Hydroelectric dams don’t reduce carbon dioxide emissions. Dams currently hold off about 15% of global freshwater runoff. If we want to sustain biodiversity and river-related goods and services, we can’t easily increase the number of dams. [1] Also: building dams requires cutting down forests, which themselves are important tools in combating greenhouse gases, since they consume and ‘lock up’ carbon dioxide. Plus, the construction of the dams themselves releases carbon dioxide. Finally: global energy demand is expected to continue increasing, [2] meaning that hydropower will probably just be added to the supply and not replace coal.  [1] International Rivers, Frequently Asked Questions.  [2] IEA, World Energy Outlook, 2010, Executive summary