Greenhouse gas balances of microalgal biofuels

Life cycle emissions of greenhouse gases associated with microalgal biofuels reported in peer- reviewed papers vary widely. For microalgal biodiesel, they range from -75 g CO2eq to + 534 g CO2eq MJ-1 biodiesel. Available studies on life cycle dealing with greenhouse gas emissions are consequential and subject to relatively large uncertainties. Thus, the results of life cycle assessments of specific combinations of product(s) and process(es) should be expressed as ranges rather than as one-point values. Choices about the application of credits, system boundaries, expected microalgal yields, allocation and the decarbonization of energy supply matter substantially to the results of life cycle assessments. The way emissions of methane are dealt with may also be important for estimated greenhouse gas emissions linked to microalgal biofuel lifecycles. Substantially reducing uncertainties in the outcomes of life cycle assessments awaits the availability of attributional life cycle assessments which deal with well-monitored existing commercial microalgal fuel productionconsumption chains.