INCORPORATING BIOFUELS INTO ECONOMIC MODELS
All four models were developed before the implementation of RFS2, but each has been modified in recent years to include varying degrees of biofuel coverage. FAPRI and GTAP include only first-generation biofuels (corn-grain ethanol, sugarcane ethanol, and oilseed-based biodiesel). The institutions that run these models plan to expand the database to include second-generation biofuels, and in recent years, FAPRI has increased the level of detail of its representation of Brazil to account for changes due to biofuels. FASOM and POLYSYS include second-generation biofuels from cellulosic feedstocks presently used. FASOM is the only model of the four to include electricity generation from cellulosic feedstocks. Incorporating biofuels into the models is complex. The difficulty of using the models for projections of biofuel feedstock production centers around three areas of insufficient detail.
CAPTURING WORLD MARKET EFFECTS VERSUS AGRICULTURAL PRODUCTION DETAILS
Whether global trade interactions and linkages with other sectors are needed is largely an empirical question. Both GE and PE models have advantages and disadvantages. FAPRI, FASOM, and POLYSYS enter some macro-variables exogenously and consider world trade linkages and flows. The comparative static version of GTAP does not project changes over time; rather, it compares the situation at two points in time given whatever shock has been applied to the model. The PE models normally simulate through time and compare the results with and without a given shock to a predetermined baseline.
CELLULOSIC BIOMASS NOT INCLUDED IN MOST MODELS
Cellulosic biomass crops, including crop residues from grain crops, perennial grasses, bagasse from sugarcane, and woody crops and residues, are being considered as a source of feedstock for liquid transportation fuels. The production and cost estimates that exist vary widely over time, space, and studies. Of the four models being discussed, FASOM includes woody biomass and POLYSYS includes cellulosic biomass crops and woody biomass. GTAP is in the process of adding cellulosic biomass, and FAPRI is in process of including cellulosic crop residue and switchgrass.
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