The food-water-energy nexus and green roofs in Sao Jose dos Campos, Brazil, and Johannesburg, South Africa

Green roofs affect the urban food-water-energy nexus and have the potential to contribute to sustainability. Here we developed a generalizable methodology and framework for data-sparse cities to analyze the food-water-energy nexus of green roofs. Our framework integrates the environmental costs and benefits of green roofs with food-water-energy systems and makes it possible to trace energy-water-carbon footprints across city boundaries. Testing the framework in São José dos Campos (SJC), Brazil and Johannesburg, 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. Our results show that SJC and Johannesburg could direct their green roof development from local food production and energy saving, respectively and highlight opportunities for green roof practices in cities.