Identifying the relative C benefits of biofuels and forests across a range of land use changes
Greater demand for low carbon (C) energy has increased interest in cellulosic bioenergy as an alternative to fossil fuels. Bioenergy crop expansion could directly or indirectly clear forests, which could release large biospheric C stocks. In that case, bioenergy would have a large initial C debt and high near-term net greenhouse gas emissions relative to fossil fuels. Land sparing due to changes in food production, consumption, or waste that reduce agricultural land requirements would avoid forest clearing and theoretically make land available for bioenergy crop production or afforestation. These alternative land use changes are usually not considered when comparing bioenergy production scenarios. We use geospatial models of energy crop productivity to develop estimates on U.S. cropland and pastureland east of the 100th parallel. We include life cycle emissions associated with fossil fuels and production of bioenergy. Using a C budget box model, our goal is to compare the relative benefits of forests and fossil energy to cellulosic bioenergy under a variety of potentially emerging land use changes. For this purpose, we used published data to characterize forest standing stocks and net ecosystem productivity to develop estimates of forest C clearing penalties and sequestration opportunities.
Three central insights emerged: 1) Direct or indirect clearing of forests due to bioenergy crop expansion creates bioenergy with high emissions relative to fossil energy. 2) On spared land, bioenergy creates a closed-loop cycle of C sharing between the biosphere and atmosphere, such that biogenic emissions are offset by bioenergy crop photosynthesis, and sometimes form a soil C sink, resulting in in low C biofuels (9.4 g CO2e/MJ) when compared with equivalent fossil energy (101 g CO2e/MJ); but the benefit relative to forest sequestration differs depending on whether the spared land comes from cropland or pasture. 3) On spared cropland, we estimate that biofuels have greater emission reductions than forest-fossil systems during the first 30 years (-9.94 Mg CO2e/ha/yr vs. -8.25 Mg CO2e/ha/yr), whereas spared pasture is predicted to have lower bioenergy crop yields and less potential as a soil C sink. As a result, we find emission reductions from bioenergy on spared pasture (-5.27 Mg CO2e/ha/yr) were much less than for forests (-8.84 Mg CO2e/ha/yr). If forest clearing can be avoided, then depending on the type of land sparing, results suggest a combination of biofuels and afforestation may offer the greatest C benefit.