Many jurisdictions are focused on achieving very low or net-zero greenhouse gas (GHG) emissions by mid-century, bringing a spotlight to the biggest challenges in decarbonization.
The transportation sector is responsible for about one-quarter of global GHG emissions and emissions are growing, even in the developed world where other emissions are generally flat. Liquid fuels made from oil dominate the sector; they are easy to transport and store, contain a great deal of energy for their weight and volume, and enable use of internal combustion engines. The degree of difficulty in decarbonizing transport varies across the sector. Electrification is relatively easy for smaller vehicles that travel shorter distances carrying lighter loads. For these vehicles, the added weight of a battery is less of a hindrance and the inherently simpler and more efficient electric motor and drivetrain (the system that delivers power from the motor to the wheels) make up for some of the weight penalty. However, the heavier forms of transportation are among the fastest growing, meaning that we must consider solutions for these more difficult vehicles as well. The challenge of decarbonizing these sectors and the technologies to overcome these challenges are global, but this paper focuses on policy options in the United States.
Medium and heavy trucking and other forms of heavy ground transportation represent a middle ground in the decarbonization challenge. Vehicles that travel set routes in limited areas represent the low-hanging fruit for electrification. City buses, urban delivery vehicles, and equipment at ports can be recharged at a central location or at wireless pads along the way, and these vehicles are leading the way in heavy vehicle decarbonization. Longer distances and heavier loads bring additional challenges, especially the weight of the battery and the very high power needs for fast charging. Chargers rated as high as 3 megawatts are under development to charge tractor-trailers and West Coast utilities are looking at building charging stations with a maximum load of 23.5 megawatts. Such heavy loads for vehicle charging will require grid upgrades, especially in rural areas.
Aviation and maritime shipping share important characteristics, despite being the most and least GHG-intensive forms of transport, respectively. These modes carry heavy loads with little or no opportunity for frequent refueling, except for short shuttle flights for airliners or ferries for maritime transport. The energy density of oil-based fuels is particularly important in these sectors. Low carbon fuels that can be dropped into the current fuel mix are likely to be important in decarbonizing both sectors, allowing progress despite 25- to 30-year lifespans of airliners and container ships. In aviation, efficiency is already reducing per-mile emissions; new planes are as much as 25% more efficient than older models and more improvements are expected. Biomass-derived jet fuel is available today, but the supply of waste oil feedstock is not sufficient to meet demand. Biofuels from cellulosic crops and agricultural wastes are possibilities for the future, as are hydrogen and fuels made from hydrogen and captured carbon dioxide. Liquified natural gas (LNG) is a lower-carbon option for maritime shipping that also meets the low-sulfur fuel requirement that took effect in January 2020. Bio- and waste-based fuels are also longer-term options in shipping, similar to aviation.
Technology exists to decarbonize the heavy transport sector, although many advanced technologies are expensive and not proven at scale. The challenge for policymakers will be keeping technology advances and policy in alignment as the technology advances.
Decarbonization of heavy transport lags behind other sectors, but spillover effects can help. For example, some advanced biofuel technologies produce a range of fuels, similar to making a range of fuels from crude oil. Today’s supply of bio-jet fuel comes from such processes, despite a lack of policy for jet fuel decarbonization. More synergies could emerge if carbon capture becomes a common way to decarbonize difficult stationary sources of GHGs, like some industrial processes. Captured carbon dioxide (CO2) can be combined with hydrogen produced with renewable electricity to make liquid fuels. Technology exists to decarbonize the heavy transport sector, although many advanced technologies are expensive and not proven at scale. The challenge for policymakers will be keeping technology advances and policy in alignment as the technology advances. The COVID-19 pandemic adds a degree of difficulty, since it is unclear how it may shift demand and consumer preferences in transport. For example, consumers may remain reluctant to use urban public transport, and shorter supply chains may be attractive to businesses seeking to become more resilient in the face of a global disruption.