Evaluating Nature-based Solutions as urban resilience and climate adaptation tools: A meta-analysis of their benefits on heatwaves and floods.

Extreme weather events driven by climate change threaten the resilience of urban structures and urban dwellers. Nature-based Solutions (NbS) are an effective tool to reduce urban vulnerability to climate risks and, at the same time, develop more liveable urban areas. Despite the acknowledged positive impacts of individual observed NbS, numerous questions persist unanswered. While existing research supports NbS' positive influence on urban climate adaptation, the extent of their impact remains insufficiently studied. Understanding the magnitude of NbS impact is crucial for justifying their preference over non-NbS alternatives and, consequently, for securing public investment. Via a meta-analysis, this paper aims to contribute to research and practice by providing a more systematic assessment of NbS effects, offering urban planners and decision-makers a robust justification for their incorporation in climate change adaptation, urban resilience, and enhanced liveability. The results of the meta-analytic model indicate that the effect of NbS is indeed positive. When assessing the impact on temperature and flood protection, there is a general positive effect across the studied NbS. However, when evaluating an average effect, the task appears to be more complex due to methodological issues and limitations. The need to increase the formalisation of how the impact of NbS is measured and reported also emerges as a result. Replicable protocols would positively impact the formalisation of the literature on the topic and positively affect the evidence-based support for the implementation of NbS by urban decision-makers.