## Introduction

Current California environmental quality goals are deeply intertwined with the clean energy systems and fuels both deployed and evolving, to meet current and future energy demands. In particular, a dramatic reshaping of California energy systems and fuels is underway including transitions from fossil fuel combustion to low carbon technologies and fuels, with a priority on zero emission and renewable energy sources in nearly every emitting sector of the economy1. The transition cannot occur soon enough. California continues to experience challenges associated with degraded air quality including elevated concentrations of both ozone and fine particulate (PM2.5)2. Furthermore, California has established commitments requiring reductions in emissions of greenhouse gases (GHGs) including the original targets of 40 and 80% below 1990 levels by 20303 and 20504, and the more recent executive order “B-55-185” requiring the state to reach carbon neutrality no later than 2045. California is already experiencing significant economic and health damages from the effects of climate change, including an increase in intensity and incidents of wildfires6,7 and deteriorating air quality8,9. Concerns over the damages brought by air quality and climate change are further exacerbated by the disproportionate shares both of these risks imposed on socially and economically disadvantaged populations10.

How these targets will be met is still uncertain due to the wide range of technologies, fuels, and other considerations that can provide GHG reductions. Electrification of fossil fuels depended sectors in tandem with electricity generated from renewable sources is certain to play a central role in California11. However, different strategies existed to achieve the same GHG reductions by adjusting the level of electrification penetration in different sectors. Further, resource availabilities can yield tradeoffs for allocating some low or net-zero carbon fuels (biofuels are a prominent example) across end-use sectors, particularly for those that are challenging to electrify12. One source of uncertainty is the future of the natural gas system, which represents an installed asset that could be managed in different ways to store and distribute low carbon gaseous fuels including renewable hydrogen and methane13. In this work, two different scenarios are considered to provide insight into the impacts of technology innovation, policy, and other drivers that will shape the evolution of energy sectors in California including a focus on the relative tradeoffs of decarbonizing the built environment and heavy-duty vehicles.

## Discussion

### Mitigation cost analysis

The mitigation costs for each scenario are taken from Aas et al.32 and were estimated by E3 using the California PATHWAYS model. PATHWAYS is an energy and infrastructure model that can be used to assess the cost and GHG emissions of California’s energy demand and supply decisions designed to achieve long-term climate targets64. PATHWAYS is used extensively in California to develop and assess economy-wide scenarios meeting climate policy targets using differing combinations of mitigation measures11,65,66,67. Given the focus on decarbonizing natural gas, a detailed technoeconomic assessment for each truck electrification pathway considered was conducted including costs (energy, capital, and feedstock) and resource potential as described in Appendix E of the referenced report67 and used to update PATHWAYS for this work. The economy wide costs estimated by PATHWAYS represent a total resource cost that includes all direct energy system costs within the California economy resulting from fuel consumption and from capital costs from energy infrastructure associated with purchase of building appliances or vehicles, as well as incremental energy efficiency or fuel-switching capital costs. To calculate mitigation cost for each census tract, it is assumed that costs to each individual are equally distributed across the California population. The mitigation cost for each census tract is then estimated by calculating the sum of individual costs within each census tract.

### Environmental justice assessment

Economist Daniel B. Suits41 initially proposed the Suits Index as a measure of tax progressiveness. Here, the Suits Index is used as an indicator of progressivity in public health benefits distribution among communities with different environmental justice statuses. Figure S3 illustrates how the Suits Index is calculated to assess each mitigation scenario with the x-axis corresponding to the socio-economic status of a census tract ranked by CalEnviroScreen 4.025 where higher rankings are considered more disadvantaged disadvantaged to environmental hazards. The y-axis represents the cumulative percentage of total environmental benefits that accrue from air quality improvements resulting from the deployment of mitigation strategies within each scenario. OB represents a proportional or equal distribution of environmental benefits among all communities. At the same time, OCB shows a distribution in favor of disadvantaged communities, and ODB shows a distribution favoring the general population of non-disadvantaged communities. The Suits Index is the ratio between the area that a Lorenz-Curve (OCB or ODB) deviates from the proportionality line (OB) and the area given proportionality (between OB and OAB). For a curve (OCB) under the proportionality line, the area is considered positive, so its Suits Index will be positive. For a curve (ODB) above the proportional line, the area is deemed negative, so its Suits Index will also be negative. The Suits Index is a value between −1 and 1, with 0 indicating for equal distribution, positive values for progressive distribution, and negative for regressive distribution.

In this study, the CalEnviroScreen 4.0 environmental justice screening tool developed by California’s Office of Environmental Health Hazard Assessment (OEHHA) is employed to rank all California communities at the census tract level (from 0 to 100). CalEnviroScreen identifies communities risked by a disparate share of air pollution in addition to socioeconomic and health challenges that increase their risk to environmental health effects. CalEnviroScreen ranks each of the state’s 8000 census tracts according to multiple endpoints associated with pollution, environmental quality, and socioeconomic and public health conditions. Organizations ranking within the final 25% (score ≥ 75) are considered disadvantaged communities. Health benefits estimated from the air quality co-benefits of different technology pathways are used for the environmental benefits. The MATLAB v9.8 R2020a is used for data processing, visualization, and suit index calculations.

### Reporting summary

Further information on research design is available in the Nature Research Reporting Summary linked to this article.