Summary
Reduced emissions from deforestation and forest degradation (REDD+) projects and programmes promise to deliver performance-based, cost-effective climate change mitigation. Fifteen years after its conception, we analysed the rigorous counterfactual-based evidence for environmental and welfare effects from such national and subnational initiatives, along with a Theory of Change. Using machine learning tools for literature review, we compared 32 quantitative studies including 26 primary forest related and 12 socioeconomic effect sizes. Average environmental impacts were positively significant yet moderately sized, comparable to impacts from other conservation tools, and mostly impermanent over time. Socioeconomic impacts were welfare-neutral to slightly positive. Moderator analysis showed that environmental additionality was likely restricted by project proponents’ adverse spatial targeting of low-threat areas. Scarce funding flows from carbon markets and ill-enforced conditionality probably also limited impacts. Hence, important policy and implementation lessons emerge for boosting effectiveness in the current global transition towards larger-scale, jurisdictional action.