Greenhouse gas emissions from biofuels
Reduced greenhouse gas (GHG) emissions are among the explicit goals of policy measures to support biofuel production. Bioenergy crops can reduce or offset greenhouse gas emissions by directly removing carbon dioxide from the air as they grow and store it in crop biomass and soil. Fuels from bioenergy crops are thus considered emission-neutral when burnt. In addition to biofuels, many of these crops generate co-products such as protein for animal feed, saving on energy that would have been used to make feed by other means, and thus avoiding GHG emissions.
Despite these potential benefits, however, scientific studies have revealed that different biofuels vary widely in their greenhouse gas balances when compared with petrol. Depending on the methods used to produce the feedstock and process the fuel, some crops can even generate more greenhouse gases than do fossil fuels. For example, nitrous oxide, a greenhouse gas with a global warming potential around 300 times greater than that of carbon dioxide, is released from nitrogen fertilizers. Moreover, greenhouse gases are emitted at other stages in the production of bioenergy crops and biofuels: in producing the fertilizers, pesticides and fuel used in farming, during chemical processing, transport and distribution, up to final use.
Greenhouse gases can also be emitted by direct or indirect land-use changes triggered by increased biofuel production, for example when carbon stored in forests or grasslands is released from the soil during land conversion to crop production. For example, while maize produced for ethanol can generate greenhouse gas savings of about 1.8 tonnes of carbon dioxide per hectare per year, and switchgrass – a possible second-generation crop – can generate savings of 8.6 tonnes per hectare per year, the conversion of grassland to produce those crops can release 300 tonnes per hectare, and conversion of forest land can release 600–1 000 tonnes per hectare.
Life-cycle analysis is the analytical tool used to calculate greenhouse gas balances. The greenhouse gas balance is the result of a comparison between all emissions of greenhouse gases throughout the production phases and use of a biofuel and all the greenhouse gases emitted in producing and using the equivalent energy amount of the respective fossil fuel. This well-established, but complex, method systematically analyses each component of the value chain to estimate greenhouse gas emissions.
(Source: Food and Agriculture Organisation (2008): The State of Food and Agriculture. Biofuels: prospects, risks, and opportunities.)