Final Report of the CARB Expert Subgroup on " Comparative and Alternative Modeling Approaches

Disclaimer: While the subgroup benefited from the active participation of its members, not all members had the opportunity to fully participate in the drafting of this final document. Therefore, these recommendations do not necessarily represent full concurrence by all subgroup members and in general should not be considered as necessarily supported by the organizations they represent. As land use change occurs constantly, and has always done so for a variety of interacting reasons, it is inherently difficult to isolate a single causative factor from many in a complex system involving diverse social, economic and ecosystem processes. Most current estimates of GHG releases caused indirectly by biofuels use economic equilibrium or partial equilibrium models to attribute conversion of uncultivated land to replace biofuel feedstock production on existing farmland.. These models were created primarily to analyze domestic farm policy, international trade and trade policies, with land use change mechanisms constructed to help achieve this analytical end. For CARB, however, the estimation of land use change is the analytical objective. The use of estimates of LUC for regulatory purposes is new and the basis for the estimations rapidly developing. Given the evolving nature of LUC estimation and the lack of a consensus model for LUC assessement, multiple, independent methods should be considered. Committee Recommendations: CARB should develop protocols for evaluating and comparing different types of methods for estimating ILUC. 1. Continue to improve GTAP's capacity to predict land change response globally in response to a change in domestic biofuel production. In particular, continuous improvements in data on land use and availability, estimation of important parameters governing land conversion and technology improvement and adoption are necessary. Committee recommendations: If CARB continues to rely soley on GTAP to estimate ILUC GHG values, then GTAP must be continuously improved for this purpose, within the limits of regulatory stability and predictability. All models need continuous updating and improvement to reflect the latest knowledge and data available. Particularly needed are improved data on global