Green roofs towards urban sustainability from a nexus perspective

Food-water-energy nexus thinking provides a systemic insight for urban sustainable governance, and green roof is an innovative strategy to compact the urban challenges by providing benefits in relation to the food-water-energy nexus.
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Green roofs towards urban sustainability from a nexus perspective
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Urbanization processes are accelerators of global changes in biophysical processes. Despite apparent economic growth, new risks emerge in urban areas, such as food security, flooding, water scarcity, and heat islands. Food, water, and energy (FWE) are indispensable lifeline resources for cities that are emphasized as pressing concerns in the Sustainable Development Goals (SDGs) of the United Nations. The interactions between FWE systems, referred to as the FWE nexus, increasingly reshape the shocks that were previously contained within a geographic area or a sector but now are becoming globally interconnected. Since local-scale actions impact regional and global development, the cities take on more responsibilities to seek powerful governance approaches for the wicked FWE nexus challenges.

In 2019, When I worked at Yale University as a Postdoc Associate, I took the role of the core member participating in the IFWEN project "Understanding Innovative Initiatives for Governing Food, Water and Energy Nexus in Cities - IFWEN" (https://ifwen.org/) led by Jose A. Puppim de Oliveira and Karen C. Seto. Green and Blue Infrastructure (GBI) can improve the FWE nexus and would be an innovative solution to promote the practical application of FWE nexus theory to guide urban sustainable management. We targeted constructing a methodology framework to simulate the dynamic interplays between GBI and the FWE nexus in cities. Prior to this, we made a synthesis of the literature to examine how urban GBI affects the FWE nexus and developed a conceptual framework of their key links and the direction and magnitude of the relationship (Bellezoni et al., 2021).

When I finished my postdoc at Yale, I joined the School of Environment at Beijing Normal University as an Associate Professor. I led my research team and cooperated with my master’s student Qiuling Yuan to continue this topic “GBI-FWEN” research and obtained funding from the National Natural Science Foundation of China. We identified the detailed interactions between GBI and FWE nexus and made an overview of the available methods to quantify the FWE flows and trade-offs of GBI based on systematic review of the methodological articles (Meng et al., 2023), which provided solid bases for us to build an assessment model of the multiple linkages between GBI and the FWE nexus in the cities. Based on the previous work, we developed a generalizable methodology and framework with geographic information system (GIS) and urban metabolism approaches, to capture the detailed flows between GBI and urban FWE nexus. The presented method supports the systemic trade-offs analysis of GBI through the quantifications of life cycle environmental impacts, direct FWEC-related benefits in the operational stage, together with the avoided transboundary environmental footprints induced by the local direct benefits. Such an analysis for trade-offs encompasses multiple elements within and outside cities, stressing that an isolated lens of linkages between GBI and FWE nexus would lead to the understanding bias by over- or underestimating the implications of GBI.

Fig. 1 Generalizable methodology and framework for urban FWE nexus of green roofs.

Taking green roofs, we applied the method to two representative data-sparse cities in different continents with distinct sizes and features to demonstrate the utility of the method, i.e., São José dos Campos (SJC) in Brazil and Johannesburg in South Africa. We found that green roofs are essentially carbon neutral and net energy consumers from a life cycle perspective. SJC is a net water beneficiary while Johannesburg is a net water consumer. Rainwater utilization could save irrigated water, but requires 1.2 times more energy consumption. The micro-scale green roofs within city boundaries can indirectly contribute to regional environmental footprint savings by enhancing urban self-sufficiency of resources and weakening the cities’ import dependence. The results show that all of the avoided transboundary environmental footprints induced by green roofs’ benefits in SJC and Johannesburg exceed their corresponding life cycle environmental impacts, enabling green roofs to benefit from the indirect effects beyond cities. To be specific, the avoided transboundary environmental footprints of green roofs in SJC and Johannesburg are respectively 1.6–384 and 1.4–7.8 times their life cycle impacts. The driving forces to avoid the transboundary energy, water, and carbon footprints of green roofs diverge in SJC and Johannesburg, which can be interpreted as their trade structure heterogeneities in the upstream supply chains for urban FWE resources. SJC and Johannesburg could direct their green roof development from local food production and energy saving, respectively.

The integrated framework proves to be excellent in revealing the interactions between green roofs and the FWE nexus and is helpful for urban and regional development decisions such as greenery redevelopment, FWE governance initiatives, as well as coalitions among cities. In particular, the research results were directly integrated into a guidebook on sustainable management of urban resources (An Implementation Guide for Cities and Subnational Governments). The guidebook outlines an integrated management vision in providing environmental services to its inhabitants, which is a tool for public managers, leaders, and concerned citizens committed to sustainable development and management of cities. It has been officially launched at the 11th World Urban Forum (Poland, 2022.6) and the 15th Conference of the Parties (COP15) of the Convention on Biological Diversity (Montreal, Canada, 2022.12), and made a significant impact in the field of urban governance. The guidebook has also been reported and disseminated by the local TV stations in Brazil, attracting wide attention from the government and other stakeholders.

Urban green roof is an innovative nature-based solution combating urban nexus challenges. Although we have made several advances in quantifying multiple interactions of green roofs on the FWE nexus for SJC and Johannesburg in this study, it is noted that current research tends to rely on selective samples and the majority of urban settlements globally remain unknown. In addition, this imbalance is replicated geographically between north and south, and between cities with different sizes (population) and income levels (per capita GDP) located in the global regions. Here we call for the explorations of the linkages of green roofs on FWE nexus based on urban heterogeneities. Further, another stream of research to be considered is the framework and assessment tool for wide kinds of FWE nexus implications from an urban lens involving other green and blue infrastructures, such as street trees, urban wetlands, and green spaces.

Bellezoni, R. A., Meng, F., He, P. & Seto, K. C. Understanding and conceptualizing how urban green and blue infrastructure affects the food, water, and energy nexus: A synthesis of the literature. J. Clean. Prod. 289, 125825 (2021).

Meng, F. et al. Quantification of the food-water-energy nexus in urban green and blue infrastructure: A synthesis of the literature. Resour Conserv Recycl. 188. 106658 (2023).

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