The Mekong River Basin in Southeast Asia is undergoing extensive hydropower development, but the magnitude of related greenhouse gas emissions (GHG) are not well known. We provide the first screening of GHG emissions of 141 existing and planned reservoirs in the basin, with focus on atmospheric gross emissions through the reservoir water surface. The emissions were estimated using statistical models that are based on global emission measurements. The hydropower reservoirs (119) were found to have an emission range of 0.2-1994 kg CO2e/MWh over a 100-year lifetime with a median of 26 kg CO2e/MWh. Hydropower reservoirs facilitating irrigation (22) had generally higher emissions reaching over 22,000 kg CO2e/MWh. The emission fluxes for all reservoirs (141) had a range of 26-1,813,000 t CO2e/yr over a 100-year lifetime with a median of 28,000 t CO2e/yr. Altogether, 82% of hydropower reservoirs (119) and 45% of reservoirs facilitating also irrigation (22) have emissions comparable to other renewable energy sources (<190 kg CO2e/MWh), while the rest have higher emissions equalling even the emission from fossil fuel power plants (>380 kg CO2e/MWh). These results are tentative and they suggest that hydropower in the Mekong Region cannot be considered categorically as low-emission energy. Instead, the GHG emissions of hydropower should be carefully considered case-by-case together with the other impacts on the natural and social environment.

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