How can we assess the role of nature in the metabolic requirements of a city?: An integrated Urban Metabolism and Ecosystem Service analysis

As cities have become the primary host of the majority of the world's population, the associated energy and resource consumption has increased rapidly, resulting in worldwide scale environmental impacts. Thus, understanding the importance of the city's physical, biological, and social heterogeneities, as well as the assessment of processes and activities is necessary given the global environmental agenda. In response, urban policy makers and planners have adopted the concept of Ecosystem Services as a way to recognize the role of nature and ecosystems in cities. However, despite this recognition, the relationship between human activities and ecological processes in urban areas is not yet thoroughly explored, generating a research gap. Since its fundamental objective is to examine the interaction between natural environments and anthropogenic systems in cities, the Urban Metabolism (UM) field is designed to overcome environmental challenges and propose novel resource mitigation solutions. Additionally, the Ecosystem Service (ES) concept and classifications provides novel alternatives and techniques that contribute to these environmental challenges by including ecosystem processes and functions. The objective of this research is to assess the contribution of ES in UM assessments. For that, we propose a methodological framework that builds on the interrelationships of Natural and Social capital, the foundation of the ES concept, and utilizes "Pressures”, “Drivers” and “State” factors to explain the relationships between anthropogenic and natural systems, and assesses the supply and demand ES flows, which can be embedded in the Economy-Wide Material Flow Analysis (EW-MFA). London is the subject of this framework application and testing due to its economic (20% of UK's GDP) and demographic (15% of the population) importance to the country. The following ES are identified as flows that contribute or reduce (offset) resource flows and emissions: Changes in energy demand due to the cooling effect of green areas, urban agriculture (biomass), organic waste and biofuel crops as energy source, and carbon sequestration from vegetation. Biophysical and ecological models, dynamic datasets, official statistics and databases are used to determine the ES supply and demand flows. An UM analysis of the city is elaborated as a base case (2015), and different scenarios will be used to determine the amount of resource flows and emissions, as well as the change in ES supply until the year 2050. The scenarios consider city policy targets (The London Plan), official projections of main socio-economic indicators (OECD, Energy Projections-BEIS), and climate change projections (General Circulation Models of biophysical indicators) which can change the outcomes of the analysis over time. Preliminary results of the UM analysis showcase main metabolic inflows in the base case (2015) such as energy (11.9 Mtoe), water (8.25 Mton) and biomass (10.3 kton), as well as metabolic outflows such as waste (3.6 kton) and emissions (33.9 kton CO2eq). Moreover, we intend to compare the total contribution of ES supply flows to demand in order to understand how much cities can meet their metabolic requirements. Results to date propose that the urban ES supply does not satisfy the demand in the base case: 0.23% of carbon sequestration demand and 0.05% of biomass (food) demand. These findings can shed light on the imporance of nature in urban environments and highlight the significance of incorporating the integrated UM-ES knowledge into sustainability policy and urban planning