We are getting used to hearing that ‘we live in a world of cities’. How to make city living sustainable is one of the key challenges of our times. The United Nations set a stand-alone goal on cities within the Sustainable Development Goals, stating to ‘make cities and human settlements inclusive, safe, resilient and sustainable’. As exemplified by this goal, it is becoming common for concepts such as sustainability and resilience to be used synergistically, or interchangeably. In order for cities to become more sustainable they must change the linear to a circular self-regulating sustainable relationship with the biosphere.
Nature essentially has a circular zero-waste metabolism where every output by an organism is also an input which replenishes and sustains the whole living environment. In contrast, the metabolism of many modern cities is essentially linear, with resources flowing through the urban system without much concern about their origin or the destination of wastes. Inputs and outputs are considered as largely unrelated.
Still, urban metabolism is a powerful metaphor that helps us think about the man-made environment in a completely different way, which feels less judgmental than certain other perspectives. All cities, like all nature’s organisms, have a definable metabolism: in order to satisfy their needs they need to produce waste. Urban metabolism can be described as ‘the sum total of the technical and socio-economic process that occur in cities, resulting in growth, production of energy and elimination of waste’. The characterization of these flows and the relationships between anthropogenic urban activities and natural processes and cycles defines the behavior of urban production and consumption. Material and energy flows arising from urban socioeconomic activities and regional and global biogeochemical processes. This metabolism determines the health and resiliency of a neighborhood, the city, and the surrounding region. Urban metabolism is therefore a deeply multi-disciplinary research domain focused on providing important insights into the behavior of cities for the purpose of advancing effective proposals for a more humane and ecologically responsible future.
While research has tended to focus predominantly on quantifying various flows in and out of cities, it has become a widely accepted notion that more than an organism, cities are human-dominant, coupled, complex ecosystems Recent urban metabolism studies have extended far beyond the original approach to quantify the flow budget, and started to reveal important characteristics of urban system features and interactions. Because of the human dominant feature of urban system means the concepts, theories, and approaches developed for, and knowledge obtained from, natural ecosystems are –also- unlikely to be sufficient to explain an urban ecosystem.
The magnitude, distribution, and internal interactions and feedbacks are regulated by policy, governance, culture, and individual and collective behavior of the urban system. In addition, while natural ecosystems may not have ‘a set points of control’ (a number of quantities the organism tries to keep at a particular value) like organisms, cities as human dominant systems do have stronger regulating and governing functions and mechanisms, such as the existence of city level government, which is embedded within the country’s government system and linked across various actors and agencies within and beyond the city. This regulating and governing mechanism plays a critical role in urban complex systems, through making policy, planning, and management decisions that influence both anthropogenic and ecological processes within and beyond the city.
Drawn from the rich and rapidly growing empirical findings of urban metabolism studies a new resilience theory has the potential to understand the behavior of cities as human-dominated, complex systems.
An urban system is human-dominated, and is governed by complex interactions among components as well as a unique regulating and governing mechanism that shape social and ecological processes. The resources that flow into cities shape and alter the structure of urban ecosystem, enable, and drive urban functions with influence on natural ecological processes of cities, and eventually produce intended or unintended outputs that either stay within the system boundary or exported beyond the boundary. In most if not all situations: shorten the production-supply consumption chain, increase resource use efficiency, minimize externalities, maximize internal regenerative capacity, balancing tradeoffs among different ecosystem services, and enhance the sense of place.
Within urban transitions there is far more fluidity in relationships where the knowledge is flowing within, across and between organizations, people, economics. The boundaries are blurring, that increasing fuzziness needs shifting our style of decision-making and solution finding. Urban development closely resembles the recent past, relying on existing models to determine appropriate actions and investments is rational. Struggling with the dominant linear logic constrains innovation, restricts to provide radically different urban transition and limits our abilities to change fast enough. Cities that become capable of managing the constant change and disequilibrium will thrive.
For building a science and resilience theory of cities, it is vital to recognize that urban systems can be truly complex. Cities display emergent properties, have dynamics that are far from equilibrium, and require enormous amounts of energy to maintain themselves. Many urban systems display key properties of complex adaptive systems, meaning that they can be highly interconnected and unpredictable while having modular subsystems that confer redundancy and are capable of resiliency.
Some components of cities or even some cities as a whole may be viewed as ‘novel ecosystems’. We may consider the city as a key laboratory for human-environment interactions and urbanization as a global experiment on sustainability. The coevolution continues. Future cities will reflect who we are, what we value, and how well we can ‘remake’ our world.