The global warming potential index (GWP) has been proposed to quantitatively compare the integrated greenhouse effect of different gases. However, the use of the GWP index is still subject to major conceptual difficulties. Here we revise and generalize this index and then apply our alternative index to the case of emissions from some hydroelectric reservoirs in Brazil. Our results suggest that, though the cumulative heating effects of emissions from hydroelectric reservoirs may be far from negligible, for the cases studied hydroelectricity in general contributes less to the greenhouse effect over a long time horizon than fossil fuelled electricity generation.