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

In response to multiple societal challenges faced by urban areas, 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 361 scientific publications that investigated the effects of NbS applied in urban contexts worldwide. We mapped and analyzed 547 empirical cases of NbS in 197 cities globally, yielding 799 outcomes. To structure this evidence we developed an urban NbS classification. With the aid of this classification, we analyzed 23 urban challenge categories and 30 outcome domains encompassing biodiversity, health well-being, and regulating ecosystem services. The established linkages between urban challenges, NbS, and their outcomes indicate that specific urban challenges were predominantly addressed through specific sets of intuitively related outcomes and secondary 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, we show that urban NbS generally yielded overwhelmingly positive effects compared to non-NbS, particularly in relation to microclimate mitigation and mental health outcomes. However, our analysis underscores a notable scarcity of evidence of multifunctional NbS that report multiple outcomes related to biodiversity and well-being simultaneously. In combination, 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. ### Competing Interest Statement The authors have declared no competing interest.