Sugarcane HarvestU. S. renewable fuel policy has two primary objectives: 1) to reduce petroleum imports, increasing energy security and 2) to reduce greenhouse gas generation in the transportation sector. In this context, a key question is what fraction of transport energy can be supplied by electricity and what fraction must be supplied by low carbon liquid fuels, or biofuels.

Two recent papers, one focused on the U.S. and another with a global perspective, show that the ability of electricity to serve the light duty fleet is much less than previously thought if both energy security and GHG reduction are to be achieved. Importantly, both studies conclude that by 2050 approximately 80% of total transportation fuel globally and in the U. S. will still be energy dense liquid fuels, even with aggressive efforts to electrify the light duty fleet.

The implications are daunting – successfully de-carbonizing light duty transportation requires simultaneous “successes” around key challenges. First, growth in travel would need to be moderate at most, unless nearly all petroleum is replaced with alternative biofuels. Secondly, nearly all light duty vehicles would need to be hybrid or electrified by 2050. Third, U.S. electricity supply would have to be massively decarbonized. Changes of this magnitude may still be insufficient to meet an 80% GHG reduction without additional low-carbon cellulosic biofuels replacing gasoline.

Current U.S. policies with respect to low-carbon, renewable fuels are highly conflicted. The “blend wall” limiting ethanol to about 10% of gasoline is an artifact of federal policy. It is not a technical barrier. The blend wall effectively pits first generation corn ethanol against potentially more abundant and low-carbon ethanol from cellulosic biomass. The fate of second generation biofuels is dominated by the market availability for these fuels; a market the Renewable Fuel Standard (RFS) was intended to require, but has not due to EPA’s inconsistent implementation. The intent of the RFS must be fully realized if we are to move toward both energy security and low-carbon fuels.