Presidential candidates, state governments, and utilities are promoting “net zero” emissions targets to rapidly reduce greenhouse gas (GHG) emissions and preserve a safe climate future by helping to limit global warming to well below 2°C. But few of them are exploring exactly how the U.S. could achieve the ambitious goal of remaking its energy economy.
Energy Innovation previously modeled a scenario to achieve the U.S. Paris Agreement pledge using the peer-reviewed, free, and open-source Energy Policy Simulator (EPS); it now uses the EPS to explore an illustrative policy package to achieve net zero U.S. emissions. By 2050, the net zero pathway abates more than 6 Gt of emissions a year and saves more than 120,000 American lives per year due to reduced particulate pollution.
This article focusing on the transportation sector is part three in a series of four identifying policy combinations that can achieve net zero U.S. GHG emissions by 2050. Other articles in the series cover the electricity, buildings, and industry sectors. (While emissions reductions from land use and agriculture represent roughly a tenth of those needed to reach net zero emissions, this series does not focus on that sector.)
The Paris Agreement targets net zero emissions by 2050. More than 60 countries, including Germany and the United Kingdom, have set net zero by 2050 goals, showing that political momentum can spur climate action – if the policy path forward is clear.
Net zero emissions: U.S. transportation
Decarbonizing the transportation sector is crucial to achieving the Paris Agreement’s goals: The U.S. transportation sector was responsible for over a quarter (27%) of 2018 U.S. GHG emissions (Figure 1).
Figure 1: U.S. GHG Emissions by Sector in 2018, with electricity sector emissions assigned to demand … [+]
Assuming power grid infrastructure and urban planning can keep pace with organic demand, without substantial additional policy intervention, transportation emissions from light-duty vehicles are projected to steadily decline more than half by 2050 (Figure 2). This decline comes as consumers increasingly favor electric vehicles (EVs) for their falling prices, lower maintenance, and impressive performance. However, emissions from other vehicles, including non-road vehicles like aircraft and ships, remain flat or increase through 2050. The U.S. must rapidly reduce transportation sector emissions to limit warming to 2°C.
Figure 2: U.S. CO2 emissions from transportation by vehicle type in a business-as-usual scenario … [+]
Policies to decarbonize the transportation sector
A number of policies can hasten the clean vehicle transition and encourage walking, biking, and public transit. Figure 3 depicts transportation sector emissions reductions caused by each measure in this net zero pathway. Transportation sector emissions do not reach zero in 2050, and in this net-zero scenario, the residual transportation sector emissions are counterbalanced by carbon sequestration in land use and forestry.
Figure 3: Policies’ contribution to abatement of direct U.S. transportation emissions in the Net … [+]
Ordered from largest to smallest contribution to cumulative 2018-2050 emissions abatement in this net zero scenario:
Electric vehicle sales requirement (73% of abatement): Today’s vehicles primarily burn petroleum fuels that emit CO2, nitrogen oxides, and particulates, worsening global warming and contributing to lung disease, stroke, and premature death.
Vehicles have long lifetimes: A typical car, SUV, or motorcycle may be operated for one or two decades; a medium/heavy truck or airplane for two or three decades; a rail locomotive or freight ship for three or four decades. Therefore, decarbonizing the transportation sector by 2050 requires phasing out sales of new, petroleum-burning vehicles almost immediately. (If a ban on new fossil fuel vehicle sales only comes into force in 2050, many polluting vehicles will still be on the roads in that year.)
Accordingly, the net zero pathway requires 100% of all newly-sold cars, SUVs, motorcycles, buses, and rail locomotives, as well as 50% of medium- and heavy-duty trucks, to be all-electric via electric vehicle policies by 2030. This is an extraordinarily aggressive timeline, but it is not unprecedented: Seven countries have announced bans on new fossil fuel vehicles that take effect in 2030 or earlier, and a further five countries have announced bans that come into effect by 2050 (or in the case of China, no date has yet been set).
Applying EV requirements to non-road vehicles is more difficult. Rail can be electrified with overhead catenary wires or a third rail, obviating the need for locomotives to carry batteries. Electrified rail networks carry passengers throughout Europe, China, and Japan, and electric freight locomotives operate in Europe, Russia, and China. Electricity may not be viable for aviation or long-haul freight shipping, due to difficulties storing and carrying sufficient energy in batteries, so the EV sales mandate is not applied to these vehicles.
Transportation demand management (TDM, 13%): TDM is an urban mobility policy representing measures to shift travel from modes with large emissions intensity (light-duty passenger vehicles, freight trucks, and aircraft) to modes with zero- or low-emissions intensity (walking, biking, buses, rail, and ships). Component policies include:
- Zoning for higher density residential and commercial development near transit hubs and along major transit corridors
- Zoning for mixed use development, so people can live near where they work
- Promoting infill development and avoiding urban sprawl – for instance, by adopting an urban growth boundary, as is used by all cities and metro areas in Oregon
- Properly funding public transit, and adopting measures to make it an attractive, first-choice option (ensuring service is frequent and predictable, with clean and safe vehicles and stations)
- Using congestion pricing to reduce the number of vehicles in urban centers, as used in London and scheduled to start in New York City in 2021
- Altering building codes to specify a maximum, not a minimum, number of off-street parking spaces per new housing unit, as London did in 2004
- Providing high-quality intercity passenger rail service (ideally fully grade-separated, high-speed rail) to reduce aircraft use
- Providing faster permitting and better infrastructure for rail and ship freight to reduce freight truck use
TDM’s total GHG abatement is limited in the net zero pathway because most vehicle types transition to 100% clean energy by 2050. TDM will drive more emissions reductions if on-road vehicles are not using 100% clean energy by 2050, and even if the target is achieved, TDM lowers overall costs by reducing the number of vehicles that must be purchased and amount of electricity generation capacity that must be built by 2050.
Hydrogen vehicle sales mandate (12%): Hydrogen is useful because batteries’ weight and comparatively low energy density may make them less economical for certain long-haul transport modes. In the net zero pathway, this policy specifies 50% of newly-sold, medium- and heavy-duty trucks must use hydrogen fuel cells by 2040. (This complements the 50% EV mandate requirement, fully decarbonizing medium and heavy duty new truck sales by 2040, since the net zero policy package also shifts all hydrogen production to electrolysis.) This is more aggressive than existing projections, although some groups, such as the California Air Resources Board, project a significant role for hydrogen fuel cell vehicles by 2050 in California, and most of these are likely to be medium- or heavy-duty vehicles.
Vehicle fuel economy or CO2 emissions standards (3%): Vehicle performance standards specify a maximum permitted amount of fuel consumption or CO2 emissions per vehicle mile traveled. The U.S. has a long history of fuel economy standards, making this a well-understood policy with an existing vehicle testing and regulatory apparatus in place. Fuel economy standards save owners thousands of dollars per vehicle, while reducing GHG emissions and improving U.S. energy security. Historically, when standards have stagnated, the average fuel economy of newly sold vehicles has slightly fallen, illustrating the essential role standards play in technological innovation.
Standards only contribute a small share of emissions abatement in the net zero pathway because many of the vehicles covered by standards (on-road vehicles and rail) transition to 100% clean energy by 2050 due to the EV and hydrogen vehicle sales mandates. Standards will drive more emissions reductions if these vehicles are not using 100% clean energy by 2050, and even if the target is achieved, standards lower overall costs by reducing the amount of clean electricity generation capacity that must be built by 2050.
In the net zero pathway, standards are also applied to aircraft and ships (25% and 20% fuel economy improvements, respectively, by 2040). These vehicles do not use clean energy in the net zero pathway, so standards are particularly important for them.
Reducing fuel use and residual emissions
The net zero pathway reduces overall transportation sector energy use about two-thirds from 2018 to 2050, and well over half of the energy used in 2050 is zero-emission electricity or hydrogen (Figure 4).
Figure 4: U.S. transportation sector fuel use by fuel type in the net zero emissions scenario, … [+]
Under the net zero pathway, the remaining emissions come predominantly from aircraft, with a significant contribution from medium and heavy trucks, as their long lifetimes mean the fleet has not fully turned over after the 100% clean energy requirement took effect (Figure 5). Other vehicle types, particularly freight ships, also continue to emit GHGs through 2050.
Figure 5: U.S. transportation GHG emissions by vehicle type in the Net Zero Emissions scenario, … [+]
Mitigating remaining freight truck emissions simply requires allowing enough time post-2050 to complete the U.S. truck fleet turnover. Reducing aircraft emissions may be accomplished by strengthening international aircraft fuel economy standards, reducing air travel, and development and deployment of aviation biofuels. For ships, zero-carbon hydrogen or hydrogen-derived fuels are a promising technological route.
Achieving low-carbon transportation for net zero emissions
Rapid transportation sector decarbonization is challenging due to long vehicle turnover times and the difficulty of using clean energy in certain non-road vehicles. Sales of most new, on-road fossil fuel vehicles must cease within approximately ten years for the U.S. to achieve zero net economy-wide emissions by 2050. But transforming U.S. transportation is possible, and the net zero pathway illustrates a set of policies that would enable the U.S. to achieve this very ambitious target.
