What makes infrastructure green? A four-criterion functional diagnostic framework

Green infrastructure (GI) and nature-based solutions (NbS) are widely used in climate adaptation, stormwater management, biodiversity conservation, landscape planning, and disaster risk reduction. Yet GI remains defined through competing network-, planning-, and technology-oriented traditions, and its relationships with NbS and green–grey hybrid infrastructure remain unclear. Drawing on a structured, definition-oriented review, this paper develops a functional and welfare-based definition of GI and a diagnostic framework for its application. GI is defined as an infrastructure arrangement comprising natural, semi-natural, or human-created ecological elements that, at the relevant spatial and temporal scale, performs or has the capacity to perform one or more scientifically identifiable functions that attain a context-specific scale-and-quality benchmark, depend materially on ecological structures or processes, and contribute positively to human well-being. The framework distinguishes benchmark-attaining from below-benchmark functions and separates the functional status of a physical arrangement from NbS as an action or intervention. For arrangements in which ecological and engineered components both contribute, a green dependence elasticity and a normalised green–grey dominance index formalise the compensation relationship between the corresponding ecological and engineered inputs or capacities, support assessment of material ecological dependence, and characterise their relative dominance without aggregating distinct functions into a single score. Applications to illustrative boundary cases show how the framework can support GI recognition, funding, monitoring, and evaluation while distinguishing genuine ecological dependence from unsupported claims that may constitute greenwashing.