NbS Triple Win Toolkit: Economics and Finance 78 Methodological challenges with assessing cost-effectiveness of NbS The economic case for investment in NbS is reported using different metrics, as evidenced in the cases above. Some report benefit-cost ratios, which compares the monetisable benefits as well as costs102. Other studies report the value of ecosystem services per annum or per hectare108 or compare the cost of different interventions, assuming the delivery of either the same benefits or benefits of commensurate value to society65,69. Similarly, some studies report the technical unit cost for different interventions with different metrics, such as flood adaptations ranging from cost per unit of adaptation option (e.g., building or groin) to cost per area of intervention, or simply an aggregated total cost of the project or intervention119. Comparing across different metrics is not immediately possible since they are often generated to answer different research questions (e.g. cost comparison of specific NbS interventions vs net benefit valuation of a given ecosystem) and include different data types or requirements. Whilst each of these metrics are useful, this makes the evidence base for NbS disparate and challenging to draw upon for practical conclusions and recommendations. NbS, in the context of the triple win, is not immediately suited to a typical cost-effectiveness analysis. Cost-effectiveness analysis attempts to identify the relative net cost of one or many interventions capable of achieving a certain objective. Cost per CO2e ($/tCO2e) has been used to assess cost-effectiveness for NbS for mitigation-type interventions. Since the social cost of carbon is equal across the world, projects can be compared across different geographies and locations. This is particularly important since it gives decision makers key strategic information in respect of which projects around the globe generate benefits for climate at the lowest possible cost to the taxpayer. Given the widespread lack of monetisation of benefits such as poverty reduction and biodiversity, solely using $/tCO2e when assessing NbS projects risks elevating the importance of climate change mitigation by ranking more favourably those projects which sequester or avoid carbon emissions. Where cost effectiveness estimates are used, it is often noted that there are other significant benefits which have not been included in the monetisation process65,69, therefore care shouldbe taken to understand which co-benefits exist in the local context,how important they are and to whom they accrue. The fact that NbS likely deliver hard-to-monetise social benefits also makes a comprehensive cost-benefit analysis approach challenging. They require a significant investment of resources to complete (see next section), especially when complex valuation exercises are undertaken. The qualitative benefits vary largely between contexts which creates an issue of cross-comparison where there are no(or few) appropriate comparable metrics to assess.