produced from natural gas, but over time energy sources that emit less carbon are used to produce hydrogen (biomass gasification and coal gasification with carbon capture and sequestration).
Improvements in engines and other vehicle technologies continue to be implemented past 2020. The fuel economy of ICEVs and HEVs is assumed to increase according to the following schedule:
2.7 percent per year from 2010 to 2025,
1.5 percent per year from 2026 to 2035, and
0.5 percent per year from 2036 to 2050.
In addition, HEVs become much more important, comprising 60 percent of the fleet by 2050. The fleet mix is shown in Figure 4.3. Fuel economy for both types of vehicles approximately doubles by 2050 (Figure 4.4), when HEVs average 60 mpg and ICEVs are at 42 mpg.
Biofuels are introduced at a rapid rate, reaching 75 billion gallons per year in 2050 (Figure 4.5). Production of corn ethanol levels off, but cellulosic ethanol grows rapidly, reducing carbon emissions (well-to-wheels greenhouse gas [GHG] emissions for cellulosic ethanol are only 15 percent those of gasoline). Competition with food crops and indirect land use impacts on GHG emissions are not considered in this analysis.