Hydroelectric reservoirs cover an area of 3.4×105 km2 and comprise about 20% of all reservoirs. In addition, they contain large stores of formerly terrestrial organic carbon. Significant amounts of greenhouse gases are emitted2, especially in the early years following reservoir creation, but the global extent of these emissions is poorly known. Previous estimates of emissions from all types of reservoir indicate that these human-made systems emit 321 Tg of carbon per year (ref. 4). Here we assess the emissions of carbon dioxide and methane from hydroelectric reservoirs, on the basis of data from 85 globally distributed hydroelectric reservoirs that account for 20% of the global area of these systems. We relate the emissions to reservoir age, location biome, morphometric features and chemical status. We estimate that hydroelectric reservoirs emit about 48 Tg C as CO2 and 3 Tg C as CH4, corresponding to 4% of global carbon emissions from inland waters. Our estimates are smaller than previous estimates on the basis of more limited data. Carbon emissions are correlated to reservoir age and latitude, with the highest emission rates from the tropical Amazon region. We conclude that future emissions will be highly dependent on the geographic location of new hydroelectric reservoirs.
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We thank F. Pacheco, R. Almeida and D. Torsten for searching data to improve our database. We are grateful to A. Cimbleris for inspiring us with insights connecting science and societal needs and to R. Mendonça and J. Ometto for critical discussion and reading of the manuscript. This work was supported by grants from FURNAS Centrais Elétricas and from the Swedish Foundation for International Cooperation in Research and Higher Education (STINT). V.L.M.H. and F.R. are partially supported by Conselho Nacional de Investigação Científica e Tecnológica (CNPq), Brasil.
The authors declare no competing financial interests.
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