component parts are related, who the winners and losers of change are, and what the drivers are. Given the growing importance of urbanization and the concentration of populations and economies in cities, urbanization is now of global interest to researchers and policies. We believe that the moment has arrived for the development of an urbanization science that will provide the basis for and stimulate local, national and international laws, regulations and agreements.

Conclusion

Urbanization is one of the major trends and challenges of the current century. The increasing strength and level of populations located within urban areas have turned attention to global sustainable development on cities. The literature on sustainable cities has developed rapidly into separate areas of knowledge based upon the different factors influencing urban development and distinct ideologies. This fractured field with partial and biased knowledge is inadequate in addressing the grand challenges associated with current urbanization.

Despite the fracturing in the literature, there are also trends that suggest there is some convergence about how to approach urban development and urbanization. Recent literature is emerging that attempts to overcome the previous imbalances and develop a holistic approach. We call this new approach urbanization science. Specifically, recent studies on urban sustainable development have three important goals. First, models attempt to identify and understand the broad nature of urbanization processes. Given the amount of data and comprehensive nature, they provide an understanding of the contexts in which sustainability issues emerge, are developed and are reinterpreted. Second, the research provides a scientific basis for testing claims for the forms and processes of sustainable urbanization by exploring the socio-ecological effects of different development trajectories. Third, by moving away from a collection of individual city studies, urbanization science identifies different pathways incorporating local, regional and global variables that help to suppress the dominance of inappropriate (unsustainable) agendas and open up opportunities for alternative pathways. While we see much merit in the emerging urbanization science, there is also a need to incorporate an examination of who benefits urbanization patterns and processes. We have noted the valuable work to date on a variety of issues, but there is not enough emphasis, to date, on environmental justice. How to work into these studies a critical reflection of norms and values driving patterns and processes remains an important task for the development of this science.

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