NbS Triple Win Toolkit: Economics and Finance 68 EbA is often promoted, instead of hard or grey infrastructure and interventions, as a potentially more cost-effective alternative solutionto achieve climate adaptation57,59. This cost-effectiveness stems from the multiple uses, a greater flexibility than hard engineering solutions, and the delivery of co-benefits for biodiversity and people. EbA can often deliver higher benefit-cost ratios when a more comprehensiveset of ecosystem services and other monetised benefits are included in the economic assessment. Cost-effectiveness is often understated when monetising such benefits is not possible59. The inflexibility of hardened structures may not allow for adaptive management under changing conditions and future climate change scenarios57,59, whereas EbA often allows for the functioning of dynamic ecological or biogeochemical processes such as sediment flows or forest regeneration59 which are more resilient to change and capableof delivering benefits under uncertain conditions over longer timescales. There may be additional drawbacks for biodiversity and ecosystem services from hardened structures. Considering the coastal zone, ‘sprawl’ of hardened substrates can create stepping stones for invasive species60 or even increase damaging energy of storms and waves and increase shoreline erosion61. Hardened infrastructure can reduce structural heterogeneity, reducing biodiversity and other ecosystem services62,63. As with other NbS interventions, more analysis of the limitations and thresholds of efficiency for EbA – as well as unintended negative impacts – is necessary, especially under changing climatic conditions58,59. Reforestation Description of intervention Reforestation is a common NbS intervention based on there-establishment of forest through planting and/or deliberate seeding on land classified as forest64. Reforestation is an effective strategy for carbon sequestration and is often noted as an attractive, low-cost option for climate mitigation activities65,66. Reforestation can also support local communities through provisioning of non-timber forest products (NTFPs), selective harvesting, and can also be deployed for soil or riverbank stabilisation, water filtration and to support biodiversity67. Monetised benefits Carbon sequestration, storage or carbon markets Revenue from sustainably harvested products(e.g., medicines, fuel, food) Revenue from recreation or tourism opportunities Benefit-cost ratios and cost effectiveness evidence The range of benefit-cost ratios reported is from 3 (in Latin America)to 20 (in Kenya). From a global perspective, reforestation has the reported greatest total cost-effective (<$100 [USD] social cost of carbon68) potential for climate change mitigation of all NbS interventions65. Together with avoiding deforestation, they offer approximately half of the total carbon mitigation opportunities arising from natural solutions costing less than $10 (USD) per tonne of CO2e. However, a greater proportion of projects involving avoided conversion of coastal, peat and forested land is more cost-effective than reforestation65,69 since opportunity costs can be high where