PNNL

Abstract

Land use change (LUC) CO2 emissions associated with bioenergy production depend on the amount of land required to produce bioenergy crops, the carbon stored in such crops (including in the leaves, stalk, roots and soil), and the carbon emitted when other land cover is directly or indirectly displaced as a result. In this study, we use a global integrated assessment model (GCAM) to estimate LUC CO2 emissions for two crops (corn and switchgrass) used to produce ethanol in the United States under alternative assumptions about natural lands protection. Varying the latter assumptions for corn ethanol results in net LUC CO2 emissions between 7 and 41 gCO2 per MJ of ethanol based on its lower heating value (LHV), whereas varying the same assumptions for switchgrass ethanol results in net emissions between -26 and 14 gCO2 per MJ of ethanol (LHV). The differences between corn and switchgrass reflect assumed differences in yields and carbon storage between these crops. The low-end estimate for each occurs when natural lands are assumed to be fully protected everywhere, which leads to significant cropland intensification. The high-end estimate for each occurs when natural lands are assumed to be unprotected everywhere, leading to greater cropland expansion and associated conversion of forest and pasture. If natural land is protected in some regions but not others, LUC CO2 emissions are projected to fall in between the low and high values for each crop.