No single energy source affects climate change and air quality in California as much as transportation fuel. California’s transportation sector represents the largest single source of greenhouse gas emissions in the state, at 37.3 percent (see Figure 1) – greater than the approximately 33 percent nationwide. Without reductions from the transportation sector, the state will not be able to meet its goals under AB 32 to roll back greenhouse gas emissions to 1990 levels by the year 2020 (equivalent to a 15 percent cutback from the business-as-usual scenario projected for 2020) or its longer-term target of reducing emissions to eighty percent below 1990 levels by 2050.