The importance of green roofs in an urban Water-Energy-Food-Ecosystem nexus context

The modern society is facing new environmental and socio-economic challenges: population is growing fast, and it is projected to continue with this trend, reaching 9.8 billion of people by 2050, with 2/3 of them living in cities. Moreover, climate changes are leading to an increase of hot and dry periods and of short but intense rainfall events, forcing policy makers to rethink to the water management system. For these reasons, it is important to integrate in the urban planning, sustainable solutions that can help dealing with these new challenges. In this context, green roofs are powerful and flexible tools, that can play a fundamental role in the creation and development of smart and resilient cities. So far, green roofs have been generally investigated focusing on one single field at the time, following a so called “silo approach”. This approach, however, does not allow to highlight the interconnections and feedback between the different sectors, limiting the understanding of the potential of this tool. An integrated water-energy-food-ecosystem nexus approach is hence required to fully explore all the potential benefits of a large-scale installation of this tool. This work presents a review of green roofs’ benefits, following an integrated water-energy-food-ecosystem approach, with the aim to identify the potential positive impacts for the development of sustainable and resilient cities. Green roofs present multiple benefits for the urban environment, which are in line with the Development Goals proposed in the Sustainable Agenda 2030 (SDGs). Green roofs can, for example, mitigate pluvial floods, adapting to climate changes (SDG13: Climate Action) and contrasting the urbanization (SDG11: Sustainable Cities and Communities). The installation of these tools on the rooftops guarantee thermal insulation for the building, reducing the energy consumption for heating and cooling systems (SDG7: Affordable and Clean Energy). Thanks to the potential applicability of urban agriculture on its surface, green roofs can reduce the population food demand (SDG2: Zero Hunger), especially in poor countries, where many people have limited or no access to food. Moreover, the harvested rainwater, if properly stored and treated, can be reused for several domestic purposes, reducing the pressure on the water supply system and consequently increasing the availability of clean water (SDG6: Clean Water and Sanitation).The installation of vegetation in urban areas partially aims to restore the natural conditions, increasing the biodiversity and attracting different species of insects and small vertebrates, which are fundamental to guarantee maintenance of the ecosystem (SDG15: Life on Land). Moreover, the installation of this tool in an urban environment contributes to improve the mental and physical well-being of citizens (SDG3: Good Health and Well-being), which is particularly relevant in relation to the health crisis caused by the COVID-19 pandemic.