Maintaining a balance between natural, economic, and social capital has already shown itself to be one of the most significant challenges of the 21st century. For hundreds of years, economic and social capital has grown by transforming environmental resources into more ‘useful’ forms. Given that natural capital is finite, this growth is unsustainable by definition, and we are now beginning to observe the limits of this growth as our global environment comes under mounting pressure.
Cities are the epicentre for this transformation of capital. Understanding these transformations (and the outcomes they produce) is critical to shaping the future role of cities as engines of economic growth, consumers of resources, and producers of social networks. The stocks and flows of capital – whether natural, financial, or human – are in many ways defined by the nature of, and connections between, urban systems at various spatial scales. Driven by migration and urbanization, cities are rapidly expanding and mutating into polycentric metropolises with new metabolic processes. As innovative technologies, designs, and strategies are deployed in the built environment, additional interactions necessarily arise between these codependent systems. These multiplying interactions and compounding complexities, in turn, allow the metabolic processes of the city to be redefined so as to make capital transformation more or less sustainable. Indeed, with the rise of global megacities and the widespread clustering of rural populations into denser urban centres, we are now in an ‘urban age’ that to a large extent will define the sustainability of capital stocks and flows across the triple-bottom line.
Conceptualized as a balance between these three capital forms, sustainability is a state in which cities do not consume more natural capital than they produce, and consequently generate no economic or social capital gains. However, this is only one conceptualization. As pointed out by both the students and tutors of the MSc Sustainable Urban Development programme, there are a range of ideas as to what constitutes ‘sustainability’, many of which are directly contradictory. The Brundtland definition, for instance, opposes this balance-oriented view and puts forward the notion that growth can and should be sustained without drawing down on natural capital. Despite their differences, in both cases, these concepts of sustainability identify a particular state that can act as a ‘frame of reference’, which can be ontologically compared and contrasted with the state of cities at present through the use of metrics and assessment frameworks.
Other definitions take a more relativistic approach. The triple-bottom line conceptualizes sustainability as a quality determined by the economic, environmental, and social impacts of development, and like the Brundtland definition, it views growth as a component, rather than as an opponent, of sustainability. However, with no absolute ‘state’ of sustainability to use as a frame of reference, cities can only be more or less sustainable, depending on their relative impacts. In this case, the references becomes the cities themselves, not the concept of sustainability. Such relative definitions, though not useful as reference points for framing urban issues, can nonetheless be used as a lens for assessing the impacts of climate change and comparing the potential options for addressing them.
There is no one definition or concept of sustainability; in fact, it may be more appropriately thought of as an umbrella term for a broad collection of related concepts that both complement and contradict each other. These concepts can act as absolute ‘frames of reference’ in and of themselves, or relative ‘lenses’ for comparing two observable reference points. In either case, they are ultimately ontological in nature, and rely on comparisons with or between observations for their meaning. Therefore, sustainability as a concept (or set of concepts) does not constitute knowledge directly, but is rather informed by knowledge, and can only be used to interpret and analyze what is observed.
In this sense, the field of sustainability represents a new paradigm for understanding some of the most important issues facing our cities and planet today – not only in terms of where we want to go, but also how we might get there.