NbS Triple Win Toolkit: Economics and Finance 76 Large range of cost-effectiveness results From both the global and local studies, there is a large range in benefit-cost ratios, ranging from 1 up to 250, and cost-effectiveness estimates between negative cost per intervention and upwards of $80 per tonne of CO2e(USD)65. This variation appears to derive from both differences between different interventions (i.e. the benefit-cost ratios for mangrove restoration are typically higher than agroforestry or climate-resilient agriculture) as well as within ecosystem (i.e. cost per CO2e for avoided deforestation varies between $10[USD] in Democratic Republic of the Congo and $80[USD] in Malaysia)65. Both the global-scale models and NbS case studies show these substantial ranges but are particularly sensitive to carbon prices and the high value of coastal economic assets. With the possible exception of social cost of carbon, the specific characteristics and values of the benefits and costs associated with NbS are likely to vary between project sites. NbS benefits are modified by local ecosystems, which are characterised by biodiversity, interconnectivity between adjacent ecosystems, and the interaction with local communities (i.e., dependence on resources and ecosystem services). With NbS highly dependent on local contexts, the costs and benefits of the same intervention in different ecosystems or geographies can vary greatly, especially where alternative land use practices are prevalent. Whilst there is no one individual cause for high benefit-cost ratiosor cost-effectiveness estimates, three specific trends are noted: 1. Longer periods of economic assessment are typically associated with higher benefit-cost ratios. The Green Book states that ‘costs and benefits should be calculated over the lifetime of the proposal’,recommending the periods of assessment such as 30 and 60 years for building refurbishment and infrastructure projects117. Three studies with high benefit-cost ratios performed analysis over a 50 and 80-year time-period. Of the case studies identified as having prepared economic analysis, and across the global analysis, timescales are typically between 15 and 30 years. NbS timescales may require longer lifetimes to be considered since the benefits of biodiversity, carbon regulation and ecosystem adaptation are likely realised over longer periods of time. This depends in part on whether the NbS intervention concerns the protection of ecosystem or restoration, since the benefits of protecting an ecosystem before degradation are realised sooner, and perhaps with less uncertainty than those arising from the recovery of an ecosystem. However, the inclusion of future benefits comes with greater uncertainty and risk. Evidence from the case studies suggests that NbS projects begin to generate net benefits towards the end of the project implementation and often numerous years after the project begins. Several case studies projected benefits and costs beyond the project implementation period to support the economic case for intervention. Since most of the economic analysis is prepared before the project is undertaken, it is often either implicit or weakly evidenced that the project will generate sustained benefits beyond the life of the project, or that local communities will continue to use and apply new technologies or different practices without further project support.This risks overstating the benefit-cost ratio and cost-effectivenessand underlines the importance of post-project reviews to understand a) the sustainability of the NbS intervention post-implementation,b) the accuracy of the initial ecological and economic predictionsat project outset, and c) the extent to which project conclusionscan be extrapolated for other similar NbS projects. Common themes and findings