Most greenhouse gas inventories and policy strategies assume that hydro dams are essentially benign with respect to climate change, but a growing body of research suggests that the greenhouse gas impacts of reservoirs can be significant. We review a pair of papers with conflicting estimates of the impacts on Brazilian dams. In our view, these papers (and others) indicate that the largest sources of emissions within a reservoir are the least frequently studied. Because the issue is politically sensitive—especially in countries that depend on hydroelectricity for a majority of their electricity production, such as Brazil—we call for increased transparency and study. For example, a special report from the IPCC could review the extent and distribution of greenhouse gas emissions from dams and reservoirs.