The invention provides a method to decrease emission of carbon dioxide from combustion of fossil fuels or other hydrocarbons and to enhance the efficiency of methane production from anaerobic biodigesters. The invention involves feeding carbon dioxide from the exhaust gas of hydrocarbon fuel combustion to an anaerobic biodigester where biomass is anaerobically fermented to produce methane. Carbon dioxide is an electron acceptor for anaerobic fermentation, and thus some of the carbon dioxide is reduced to methane, which can again be used for fuel. In this way, at least a portion of the exhaust gas CO2 is recycled to form fuel methane instead of being released into the atmosphere. Thus, the net CO2 emission from burning a given amount of fossil fuel is decreased. Adding carbon dioxide to an anaerobic fermentation also increases the efficiency and amount of methane production in the fermentation.