Cities all over the world are confronting intertwined environmental, social and economic problems and aim to become resilient to climate change and promote wellbeing for all their citizens. Nature-based solutions have been proposed as a promising policy approach to addressing urban problems for the potential they have to deliver multiple benefits and foster wellbeing for individuals and communities. However, the evidence for their multiple benefits is rather scarce and highly fragmented, and more robust frameworks for the monitoring and assessment of their impacts are needed to guide urban policy-making. This paper focuses on the current state of impact assessment of nature-based solutions in Europe and through a systematic review of the literature identifies four conceptual problems and three empirical gaps that impede the accumulation of solid evidence regarding of the impacts of different types of nature-based solutions for different social groups; as well as of the contextual conditions that contribute to their performance and delivery of multiple outcomes. Based on the identified mis-conceptualizations and gaps, we derive a series of principles that should guide the development of robust impact assessment frameworks for nature-based solutions. We discuss the policy implications of these gaps and principles. We conclude by making a series of recommendations that should inform the design of impact monitoring and evaluation frameworks in cities, in order to develop the comparative evidence base on the effectiveness of nature-based solutions. This, in turn, can inform urban decision-making on the appropriate design, implementation, and long-term regeneration of nature-based solutions, to ensure long-term delivery of important ecosystem services for different social groups.
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