Estimating emissions associated with indirect land use change (ILUC) is a fundamental part of analysing the likely net GHG emissions impacts of biofuel mandates, and in some regulations (e.g. U.S. Renewable Fuel Standard, California Low Carbon Fuel Standard and ICAO’s CORSIA) estimates of ILUC emissions associated with specific feedstocks have been integrated into regulatory lifecycle analysis frameworks. This academic paper, co-authored by Chris Malins of Cerulogy with Richard Plevin and Robert Edwards, examines the development of one particular model for estimating ILUC emissions (GTAP-BIO) and asks how well supported by relevant data and/or analysis various adjustments and innovations introduced to the model over the past decade or so have been.
Particular attention is paid to a series of model amendments that have enhanced the role of intensive responses (increased productivity on existing agricultural land) in the model and reduced the predicted extent of land use changes from biofuel policy. The paper finds that there is a lack of compelling evidence supporting adopted assumptions, and that on more than one occasion model adjustments that have been presented as ‘neutral’ have in fact predictably resulted in reduced output ILUC values. It also notes that while the ‘cropland-pasture’ land category in the U.S. has been made central to the model outcomes there is very little evidence available to confirm that this is a realistic assumption, and that emission factors have been adopted for cropland-pasture conversion that are difficult to justify analytically. Reductions in modelled ILUC estimates ought therefore to be understood as at least as much the result of subjective decisions by the modelling teams involved as the result of any objectively demonstrable improvement in our understanding of the systems being studied.
The paper concludes that it is unclear whether more recent published ILUC estimates are likely to be closer to the ‘real’ average ILUC values for corn ethanol and soy biodiesel than higher values from earlier assessments.