Mainstreaming nature-based solutions in cities: A systematic literature review and a proposal for facilitating urban transitions

Nature-based solutions are gaining prominence in urban sustainability discourses, especially in climate adap-tation, in efforts to increase resilience, and as a means of promoting a range of social, environmental, and economic benefits. There are however barriers and inertia that slow the adoption of such solutions, and a term commonly used for overcoming such factors is mainstreaming. The term mainstreaming in relation to nature-based solutions is ambiguous, as it is entangled within or conflated with other similar concepts that also describe change processes. This lack of clarity is a cause for potential misdirection of planning with nature-based solutions towards more climate resilient cities. Therefore, this article expands and deepens the understanding of mainstreaming nature-based solutions in cities by proposing a (re)conceptualisation of the term mainstreaming. Our (re)conceptualisation explores mainstreaming in the context of multi-level governance, to unpack where mainstreaming can unfold within and across levels, and focuses on knowledge drivers, to unpack how main-streaming activities are shaped. These dimensions are important for unpacking where mainstreaming can happen and what informs mainstreaming activities, facilitating more effective discourses, policy-making, and wider adoption of urban nature-based solutions. We report on a systematic literature review and synthesis of 147 ar-ticles, which proposes a new mainstreaming framework and definition by identifying five mechanisms and four roles to explain how mainstreaming processes unfold or mobilise in cities. This adds a framework and language to the academic and policy debate that is needed for operationalising nature-based solutions mainstreaming processes, and thereby to transform urban planning practices.