Over 1 million dams exist worldwide. These structures have numerous environmental effects, and there is no shortage of research on the various ecological consequences of dams. However, the bulk of the research effort has been narrowly focused on river ecosystems. Writing in BioScience, Bridget R. Deemer of the School of the Environment at Washington State University Vancouver and an international team of researchers elucidate another major threat: the release of greenhouse gases.

Researchers first raised the possibility that dam-created reservoirs might significantly contribute to climate change some 16 years ago, but since then, efforts to pinpoint the severity of the effect have been hampered by a lack of data and inconsistent measurement techniques.

Deemer and her colleagues embarked on a systematic effort to synthesize reservoir data, with three goals in mind: "to generate a global estimate of greenhouse gas emissions from reservoirs, (2) to identify the best predictors of these emissions, and (3) to consider the effect of methodology on emission estimates."

Their results were striking. The authors estimate a yearly release of 0.8 gigaton of carbon dioxide equivalent from reservoirs. This amounts to 1.3% of total human-caused greenhouse gas releases--a quantity that Deemer and her colleagues argue should be included in global inventories of human-caused greenhouse gas emissions.

Furthermore, the authors caution: "with the current boom in global dam construction, reservoirs will represent an even larger fraction of anthropogenic carbon dioxide equivalent emissions in the coming years."

Although it is unlikely that dam construction will soon abate, the authors point to possible ways to alleviate greenhouse gas releases. For instance, by siting dams upstream of large inputs of pollution, managers may be able to create reservoirs with lower emissions. According to the authors, such careful siting of new reservoirs, in combination with better management of existing ones, "may help balance the positive ecosystem services that reservoirs provide against the greenhouse gas emission costs."