Michael O'Hare1, Richard Plevin2, Jeremy Martin3, Andy Jones2, Alissa Kendall4, and Eli Hopson3.

1 Goldman School for Public Policy, UC Berkeley
2 Energy and Resources Group, UC Berkeley
3 Union of Concerned Scientists, Cambridge, MA
4 Department of Civil and Environmental Engineering, UC Davis


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The global warming intensities of crop-based biofuels and fossil fuels differ not only in amount but also in their discharge patterns over time. Early discharges, for example, from market-mediated land use change, will have created more global warming by any time in the future than later discharges, owing to the slow decay of atmospheric CO2. A spreadsheet model of this process, BTIME, captures this important time pattern effect using the Bern CO2 decay model to allow fuels to be compared for policy decisions on the basis of their real warming effects with a variety of user-supplied parameter values. The model also allows economic discounting of climate effects extended far into the future. Compared to approaches that simply sum greenhouse gas emissions over time, recognizing the physics of atmospheric CO2 decay significantly increases the deficit relative to fossil fuel of any biofuel causing land use change.

CO2 emissions and resulting atmospheric abundance for gasoline (25 years at 94 g CO2e MJ-1) and maize ethanol (25 years at 60 g CO2e MJ-1 plus iLUC discharge of 776 g CO2 MJ-1 and foregone sequestration totaling 102 g CO2 MJ-1; post-cultivation recovery of 50% of the lost biomass carbon over 30 years). (Click on figure to enlarge.)