Urbanization significantly impacts global biophysical processes, leading to challenges like food insecurity, flooding, water scarcity, and heat islands. Food, water, and energy (FWE) are crucial resources, and their interconnectedness (the FWE nexus) creates complex sustainability issues. Green roofs offer a potential solution by contributing to food production (rooftop farming), water management (rainwater harvesting, waterlogging prevention), and energy efficiency (temperature regulation). While green roofs offer significant FWE benefits, their construction, maintenance, and disposal also have environmental costs. Existing research often focuses on specific aspects (life cycle impacts or individual FWE benefits) and is frequently limited to small scales, neglecting the crucial role of green roofs within the urban FWE nexus. This study addresses this gap by developing a comprehensive methodology to assess the urban-scale impacts and benefits of green roofs, considering both direct and indirect (transboundary) effects. The methodology is applied to São José dos Campos (SJC), Brazil, and Johannesburg, South Africa, two data-sparse cities with distinct characteristics, to demonstrate its applicability and reveal the heterogeneity of optimal green roof management strategies across different urban contexts.

Previous research on green roofs has often adopted a siloed approach, focusing either on life cycle assessments (LCAs) or individual FWE benefits. Studies have estimated food production potential from edible rooftops (Saha and Eckelman, 2017), evaluated the energy generation and carbon emissions of photovoltaic green roofs (Jahanfar et al., 2018), compared the environmental impacts of linear food systems and rooftop greenhouses (Sanyé-Mengual et al., 2013), and examined the ecological and economic impacts of city-wide green roof adoption (Zhou et al., 2018). However, few studies have systematically evaluated the multiple trade-offs of green roofs within the urban FWE nexus at the city scale. Existing research often lacks the data necessary for a comprehensive assessment, particularly in data-sparse cities. This paper aims to fill this research gap by presenting an integrated methodology that considers life cycle environmental burdens, direct FWE benefits, and avoided transboundary environmental footprints.

The study developed an integrated methodology and framework combining Geographic Information Systems (GIS) and urban metabolism approaches. The framework consists of four steps: 1. **Urban Rooftop Area Extraction (GIS sampling):** GIS was used to estimate the potential rooftop area in SJC and Johannesburg. Random sampling was used to estimate the proportion of building rooftops within the urban area. 2. **Life Cycle Environmental Impacts (LCA approach):** A cradle-to-grave LCA was conducted to quantify the energy consumption, water consumption, and carbon emissions of the green roof system (including structures, farming system, and rainwater harvesting). The Gabi database provided life cycle parameters. 3. **Direct FWEC-Related Benefits (Process-based model):** Process-based models were used to quantify the direct FWE benefits of green roofs during operation. The DNDC model estimated tomato yield, a proxy for food production; a green roof energy model estimated energy savings; empirical formulas calculated direct water savings from rainwater harvesting; and empirical coefficients determined direct carbon capture. 4. **Avoided Transboundary Environmental Footprints and Nexus Indexes (EIO-LCA model):** An EIO-LCA model estimated avoided transboundary environmental footprints by analyzing the supply chains of food, water, and energy. The GTAP v10 database provided input-output data. Scenario analyses were conducted by proportionally scaling up the per-square-meter results from Steps 2-4, considering different proportions of available rooftop areas converted into green roofs. Nexus Indexes (NEI, NWI, NCI) were calculated to aggregate avoided transboundary environmental footprints for each resource.

The study found that green roofs in both SJC and Johannesburg 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 can save irrigated water, but it requires 1.2 times more energy consumption than tap water. **Trade-offs Analysis:** The integrated methodology revealed complex trade-offs between life cycle impacts and direct/indirect benefits. While green roofs provide direct benefits (food production, water savings, energy savings, carbon capture), they also have life cycle environmental burdens. In both cities, life cycle carbon emissions nearly offset carbon capture, and life cycle energy consumption exceeded direct energy savings. Water consumption varied, with SJC showing a net water benefit and Johannesburg a net water loss. **Transboundary Impacts:** Notably, avoided transboundary environmental footprints significantly exceeded life cycle impacts in both cities. In SJC, local food production was the main driver of reduced transboundary footprints (energy, carbon, water), while in Johannesburg, direct energy saving was the most significant driver. **City-wide Scenario Analysis:** Scenario analysis showed that under a 30% available rooftop conversion scenario, SJC could achieve full vegetable self-sufficiency, while Johannesburg could reach a maximum of 72.37%. Rainwater harvesting significantly contributed to water savings in SJC but had less impact in Johannesburg due to the city's higher irrigation demand. From an energy perspective, both cities were net energy consumers, but avoided transboundary energy footprints partially offset life cycle energy consumption. Both cities showed carbon neutrality.

The findings highlight the importance of considering both direct and indirect impacts of green roofs on the urban FWE nexus. The integrated methodology and framework provide a more comprehensive assessment than previous studies, which often focused solely on life cycle impacts or specific benefits. The results demonstrate the heterogeneity in the optimal strategies for green roof development across cities. SJC should prioritize local food production due to its high impact on transboundary carbon and water footprints, while Johannesburg should focus on energy savings to compensate for life cycle energy consumption. The study emphasizes the potential of green roofs for enhancing urban resource self-sufficiency and reducing reliance on external resources, highlighting their role in achieving sustainability goals.

This study provides a novel, generalizable methodology for assessing the impacts of green roofs on the urban FWE nexus, particularly relevant for data-sparse cities. The findings underscore the importance of considering both direct and transboundary effects when planning green roof initiatives. Cities should tailor their strategies based on their specific FWE goals and resource availability. Future research should focus on refining the methodology by improving data availability, expanding the assessment dimensions (e.g., health benefits, educational value), and exploring the integration of other green and blue infrastructures. The integrated framework presented here offers a valuable tool for informing sustainable urban planning and resource management decisions.

The study relies on assumptions regarding available rooftop areas and uses proxy data for some parameters due to data limitations in the selected cities. The scenario analyses are based on specific assumptions about rooftop availability and conversion rates, which might not reflect real-world scenarios accurately. The choice of tomato as a representative crop might limit the generalizability of food production estimates. While the methodology aims to be generalizable, applying it to other cities may require adaptation depending on data availability and local context.