Carbon offsets from voluntarily avoided deforestation projects are generated based on performance vis-à-vis ex-ante deforestation baselines. We examined the impacts of 27 forest conservation projects in six countries on three continents using synthetic control methods for causal inference. We compare the project baselines with ex-post counterfactuals based on observed deforestation in control sites. Our findings show that most projects have not reduced deforestation. For projects that did, reductions were substantially lower than claimed. Methodologies for constructing deforestation baselines for carbon-offset interventions thus need urgent revisions in order to correctly attribute reduced deforestation to the conservation interventions, thus maintaining both incentives for forest conservation and the integrity of global carbon accounting.