Will the three Gorges reservoir function as a carbon sink? Evidence from long-term field sampling and projections

While hydropower is a cornerstone of low-carbon energy, reservoir-induced greenhouse gas (GHG) emissions, specifically CO2 and CH4, remaining a critical uncertainty for its long-term sustainability. This study utilizes an unprecedented decade-long empirical dataset from the Three Gorges Reservoir (TGR) to clarify the long-term emission dynamics and explore operational mitigation strategies. Five annual sampling campaigns post-impoundment revealed an annual emission of 359 ± 78 GgC·yr-1 of CO2 (331 gC·m-2·yr-1) and 3.81 ± 1.05 GgC·yr-1 of CH4 (3.51 gC·m-2·yr-1), primarily through air-water diffusion and regulated by terrigenous carbon inputs. CMIP6-driven machine-learning long-term projections indicate a divergent future pathway: CO2 emissions are expected to decline by up to 69.7 GgC by the 2090s, potentially turning the TGR into a net carbon sink; conversely, CH4 emissions are projected to rise due to climate-driven external loading. Integrating these insights into the Water-Energy-Carbon (W-E-C) nexus, a theoretically feasible strategy was exhibited that raising the summer water level by 5 m could reduce the hydropower carbon intensity by up to 5.1 % without compromising electricity generation. These findings provide a scalable methodology for the low-carbon optimization of large-scale hydroelectric systems, emphasizing that CH4 mitigation remains the paramount long-term management challenge for the sector.