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2 to long-term geologic storage (BECCS). The energy so released is converted either into liquid fuels to offset hard-to-replace petroleum products (e.g., aviation and long-haul transport fuels) or into electricity for diverse end uses including electric vehicles (EVs). Dedicated bioenergy crops combined with carbon capture and storage (BECCS) are a feature of almost all Intergovernmental Panel on Climate Change (IPCC) mitigation scenarios that constrain the global temperature increase to 1.5 °C by 2100. (1,2) In these scenarios, biomass is used initially to offset transport sector fossil fuel use by substituting biomass-based fuels for the petroleum (gasoline and diesel) now used to power light-duty vehicles. (3) Later, once carbon capture and storage (CCS) technologies become available, captured biomass carbon (C) is subsequently transferred as COto long-term geologic storage (BECCS). The energy so released is converted either into liquid fuels to offset hard-to-replace petroleum products (e.g., aviation and long-haul transport fuels) or into electricity for diverse end uses including electric vehicles (EVs).

2 drawdown (negative C emissions) will depend at least in part on biomass to provide CO 2 for geologic sequestration (BECCS). Thus, future liquid fuel projections are based on a diminishing use for transportation to a point where most but not all transportation needs are met by electricity from a variety of sources in addition to biomass, including wind, solar, nuclear, and hydropower facilities. That said, (a) in the U.S., it will be decades before electric infrastructure is sufficient to deliver electricity to a substantial fraction of the entire U.S. light-duty vehicle fleet (3,4) and (b) once the fleet is converted to electric, the continued need for COdrawdown (negative C emissions) will depend at least in part on biomass to provide COfor geologic sequestration (BECCS). (5) While there are serious land availability limitations for different BECCS scenarios, (6,7) at least in the U.S., a substantial fraction of future biomass needs can come from herbaceous crops grown on former agricultural lands. (8−11) Although this is not necessarily the case elsewhere, requiring our analysis to be extrapolated with care, there is, notwithstanding, an important need globally to understand the climate implications of biomass-derived fuels for light-duty vehicles both during the expected transition to electric vehicles post-mid-century (12) and following maximum vehicle electrification, when biomass may be used for CCS while producing electricity.

2 e uptake) to C positive (i.e., net CO 2 e release): from −396 to 61 g CO 2 e MJ–1 for switchgrass, 2 e MJ–1. This same uncertainty extends to estimates of mitigation potentials for powering electric vehicles and for either ethanol- or electric-powered vehicles used with carbon capture and storage (CCS). All components of the measured climate impacts of energy production from mature stands of cellulosic bioenergy crops have yet to be compared experimentally, (9,13−16) and thus direct mitigation estimates remain largely uninformed by direct empirical evidence. When considering liquid transportation fuel as the sole end product, (17−20) life cycle models estimate emission intensities that range from C negative (i.e., net COe uptake) to C positive (i.e., net COe release): from −396 to 61 g COe MJfor switchgrass, (21,22) −139 to 13 for miscanthus, (23,24) and −150 to 164 for the maize stover. (22,25,26) The U.S. Environmental Protection Agency (27) estimates an overall range of −10 to 27 g COe MJ. This same uncertainty extends to estimates of mitigation potentials for powering electric vehicles and for either ethanol- or electric-powered vehicles used with carbon capture and storage (CCS). (28)

2 e flows. Here, we report the first empirical assessment comparing direct climate impacts of multiple cellulosic bioenergy crops for different bioenergy end uses. Earlier studies, e.g., (15,25,29−34) while crucially important for informing specific life cycle assessment (LCA) attributes in aggregate, have either not compared feedstocks side by side, or on different soils, or have not used field-based measurements of all important COe flows. (8) Rather, most if not all LCA comparisons to date have drawn on aggregated estimates from multiple studies and/or simulated values from biogeochemical models. (35−37) The absence of comprehensive field-based studies adds to already large uncertainty due to potential indirect effects of land-use change resulting from biomass production on lands now used for food production. (38−41) Thus, current debates on the climate impacts of large-scale implementation of biomass-based renewable fuels suffer from a lack of empirical knowledge of soil greenhouse gas (GHG) emissions, soil organic C (SOC) dynamics, and spatial variability in yields of bioenergy crops.

2 e balances of biomass production. The current LCA models based on average numbers and specific assumptions are producing multiple more or less plausible scenarios, while failing to predict verifiable real-world effects. (42) And only a handful of studies to date have provided comprehensive measurements of all major components contributing to net COe balances of biomass production. (9,13,43,44) Moreover, we are aware of no empirical studies that have considered alternative end uses—ethanol vs electric, both with and without CCS—of potential value for integrated assessment and earth system models that include a variety of alternative transportation and fuel switching strategies, (2) including ethanol- and electric-powered vehicles.