Inputs

The standard functions of a city require huge amounts of energy, raw materials and water, and these resources are mostly finite. Energy is necessary for transportation, industry, commerce, construction, infrastructure, water distribution and food pro-duction. Globally, cities consume 75 % of the total energy generated (^{UN-Habitat, 2012}), but each city requires a different amount of energy depending on characteristics such as scale, area, population and level of development. This energy is consumed in variable proportion by three main urban sectors: construction, industry and transport (Table 1).

Table 1 Energy consumed in four US cities (in MMBtu^*). 

Source: Prepared by the authors based on the American Council for an Energy-Efficient Economy (ACEEE) (2017).

^*MMBtu = one million Btu; Btu = British Thermal Unit.

In cities in developed countries, more than half of the energy is consumed by the operation and maintenance of the built infrastructure, about 30% by the transportation system and the rest is consumed by industry (Figure 1). Bogotá, the capital of a developing country consumed a total of 183,715,392 MMBtu in 2010, slightly more than the energy consumed by Las Vegas in 2014 (^{Alfonso and Pardo, 2014}).

Source: Prepared by the authors based on United Nations (2008).

Figure 1 Energy consumed by four cities in developed nations (by %). 

In cities from intermediate economies, more than half the energy is consumed by transportation, the rest is distributed in varied proportion between construction and industry (Figure 2).

Source: Prepared by the authors based on ^{United Nations (2008)}.

Figure 2 Energy consumed by four cities in nations with intermediate incomes (by%). 

In the megacities of developing nations, like China or India, industry consumes up to 80% of the energy (Figure 3).

Source: Prepared by the authors based on ^{United Nations (2008)}.

Figure 3 Energy Consumed by three megacities in Asia (by%). 

On the other hand, the main materials that cities require are: biomass for the provision of food and for the production of some organic items; fossil fuels-coal, oil and natural gas-to generate energy; metallic and non-metallic minerals for industry; and materials for the construction industry: sand, stone, clay and cement supplies (^{Schaffartzik et al., 2014}).

Agrarian societies mainly consumed organic materials (biomass) for food, firewood and shelter (Franke, Busch and Zeitz, n.d.). With the evolution of civilizations, not only has there been a change in the materials that cities require, but demand has multiplied as well. The global transition from agrarian societies to industrialized regimes is linked to the massive exploitation of natural resources, such that the human race currently uses approximately 68 gigatons¹ (GT) of materials per year-ten times more than one hundred years ago (^{Schaffartzik et al., 2014}). In the last seventy years, the extraction of materials for cities has grown at an exponential rate, especially in Asia and Latin America (Figure 4). With respect to Bogotá, in 2010, a total of 2.9 billion tons of biomass (food) and 6 million tons of construction materials were consumed in the city (^{Alfonso and Pardo, 2014}).

Source: Prepared by the authors based on ^{Schaffartzik et al. (2014)}.

Figure 4 Consumption of domestic materials by region (in gigatons per year). 

Regarding annual water use, New York stands out globally with 10.9 million megaliters² (ML) followed by Cuangzhou with 9.8 million, Shanghai with 7.5 million, Los Angeles with 6.6 million, Tokyo with 4 million, Mumbai with 3.9 million, México City with 3 million, Beijingwith 2.9 million, Seoul with 2.7 million, Osaka with 2.5 million, Buenos Aires with 2.2 million, Cairo with 2.1 million, and Sao Paulo with 2.1 million. The rest of the cities in the world each consume less than 2 million ML per year (^{Kennedy et al., 2015}). According to officials from the Bogotá Aqueduct and Sewer Company an effective net water supply of 110 liters per inhabitant/day was provided in 2016 (^{Jiménez and Santana, 2017}). That is, the city consumed 0.321 million ML that year (Figure 5).

Source: Prepared by the authors based on ^{Kennedy et al. (2015)} and ^{Jiménez and Santana (2017)}.

Figure 5 Comparison: water consumption in seven cities. 

Outputs

In cities, energy, materials and water are trans-formed to generate goods or services that sustain the normal functions of urban life. Yet, trans-forming them generates a series of unwanted externalities: i) air pollution generated by fixed and mobile sources; ii) solid waste (organic and inorganic) resulting from industrial, commercial and residential processes; iii) debris generated by the construction industry or by demolition of obsolete buildings; and iv) liquid waste from industry, housing, commerce and other activities, such as recreational or educational activities.

In direct relation to the characteristics of each city-scale, area, population and level of development-a specific amount of externalities are generated. For example: regarding greenhouse gas emissions, New York produced the equivalent of 55 million tons of CO2 in 2011; Mexico City, 30.7 million in 2012; Buenos Aires, 15.6 million in 2008; and Bogotá, 6.7 million in 2014. Regarding solid waste, New York produced 14 million tons in 2014; Mexico City, 4.7 million in 2015; Buenos Aires, 4.8 million in 2010; and Bogotá, 2.3 mil-lion in 2014 (Figure 6). In addition, Bogotá generated 223 million cubic meters of wastewater in 2014, and 12 million tons of debris that same year (Government of the City of Buenos Aires, 2009; ^{González, 2010}; City of New York, 2011; Government of Mexico City, 2012, 2015; ^{Alfonso and Pardo, 2014}).

Source: Prepared by the authors based on various sources (2018).

Figure 6 Comparison: production of solid waste in four cities. 

Bearing in mind that the massive exploitation of natural resources is totally unsustainable because the global system is confined to a limited area and is therefore finite, and considering that emitting pollutants into the atmosphere, pouring liquid waste into lakes, rivers and oceans, and dumping solid waste into the biosphere are all detrimental, not only to ecosystems and biodiversity, but also to the human species, this article sets out the model for a Multidisciplinary Loop for Urban Sustainability.

1. The Closed Loop

The concept of a closed loop arose in the fertile fields of eighteenth-century mathematics and geometry. In 1735, the Swiss mathematician Leonard Euler proposed a circuit, or "closed path," that passes through each corner of a given polygon exactly once. This provided an answer to the famous problem of the Königsberg bridges⁶ while simultaneously creating the foundations for graph theory. More recently general systems theory as proposed by ^{Bertalanffy (1993)}, has formulated two types of systems: open and closed. Thermodynamics is applied specifically in closed systems.

The closed loop concept is currently applied to countless disciplinary fields: robotics, computer science, artificial intelligence, medicine, thermal comfort, mechanical ventilation, thermodynamic control and value networks, among others. According to Guide and van Wassenhove, the foundations of the closed loop concept in the field of industrial engineering were established in the mid-1990s, and since then they have been applied to a variety of recycling and reuse efforts, e.g. acquisition of used products, reverse logistics, remanufacturing, repair, and remarketing. These authors propose five phases in the evolution of the concept and its application in industrial engineering: i) the golden era of remanufacturing, which had its peak in the early 1990s; ii) from remanufacturing to an appreciation for reverse logistics, which gained importance during the first years of this century and fundamentally contributed to solving a series of economic and ecological problems; iii) coordination of the reverse supply chain, which provided a better understanding of how to design downstream and upstream channels, led to better decisions in the flow of upstream products, facilitated interaction between new and remanufactured products, and reduced the return rates for blue-ribbon clients; iv) Closing the cycle, an integrated design perspective, which has important implications for the market, but requires the recognition and participation of a large number of independent actors who need to be coordinated to be able to fulfill the economic potential of the system; and v) prices and markets, the current phase linking the concept to a wide variety of disciplines because, if prices and markets are not fully understood, they can become barriers or constraints, no matter how well-designed the operating system may be (2009). This phase is just beginning and is spearheaded by green markets.

2. Urban Metabolism

The concept of "urban metabolism" was coined by the American scientist, inventor and professor Abel ^{Wolman. He presented his ideas in an article published in 1965} entitled "The Metabolism of Cities." Wolman proposed two ideas: i) that cities have a set of metabolic needs that support their inhabitants, such as food, fuel, clothing, durable goods, construction materials, or electrical energy; and ii) that the metabolic cycle is only complete when the byproducts and waste caused by daily activities are removed and disposed of without creating danger or discomfort for the inhabitants of a city. Wolman questioned the traditional methods used in American cities to dispose of waste. He noted the high rates of air and water pollution and focused his study on what he called the three most acute metabolic problems: providing an adequate water supply, effectively disposing of wastewater, and controlling the contamination of water and air (1965). Wolman's proposal was dismissed by the scientific community for several decades, but recently it has garnered a lot of interest from urban planners and other scholars of urban phenomena who have now rescued and reevaluated urban metabolism. It is currently considered essential for the development of sustainable cities and com-munities. Urban metabolism has been defined as the total sum of technical and socio-economic processes that occur in cities, which is expressed through urban growth, energy production and the elimination of all types of waste (^{Kennedy, Pincetl and Bunje, 2011}). Apart from materials, energy and water, other authors link socioeconomic variables to urban metabolism, as well as pollutant vectors like carbon emissions (^{González, Donnelly, Jones, Chrysoulakis and Lopes, 2013}). Another line of research uses it as a tool to find urban and regional planning systems that are more sustainable, because it allows us to understand the way in which urban development impacts the local, regional and global environment (^{Conke and Ferreira, 2015}). Finally, other researchers propose a very recent concept: "intelligent urban metabolism." Thanks to ICT, this concept provides a way to assess the real time flow of matter and energy in an urban setting (^{Shahrokni, Lazarevic and Brandt, 2015}).

3. Circular Economies

In 1970, Kneese, Ayres and D'Arge published the book Economics and the Environment: A Materials Balance Approach. In it, they under-score the negative environmental effects of industrial externalities. For primitive man, the world and its resources were unlimited, but for twentieth-century civilizations they had ceased to be. They introduce the concept of "material balance" by proposing that the flow of materials be managed and channelled in direct relation to their economic value. In 1990, the English economist David Pearce introduced the concept of sustainable, green or environmental economics, which went against the characteristic anthropocentrism of neoclassical economic theories. Based on this new environmental perspective, he defined four basic functions of the environment: i) comfort; ii) natural resources; iii) waste absorption, or the "sink function;" and iv) the support of all life forms. Pearce, in collaboration with other researchers, laid the foundations for the circular economy through a series of books where he identified twelve variables: production, consumption, capital goods, utility, natural resources, recycling, waste, exhaustible resources, recyclable resources, assimilative capacity, harvest and yield (Pearce, Markandya and Barbie, 1989; Pearce and Turner, 1990).

Recently, the field of circular economics has significantly expanded. In brief, its main benefits are: i) a transformation of the role of resources within the economy; ii) the conversion of industrial waste into material for other industries; iii) the reparation, reuse or improvement of products at the end of their life cycle, instead of discarding them; iv) in a world characterized by high prices and volatile markets for resources, it offers huge business opportunities; v) if its implementation is accelerated through public policies, it can mitigate climate change, water scarcity and other global challenges; and vi) it can alleviate tensions around access to resources and supplies (^{Preston, 2012}). As is, the transition to a circular economy has only begun. Its inter-disciplinary framework offers good prospects for improving, or changing, current production and consumption models that are considered obsolete due to their environmental impact and the social inequality they generate (^{Ghisellini, Cialani and Ulgiati, 2016}). Ideally, the circular economy should create "autonomous production systems in which materials are used again and again" (^{Genovese, Acquaye, Figueroa and Koh, 2017}, p. 344). These authors, like many others, highlight the close relationship between circular economics and industrial ecology.

4. Industrial Ecology

According to Watanabe, the term "industrial ecology" was coined in 1971 by a research group from the Japanese Ministry of International Trade and Industry (MITI) (1994). Since then, it has come to define the industrial policy of that country according to the following precepts: i) a recognition of the limits of the system (the world), because it is ultimately confined to a limited area: planet earth; ii) a recognition of the internal relations of the system, because each organic and inorganic substance contributes to the stability of natural planetary cycles through complex relationships; iii) a recognition of the externalities of the system, because they play a very important role in maintaining the balance or imbalance of the system; iv) a recognition of the cause-effect relationships of the system in order to maintain balance, especially relationships established between human activities and the environment; and v) a recognition of the need for self-control, seeking an ideal balance between human activities and the operative limits of the system.

It should be stressed that the concept of industrial ecology is derived from two disciplines: ecology and systems theory. Under the postulates of these two disciplines, it seeks to study the development and behavior of industrial systems from the perspective of the evolutionary patterns of natural systems, which include: a closed loop of materials, evolutionary principles, system resilience and dynamic feedback (^{O'Rourke, Connelly and Koshland, 1996}). Although there are many authors who have contributed to the concept of industrial ecology, the objectives established by John ^{Ehrenfeld in 1994} remain current: i) the improvement of metabolic pathways for industrial processes and the use of materials; ii) the creation of closed loop industrial ecosystems; iii) the dematerialization of industrial production; iv) the systematization of energy use patterns; v) a balance between the capacity of natural ecosystems and materials and industrial products; vi) aligning policy with the long-term evolution of industrial systems; and vii) the creation of new structures for coordinated action using communicative and informative connections (p. 16).

5. Value Networks

The "value network" originated in the distribution and marketing fields of the 1980s, and it subsequently began to include other links in the production chain (Gibson, Hanna, Defee and Chen, 2014). From an organizational perspective, the value network emerged from the integration of a wide variety of interrelated activities that were initially fragmented. Regarding definitions, early on it was defined as a network of organizations that are involved through up and down links, in the different processes and activities that produce value in the form of products and services delivered lo the final consumer (^{Christopher, 1992}). For its part, the Council of Supply Chain Management Professionals under-stands the concept as the exchange of materials and information in the logistics process, which extends from the acquisition of raw materials to the delivery of the finished product to the end user. All vendors, service providers and customers are links in the value network (2010). More recently the value network has been defined as a series of integrated companies that must share information and coordinate physical execution to ensure an effective flow of goods, services, information and money (^{Coyle, Langley Novak and Gibson, 2013}). Other authors link a multiplicity of members and channels to the concept of value network. They highlight the flow of resources back and forth-"downstream" and "upstream"-and point out that customer requirements are met thanks to the operation and dynamics of the system, which involves all participants in the network(s) (^{Stock and Boyer, 2009}). Finally it should be stressed that there are no networks of equal value. Their differences depend on factors such as their structure, the industry sector, the geographical scope of the activity and the variety of products, compliance methods and patterns of demand.

6. Reverse Logistics

One of the first descriptions of "reverse logistics" was made in regards to industrial engineering: "Going the wrong way on a one-way street, because the vast majority of product shipments flow in one direction" (^{Lambert and Stock, 1981}). This description resembles the one put forward by ^{Murphy and Poist in 1989}: "the movement of goods from the consumer to the producer, along a distribution channel" (cited in ^{Rogers and Tibben-Lembke, 2001}, p. 129). Three more definitions were coined in 1998: i) it is the role of logistics in product returns, supply reduction, recycling, material substitution and reuse, waste product elimination and renewal, repair and remanufacturing (^{Stock, 1998}); ii) it is the process by which companies can be more environmentally efficient through recycling, reuse and reducing the amount of materials they use (^{Carter and Ellram, 1998}); and iii): "It is the process of planning, implementing and con-trolling the efficient and profitable flow of raw materials, in-process inventory finished products and related information from the point of consumption to the point of origin, with the purpose of recovering the associated value or defining its adequate regulation" (^{Rogers and Tibben-Lembke, 1998}).

In the 21st century, the concept has evolved quite a bit. In 2003, the Council of Logistics Management defines it as, "The process of moving goods from their final destination to another point, with the purpose of capturing value that a product would not otherwise be able to appropriate" (^{Don and Doldan, 2010}, p. 220). On the other hand, Cure, Meza and Amaya conceive it as:

The process of planning, developing and efficiently controlling the flow of materials, products and information, from the last link of the value network to the point of origin, so as to satisfy the needs of the consumer, recovering the waste obtained and managing it so that its reintroduction into the supply chain is possible, obtaining an added value or adequately eliminating it (2006, p. 186).

For Cabeza, reverse logistics "covers the set of logistic activities that include collection, dis-assembly and dismemberment of already used products or their components, as well as different material types and states, in order to make maximal use of their value in the broad sense of their sustainable use and their ultimate destruction" (2012, p. 26). Finally, Dyckhoff, Lackes and Reese consider that reverse logistics includes all activities related to the handling, processing, reduction and disposal of all hazardous and non-hazardous waste generated by the production, the packaging and the use of a product, which includes the inverse distribution process. They also highlight the ecological function, because reverse logistics can aid in the elimination of innumerable negative environmental impacts (2013).

7. Environmental Psychology

The term "environmental psychology" was coined by the Hungarian psychologist Egon ^{Brunswik in an article published in the Psycho-logical Review in 1943}. Thus began the first phase of this discipline, which was primarily focused on "environment-behavior" relation-ships, i.e. behavioral changes motivated by the environment. In the mid-1970s, a second phase was established, one specifically oriented toward the behaviors that architectural and urban spaces induce, and in this subsequent development it was given the name "architectural psychology." However, in both phases, only environment-behavior relationships were studied. It was not until the 1990s that a form of environmental psychology developed a relationship to environ-mental conservation. Thereafter, the preceding relationship was reversed to begin studying "behavior-environment" relationships, i.e. environmental effects caused by human behaviors. So there are two very different approaches to environmental psychology: i) one that analyzes the effects of natural or built environments on human behavior; and ii) one that studies the effects of human behavior on the physical and natural environment.

In relation to the second approach, there are two kinds of fundamental behaviors: responsible behavior with the environment (or sustainable behavior), which seeks to conserve and protect it, and irresponsible behavior (or unsustainable behavior), which is destructive and results in environmental degradation. To be clear, human beings are the global agents of environmental imbalance, and because the environmental problems that arose in the sixties and seventies could not be solved by the natural sciences (which had been assumed to be the field that would take charge in these issues), environmental psychology began to play a decisive role in the search for environmental equilibrium and urban sustainability (Baldi and García, 2006; ^{Berroeta; 2007}; ^{Pol, 2006}, ^{Roth, 2000}). Environmental psychology can be defined as "the study of the interrelation of an individual and his or her physical and social environment in its spatial and temporal dimensions," and it aims to identify the processes that regulate the relationship between an individual and his or her environment by revealing the individual's environmental imaginary and attendant behaviors; (^{Moser, 2003}, p. 14).

8. Ecodesign

Ecodesign (also known as environmental design or green design) emerged in the Netherlands in the early 1990s, and the first experiments to apply its principles on a large scale were funded, in that same decade, by the governments of Australia and the Netherlands. A huge number of guides for ecodesign soon emerged from the fields of design and industrial engineering, and by 2003, there were already 26,000 websites dedicated to this kind of environmentally friendly design (^{Ryan, 2003}, ²⁰⁰⁴). Regarding its definition, Glavic and Lukman conceive of it as "a product development process that takes into account the entire life cycle, and considers environmental aspects at all stages, in a search for products that generate the lowest possible environmental impact throughout their life cycle" (2007, p. 1875). They also claim that ecodesign seeks to reduce the input materials, to minimize negative externalities at the release, and to reduce human health risks, and that it is closely related to the evaluation of the life cycle, to environmental or green engineering, and to processes of reuse, recycling and remanufacturing. On the other hand, Balboa and Domínguez connect it to the circular economy (2014).

In 1999, the Malaysian architect Ken Yeang published the book Designing with Nature: The Ecological Basis for Architectural Design, where he addressed all the principles that have hitherto been mentioned about ecodesign. With this publication, he renewed the foundations of architectural design, landscape design and urban and regional planning. In collaboration with Lillian Woo, he also authored the Dictionary of Ecodesign: An lllustrated Reference, a book aimed at professionals belonging to a wide range of disciplines to fulfill ecodesign's broad inclusivity. Here follows the entry for ecodesign from the Yeang and Woo dictionary:

Also known as sustainable design, ecological design of the built environment, green architecture and green design. It is the management of the use of the processes of an ecosystem and its non-renewable resources through ecomimicry. Its main objectives are physically and mechanically integrating constructed forms and infrastructures with the characteristics and processes of the ecosystem at a given site; preventing the depletion of energy, water and raw material resources; preventing environmental degradation caused by the facilities and their life cycle; and creating a bio-integration between the built environment and the natural environment. It includes any form of design that minimizes destructive environment ¡mpacts through a physical, systemic and temporal ¡ntegration with the living processes of the natural environment (2010, p. 79).

Comparative Analysis

Table 2 presents a summary c comparative analysis of the concepts under review.

Table 2 Theoretical concepts that strengthen urban sustainability, comparative table. 

Source: Prepared by the authors based on various sources (2018).

Multidisciplinary Loop for Urban Sustainability

The urban, economic, industrial and human processes involved in the normal functions of a city currently require extracting enormous amounts of energy, materials and water that, when processed inside cities, generate a large amount of externalities. In an attempt to reduce the exploitation of natural resources and prevent the degradation of ecosystems and biodiversity over the past three hundred years, a series of isolated concepts aimed at renewing the balance between nature and the human species have emerged within different disciplines.

Unfortunately, these theoretical concepts that are so valuable for achieving environmental equilibrium have been kept in different disciplinary fields: isolated, separated, encapsulated, encrypted, closed-in on themselves, or better: embedded within the walls of their own disciplines. To the point that, normally, they can only be accessed from within their corresponding discipline. Unfortunately, although they share the same objective, urban and global sustainability, they do not relate to or combine with each other.

The aforementioned occurs due to the way various disciplines, which only capture a fragmented part of reality, perform their detailed and specific studies. They are artificially created and compart-mentalized disciplinary fields where the dominant tendency is to observe an object in fragmented form (Baldi and García, 2006).

Consequently, it is urgent that the concepts that strengthen urban sustainability presented in this article, although historically developed within different disciplines, are now integrated, structured and interrelated so that they can act as a unified and compact multidisciplinary system. As such, they will become more solid, effective and efficient. To unify and integrate them we propose a dynamic model, the Multidisciplinary Loop for Urban Sustainability: eight concepts for urban sustainability that emerged from various disciplines, and that for decades remained isolated, will be unified by drawing a loop around the city to produce arteries of communication between the various disciplines that involve and, by working together, significantly strengthen the sustainability of cities (Figure 7).

Source: Prepared by the authors

Multidisciplinary loop for urban sustainability.

It is imperative that each of the concepts and disciplines that constitute the Multidisciplinary Loop for Urban Sustainability begin to interact, understand and communicate with others, even if they belong to or have emerged from different disciplines. When all is said and done, all of them will be integrated into the same reality: the contemporary city.