Exploring methodologies to understand the living city

“In order to translate the urban metabolism concept into a project methodology, PCA-STREAM relies on partnership-based approaches involving an entire ecosystem of experts in collective intelligence processes. … Therefore, there is a range of complexphenomena that are open to exploration by means of a more systemic approach. This approach is in itself an imaginary, impacting our representations of the city, its projects, and its forms of interaction. As a result, how this approach is implemented over time across the firm’s projects and how involved parties will take ownership will contribute to defining the urban metabolism metaphor in practice.”

This is how we presented our long-term work on the concept of the “urban metabolism” a few months ago (At the roots of Urban metabolism, PCA-STREAM, 2022). PCA-STREAM’s theoretical experiments centred on this idea place living beings at the forefront of a new paradigm for navigating our relationship to a complex world (Urban metabolisms: Combining complex Approaches, Philippe Chiambaretta, 2018).

Experimenting with existing methodologies: life cycles and urban metabolism

Two existing methodologies are currently simultaneously employed by the firm in urban-scale projects—life-cycle analysis (LCA) and urban metabolism analysis.
Life-cycle assessments examine the environmental impacts of a product or a system of products across its entire its life cycle, employing a comprehensive multi-criteria approach. This “system of products” is computed through aggregation at the building or urban scale. As for urban metabolism, it serves as a tool to quantify flows (inputs, outputs, and stocks) of energy and materials within a specific territory, and is also adaptable to different scales.

At first glance, when swiftly outlined in the context of urban design, these two methodologies may appear somewhat redundant. Both leverage the metaphor of the living, delving into the analysis of distinct stages in the life of entities such as materials, buildings, neighborhoods, or cities. They also both involve quantifying the flows of materials, energy, water, and waste associated with their life cycle—from raw material extraction to disposal (whether discarded, recycled, or reused), essentially covering the journey “from cradle to grave.” As a matter of fact, these two methodologies make use of relatively similar input data, revolving around the quantity and composition of materials used. Finally, they both share the same objective—to enhance our understanding of the quantifiable impacts of built objects on the environment, thus also highlighting any shortcomings for urban designers (and thus, implicitly, also helping identify opportunities for improvement).

Life-cycle analysis and urban metabolism do not fully overlap, however. Without claiming to conduct an exhaustive comparative analysis, it proves insightful to underscore specific differences to extract their full potential for urban projects. These include how they consider lifespan, how they account for human activities induced by projects, the graphical representations that can be drawn from them and their subsequent impact on the comparability of results, and the treatment of associated qualitative and societal aspects.

Taking into account the temporality of metabolisms

Long deemed experimental, LCA underwent a process of international standardization in 1990s, transforming it in a benchmark tool, allowing for the harmonization of methods and conducting comparative studies.

In France’s RE2020 set of environmental regulations for new buildings, life-cycle assessments are calculated over a span of fifty years, which is considered to be the average building lifespan. This extensive duration requires adjusting the environmental impacts tied to the replacement of products with shorter lifespans.

In contrast, urban metabolism analysis lacks standardization as it isn’t regulatory, and generally involves examining a shorter time frame of typically one year. This primary unit of time incorporates weighting, allowing for the case-by-case inclusion of the lifespan and necessary replacement of each constituent sub-element. Urban metabolism thus provides a comprehensive snapshot of a typical year of operation, covering the entire intricate meshing of life cycles within the territory being examined.

Visualizing results and enhancing comparability: a project tool

While the chosen unit of time might initially suggest reduced precision in assessing the environmental impacts of the various components over their life cycle, considering higher levels of detail allow for crucial time adjustments. Conventional life-cycle assessments spanning fifty years often assume consistent usage over time, overlooking shifts in the energy mix—a limitation addressed by working with greater granularity.

Furthermore, the primary unit of time comes with the benefit of allowing for a useful graphical representation that is traditionally associated with it—Sankey diagrams. These flow diagrams consist of arrows whose thickness is proportional to the flow rate of the depicted property. They are particularly effective for visualizing origin-destination patterns as they emphasize transfers, gains, and losses.

In urban projects, one application of this kind of representation is to provide arrows with spatial coherence, going beyond simply quantifying flows by also enabling the tracing of the origins of inputs and the destinations of outputs. This approach evokes the historical roots of the Sankey diagram, originally explored by engineer Charles Joseph Minard at the École Nationale des Ponts et Chaussées well before Irishman Matthew H.P.R. Sankey employed it in 1898 to visualize the energy efficiency of a steam engine (Rendgen, 2020).

“My maps do not just show, they also count, they calculate for the eye; that is the crucial point, the amendment I have introduced through the width of the zones in my figurative maps and through the rectangles in my graphic tableaus.” (Charles-Joseph Minard, 1861)

Though less suitable for quantitative comparisons, these diagrams serve as robust data visualization tools, enhancing our understanding of causal links directly tied to urban projects inviting more nuanced qualitative considerations.

How are qualitative and societal aspects taken into account

One shared methodological limitation emerges where life-cycle analysis and urban metabolism analysis intersect: they both rely exclusively on a quantitative approach to the life cycles they examine.

This is the reason why relying solely on life-cycle analysis as the sole normative methodology often falls short, as it predominantly focuses on quantifying environmental impacts through predefined indicators like air acidification, water body eutrophication, global warming potential, or photochemical ozone formation. This is even often narrowed down to a single metric related to carbon weight quantification, global warming potential, a phenomenon Jan Konietzko (2021) aptly termed “carbon tunnel vision.” This narrow focus may result in overlooking and invisibilizing other vital environmental concerns such as biodiversity and air quality, as well as social and societal issues such as inclusion.

While the urban metabolism approach may present itself as more holistic and adaptable in its metrics, it is inherently marked by its massive reliance on quantitative methods for data acquisition. The foundational metaphor of metabolism, which is rooted in the principle of homeostasis, underscores the necessity for all interactions within a metabolism to be balanced for it to operate correctly.

Transposing the concept of urban metabolism into a project methodology

But what is actually unfolding along the proportional arrows of our Sankey diagrams? What spatial, social, and political issues come into play during the transfers and transformations of matter and energy? It is precisely this qualitative approach to urban metabolism that researcher Pierre Desvaux (2019) suggests exploring through the analysis of “metabolic pathways.”

This holistic approach is one of the focal points for investigation within the “Urban metabolism” chair, aiming to unravel how interdisciplinary collaborations can effectively tackle the challenges of shaping the urban landscapes of the future.

Pauline Detavernier, Doctor of Architecture and Director of Research and Development at PCA-STREAM

Bibliography

Chiambaretta, Philippe, “Dynamiques synergétiques des métabolismes urbains” [Synergetic Dynamics of Urban metabolisms]. In Synergies urbaines : pour un métabolisme collectif des villes [Urban Synergies: For a Collective metabolism of Cities], R D’Arienzo & C Younès, 141–57. Genève: Métis Presses, 2018.

Pierre Desvaux (2019), “Pour une approche qualitative du métabolisme urbain. L’exemple des voies métaboliques des déchets plastiques au Caire (Égypte)” [], Flux 2019/2 (N° 116-117), 147–60.

PCA-STREAM (Léone-Alix Mazaud) (2022), “Aux racines de la ville-métabolisme” [At the roots of ‘Urban metabolism’], Stream Voices no .5.

Sandra Rendgen (2018), The Minard System: The Complete Statistical Graphics of Charles-Joseph Minard, Princeton Architectural Press.