Scientists and policymakers around the world agree that in order to avoid the worst impacts of climate change, global warming must be kept to less than 2° Celsius above pre-industrial levels; as of 2016, we had already warmed by about 1° Celsius (See – noaa.gov). This translates into the need for developed countries to achieve about an 80 percent reduction by mid-century in the greenhouse gas emissions that are warming the planet, with developing countries following suit as soon as possible thereafter. Deep decarbonization by mid-century is extremely challenging, but is feasible through the interplay of technology innovation, smart public policy, and market forces.
The Big Picture in the U.S. In the US, the two highest-emitting sectors are electricity and transportation. In 2016, transportation edged out electricity for the first time. Greenhouse gas emissions in the US peaked in 2007, at 7.3 billion metric tons of carbon dioxide equivalent (See – epa.gov). Since that peak, total emissions have declined 10.4 percent. US power sector emissions have declined by 20 percent since 2005, for several reasons. 
Overall power sector demand is growing very slowly due to a combination of broader shifts in the U.S. economy and improvements in energy efficiency. Natural gas, which has about half the carbon content of coal, has been outcompeting coal in the marketplace due to low natural gas prices. Cheap natural gas is a consequence of the shale gas revolution, which in turn has its roots in tax credits and R&D in the 1970s and 1980s directed toward “unconventional” gas. More recently, the market shares of wind and solar power are increasing due to research and development, federal tax credits, and state renewable portfolio standards. Wind and solar power are becoming more and more cost-competitive (See – Lazard.com).
Transportation Sector’s Role in Decarbonizing the Economy. The transportation sector is in a markedly different situation from the electricity sector, making transportation decarbonization much more challenging. US transportation is more than 90 percent dependent on oil, and the current low price of petroleum products like gasoline and diesel provide little incentive for consumers to reduce driving or choose more efficient vehicles. The key success story over the past few decades for restraining transportation greenhouse gas emissions has been vehicle fuel economy (CAFE) and greenhouse gas standards, which save consumers money and reduce US oil dependence. The standards allow vehicle manufacturers to comply on a fleet-average rather than a vehicle-by-vehicle basis, so they can respond to a range of consumer preferences (See CAFE standards explained in this 2012 article – caranddriver.com) . The interplay between these standards, technology innovation and market forces have resulted in cars and vehicles that are more efficient, powerful, low-maintenance, and comfortable. It is critical that these vehicle standards remain in place and become more stringent over time. Without them, vehicle oil use and emissions will start increasing, undermining our economic competitiveness, energy security, and environmental well-being. Countries around the world are strengthening vehicle efficiency standards as well. Thus, if the US continues to strengthen standards at home, this helps the US auto industry to compete abroad.
Electric and Hydrogen Fuel Cell Vehicles. Beyond energy efficiency, a key pathway to decarbonizing passenger vehicles is through alternative fuels. Both electric and hydrogen fuel cell vehicles are options here, and their viability depends on the interplay of technology innovation, policy and market forces. Importantly, the life cycle emissions from an electric vehicle depend on how the electricity that it uses is generated. A vehicle charged with coal-powered electricity has higher emissions than one charged with nuclear or renewable power. Therefore, decarbonizing the transportation sector through electric vehicles depends on decarbonizing the grid (See – energy.gov). Broad deployment of hydrogen and electric vehicles will require substantial build-out of hydrogen refueling or electric charging infrastructure.
Connected and Autonomous Vehicles. The impact of connected and autonomous vehicles is a wild card that could make transportation decarbonization either much harder or much easier. Mobility and safety are the key motivators of connectivity and automation, but the energy and environmental implications are huge and deeply uncertain. On the one hand, intelligent vehicles and transportation systems can drive tremendous efficiency gains through improved traffic management, constant speeds, and limited breaking and accelerating, thereby reducing fuel use and emissions. On the other hand, automation can dramatically increase travel demand, for example, as it increases access to cars for people like senior citizens and children. According to a 2016 study by three of the US Department of Energy’s national laboratories, by 2050 full connectivity and automation could have an extremely broad range of impacts, from a 60 percent decrease in fuel demand to a 200 percent increase. (See – nrel.gov). One potential unintended consequence of vehicle connectivity and automation is the increased cyber-security risk. The positive and negative impacts of connected and automated vehicles requires further research as technologies advance.
Other Modes of Transportation. Beyond passenger vehicles, other modes of transportation need to decarbonize, including heavy trucks, buses, ships, and airplanes. With some exceptions, these modes generally do not lend themselves to electrification, but can be addressed by increasing efficiency, using lower-carbon fuels such as biofuels, and shifting travel and freight movement to less carbon-intensive modes of transport (e.g., by shifting shorter airplane flights to electric-powered rail).
Conclusion. If the US and countries around the world are to reduce greenhouse gas emissions by 80 percent by 2050, and attempt to stay below 2° Celsius, the transportation sector must be a priority. The US should take the following steps, through some combination of local, state, federal and private action:
1) Continue strengthening our flexible vehicle efficiency and greenhouse gas standards;
2) Increase investment in technology innovation to improve vehicle efficiency, intelligent transportation systems, and alternative-fueled vehicles;
3) Implement smarter, more transportation-efficient growth and mobility initiatives in our cities and towns;
4) Put in place policies and financial incentives to transition to zero-emitting hydrogen and/or electric vehicles running on decarbonized electricity; and,
5) Continue to develop lower-emission options for trucks, buses, ships, and airplanes.
Through the interplay of technology innovation, smart public policy, and market forces, we improve our economic competitiveness and energy security as well as our environmental well-being.
Judi Greenwald is a globally recognized energy and climate expert. She has served as the Climate Advisor to the Secretary of Energy and Deputy Director for Climate, Environment, and Energy Efficiency for the DOE Office of Energy Policy and Systems Analysis where she led analysis on the Clean Power Plan and the U.S. nationally determined contribution to the Paris Climate Accord. Previously, she served as the Vice President for the Center for Climate and Energy Solutions, and has worked on energy and climate issues in Congress, the White House, the Nuclear Regulatory Commission, and the Environmental Protection Agency.
