Solution to what? Global assessment of nature-based solutions, urban challenges, and outcomes

Abstract In response to multiple societal challenges faced in cities, nature-based solutions (NbS) are gaining prominence as means to support sustainable and resilient urban planning. However, NbS are being implemented in cities around the globe without comprehensive evidence on their effectiveness in addressing urban challenges. Based on a systematic mapping methodology, we synthesized 547 empirical cases of NbS in 197 cities globally, yielding 799 outcomes encompassing biodiversity, health well-being, and regulating ecosystem services. To structure this evidence we developed an urban NbS classification and categories of urban challenges and outcomes. Effectiveness of NbS was assessed through synthesizing which urban challenges are addressed by NbS, which outcomes are generated, and how these outcomes perform compared to alternative solutions. Our analysis suggests that specific urban challenges were mostly linked to closely related outcomes, but rarely to multiple outcomes. Specifically, forests & trees and general parks were commonly used to enhance health and well-being, while grassland and gardens were applied to mitigate biodiversity loss. Furthermore, urban NbS generally yielded positive effects compared to non-NbS, particularly in relation to microclimate mitigation and mental health outcomes. However, we note a scarcity of evidence on multifunctional NbS, especially on studies that report multiple outcomes related to biodiversity and well-being simultaneously. Our study provides a foundation for further understanding NbS effectiveness and can inform urban planners and policymakers with measurable evidenced-based targets for the application of NbS.