Large reservoir expansion underlies the near-doubling of reservoir GHG emissions in China since 21st century

The long-term temporal dynamics of GHG emissions from reservoirs remain insufficiently examined, despite their potential to reveal the influences of environmental variation and economic development on the reservoir carbon budget. In this study, we investigated the temporal variation of GHG emissions from Chinese reservoirs since 1940. Reservoir construction between 1950 and 2000 led to only a gradual increase in total national reservoir GHG emissions. An increased rise in total GHG emissions occurred after entering the new century, driven by the commissioning of large reservoirs. This upward trend further accelerated during the 2010s, reflecting the rapid expansion of large reservoirs in southwestern China with significant CH₄ degassing emissions. The high degassing emissions are attributable to favourable local environments that provide abundant substrates for methanogenesis, coupled with large discharge volumes facilitating CH₄ degassing. Moreover, this study identified a non-linear, U-shaped relationship between reservoir age and GHG emission rates. This relationship was then incorporated into the emission model, which showed that emission rates initially decline but then rebound later due to accumulated sedimentation and eutrophication. Thus, mitigating GHG emissions from Chinese reservoirs is imperative, with particular attention given to large reservoirs in southwestern China.