Dams not to blame for all reservoir carbon emissions from retained organic carbon

Dams trap organic carbon (OC) and create carbon dioxide (CO₂) and methane (CH₄) emission hotspots within reservoirs, underlying the ongoing debate over their role in carbon emissions. However, even without dams, this labile OC would still decompose and emit CO₂ or CH₄ during downstream transport and after deposition in estuaries. Thus, relying solely on dammed reservoir observations to assess these emissions may introduce considerable bias. Here, we simulated both dammed and non-dammed scenarios for 5346 global reservoir dams to assess their impact on carbon emissions from retained OC. The results show that reservoir-only approaches overestimate dam-induced carbon emissions by neglecting baseline emissions under no-dam conditions. After baseline correction, dams increase carbon emissions by an average of 21.5% in CO₂-equivalent (CO₂-eq), which represents only 18.6% of the emissions estimated using the conventional reservoir-based accounting approach. XGBoost identified reservoir depth and hydraulic residence time as the primary controls on dam-induced carbon emissions.