[en] A recent article in Ambio identifies water regulation for hydroelectric purposes as a significant contributor to the greenhouse effect. The hypothesis presented arose from the fact that water regulation usually involves the construction of large reservoirs and hence the flooding of former terrestrial environments. It is known that wetlands and similar ecosystems, in particular, release methane during the decomposition of organic material. Methane is a so-called greenhouse gas, which means that it has the capacity to absorb outgoing thermal radiation, thus increasing the temperature of the atmosphere. However, several of the figures used to support the hypothesis are incorrect or represent extremes. This comment provides data compiled for Sweden with the purpose of adding to a more general picture of the environmental impacts of hydro power. 7 refs, 1 tab