Mind the Gap: Transportation Emissions vs. California’s 2022 Scoping Plan Pathway 

The State of California has a unique air emissions regulatory history that began in 1947 with the California Air Pollution Control Act, which authorized the creation of county Air Pollution Control Districts (APCDs). This followed the first recognized episodes of smog in Los Angeles in the summer of 1943. From that legislation and others that followed, APCDs and later the California Air Resources Board (CARB) began regulating emissions from stationary sources, fuels, and vehicles. In 1966, California established the first tailpipe emissions standards in the nation. On the federal level, the landmark 1970 Clean Air Act set the stage for nationwide regulation of emissions and, notably, authorized California to set its own separate and stricter-than-federal vehicle emissions standards in recognition of its prior regulations and air quality issues.  

Through the end of the 20th century, the focus on air emission was criteria pollutants – oxides of nitrogen (NOx), carbon monoxide (CO), hydrocarbons (HC) and particulate matter (PM) – that are the precursors of smog. In the 21st century, to address climate change, greenhouse gases (GHGs) were added and have become the central focus of air emission regulation in the state. Since then, numerous laws and executive orders have shaped California’s climate program, with the cornerstone being the Global Warming Solutions Act of 2006 (AB 32), which established the first comprehensive, economy-wide GHG reduction mandate in the United States. AB 32 required California to reduce statewide GHG emissions to 1990 levels by 2020, a reduction of roughly 15% below business-as-usual projections, forming the foundation of the state’s climate policy architecture and signaling a decisive shift toward long-term decarbonization. Lawmakers have updated AB 32 multiple times since 2006, most recently with AB 1279, which resets the state’s goal to achieve net-zero GHG emissions by 2045. 

Two key regulatory programs established under AB 32 that impact transportation fuels are Cap and Invest (C&I) – formerly Cap & Trade (C&T) – and the Low Carbon Fuel Standard (LCFS). C&I and LCFS are among the most influential market-based climate policies in the world. C&I sets a statewide, annually declining cap on covered entities that must acquire allowances to emit GHGs, while the LCFS aims to decrease the carbon intensity (CI) of California’s transportation fuels and expand the availability of low-carbon and renewable alternatives. 

A key planning tool that sets the road map for regulations under the AB 32 umbrella is the Scoping Plan in which California’s strategies for meeting the State’s GHG goals are laid out. The initial Scoping Plan was approved in December 2008. CARB, as lead agency, is required to update the Scoping Plan every five years; the most recent plan was approved in 2022, and the next version is due to be published in 2027.   

Because development of a Scoping Plan is a large and complex undertaking, requiring input from many agencies, each update takes several years, and the final years of any modeling period likely rely more on projections made a few years earlier than on recent historical data. In this article, we focus on the 2022 Scoping Plan, which now can be compared with several years of actual data from both before and after its adoption. This overview will assess how the transportation sector – particularly transportation fuels – has performed in reducing GHG emissions relative to the Scoping Plan pathway. This is the first in a planned series of articles examining California’s progress in reducing GHG emissions. Subsequent articles will dive deeper into gasoline, diesel, and electric vehicles before concluding with a wrap-up on the prospects for meeting California’s long‑term climate goals from a transportation perspective. 

Ambitious plan, stubborn transportation emissions

California’s 2022 Scoping Plan charts a path to reduce anthropogenic GHG emissions 85% below 1990 levels by 2045, consistent with AB 1279. Hitting that target requires substantial reductions across all sectors of the economy, with transportation bearing a particularly large share of the burden. The plan’s transportation pathway relies on rapid reductions in fossil fuel use – through a combination of “drop-in” renewable fuels and advanced vehicle technologies paired with low-carbon fuels – and carbon capture and sequestration (CCS). 

In the most recent statewide GHG inventory (2023 data), statewide carbon-dioxide-equivalent emissions totaled 360.4 million metric tons, with transportation accounting for the largest single source category at 37% of the total. Figure 1 illustrates the progress made in reducing statewide GHG emissions.  

Figure 1. Annual Statewide GHG Emissions and GHG LimitsSource: CARB – Current California GHG Emission Inventory Data  

Figure 2 illustrates the transportation sector’s GHG emissions reported by CARB in the 2022 Scoping Plan alongside a Stillwater estimate of the transportation sector’s actual emissions over the same period. From 2018 to 2020, the actual emissions closely tracked the Scoping Plan, indicating that actual data was used to develop the scoping plan over this period. Starting in 2021, however, the actual line deviates from the Scoping Plan with a more gradual reduction slope since 2020. 

Figure 2. Transportation Sector GHG Emissions vs. 2022 Scoping Plan (2018-2025) Sources: 2000–2023 GHG Emissions Trends Report Data and Stillwater analysis 

The more gradual decline in observed emissions suggests that the transportation sector may be veering off the modeled Scoping Plan pathway, at least for the near term. But what does this mean for the longer term? To illustrate a potential trajectory, we plot an exponential curve of transportation-sector GHG emissions – fit to the last four years of actual data – across the entire Scoping Plan horizon. As can be seen, the future “actual” pathway will have to accelerate to meet the Scoping Plan. 

Figure 3. Transportation Sector GHG Emissions vs. 2022 Scoping Plan (2018-2045)Sources: 2000–2023 GHG Emissions Trends Report Data and Stillwater analysis       

Conclusion

California’s climate goals are ambitious, and the Scoping Plan necessarily presents a scenario in which these goals are achieved. To stay on track, the transportation sector must decarbonize rapidly, which means cutting fossil gasoline and diesel consumption and accelerating adoption of vehicles that use low-carbon energy. In the remaining four articles of this five-part series, we will drill down in three key areas that are driving the transportation sector GHG emissions: gasoline fuels, diesel fuels, and electric vehicles, and conclude with a wrap up of the series. Stay tuned to this space!