Hydropower is commonly considered as a clean energy source to fuel Southeast Asian economic growth. Recent study published in Environmental Research Letters finds that hydropower in the Mekong River Basin, largest river in Southeast Asia, might not always be climate friendly. The median greenhouse gas (GHG) emission of hydropower was estimated to be 26 kg CO 2 e/MWh over 100-year lifetime, which is within the range of other renewable energy sources (<190 kg CO 2 e/MWh). The variation between the individual hydropower projects was, however, large: nearly 20% of the hydropower reservoirs had higher emissions than other renewable energy sources and in several cases the emissions equalled those from fossil fuel energy sources (>380 kg CO 2 e/MWh). The study concludes that hydropower in the Mekong cannot be considered categorically as a clean energy source; instead, the emissions should be evaluated case-by-case together with other social and environmental impacts.

Mekong River is a global hot spot for hydropower development. The economy in the region is growing rapidly and hydropower is broadly seen as a viable source for satisfying growing energy demands. However, the extensiveness of hydropower development has raised concerns on ecological and social sustainability, particularly in the case of rich inland fisheries of which millions depend for livelihoods and food. Less attention is put, however, on the potential climate impacts of the hydropower although case studies from other tropical regions show that hydropower emissions cannot be neglected. The emissions originate mainly from degradation of organic material in the reservoir but also from dam construction.

In the Mekong Basin emission measurements exist only from a few reservoirs, and the study led by Aalto University, Finland, is the first one to estimate the emissions of the entire hydropower development in the basin. The study estimated emissions of 64 existing and 77 planned hydropower reservoirs using statistical models that are based on global emission measurements. The focus was on gross emissions of CO 2 and CH 4 through the reservoirs water surface. The results are reported as CO 2 equivalents (CO 2 e) that combines the CO 2 and CH 4 emissions.

"The hydropower related emissions started in the Mekong in mid-1960's when the first large reservoir was built in Thailand, and the emissions increased considerably in early 2000's when hydropower development became more intensive. Currently the emissions are estimated to be around 15 million tonnes of CO 2 e per year, which is more than total emissions of all sectors in Lao PDR in year 2013," says Dr Timo Räsänen who led the study. The GHG emissions are expected to increase when more hydropower is built. However, if construction of new reservoirs is halted, the emissions will decline slowly in time.

"Highest emissions were generally found in hydropower projects that had large reservoirs compared to the electricity generating capacity and were located in warm sub-tropical areas. These findings suggest that the emissions of future hydropower projects can be reduced by designing and choosing the location appropriately," summarises Assistant Professor Matti Kummu. In addition, emissions can be mitigated by carefully removing the vegetation from the reservoir area before filling of the reservoir, and by minimising the amount of organic material entering the reservoir from the catchment.

The study calls for further research on GHG emissions of hydropower and improved inclusion of climate impacts in strategic environmental assessments (SEA) of hydropower in the Mekong to help to exclude most harmful projects. The results of the study are tentative and more extensive measurement and modelling campaigns are urgently needed to improve the accuracy of the emissions estimates.