A significant portion of carbon dioxide emissions comes from the cars and trucks we drive. The transportation sector contributed about 27 percent of all US greenhouse gas emissions in 2011. Within this sector, light-duty vehicles—including passenger cars, pickup trucks, minivans, and SUVs—were responsible for 61 percent of emissions.

Despite recent growth in electric vehicle sales, the dominance of conventional vehicles is unlikely to change anytime soon. The US Energy Information Administration estimates that 87 percent of US cars on the road in 2040, and 81 percent of light trucks, will be fueled with gasoline or diesel.

There is a good alternative, but it hasn’t gotten much attention: ammonia. Running vehicles on ammonia could reduce emissions to levels far below those achieved by other alternative fuels, such as natural gas or ethanol derived from corn.

Why ammonia? An ammonia molecule is composed of one nitrogen atom and three hydrogen atoms. Ammonia can be burned in internal combustion engines with minor modifications — emitting only nitrogen and water vapor from the tailpipe, even when only low-cost emissions controls are used. Unburned ammonia and nitrogen oxides in the engine’s exhaust would be removed by a selective catalyst reduction system. Ammonia can be produced, at an affordable cost, by a catalytic reaction between nitrogen (obtained from air, which is 78 percent nitrogen) and hydrogen (obtained by splitting water molecules into hydrogen and oxygen).

Ammonia-fueled vehicles operate in much the same way as gasoline-fueled vehicles: Liquid ammonia is burned with oxygen, producing energy that is harnessed to drive the vehicle’s wheels. This familiar technology means that ammonia-fueled vehicles can generally be built and maintained in the same way as the current vehicle fleet. But unlike conventionally fueled vehicles, ammonia-powered cars would not emit carbon dioxide.

Most cars on the road can run on a mixture of 90 percent gasoline and 10 percent liquid ammonia, and could be modified to run on a mixture of up to 80 percent ammonia—at a cost of $1,000 to $5,000 per vehicle. An engine that could run entirely on ammonia is currently under development.

A 2005 study by the Risø National Laboratory in Denmark concluded that ammonia would be no more dangerous than current fuels. Ammonia has been used as an industrial and agricultural chemical for more than a century, and it dissipates rapidly when released because it is lighter than air.

The infrastructure for large-scale production and distribution of ammonia already exists worldwide. Gas stations would require only modest changes to dispense ammonia. It can be stored easily in pressurized tanks at relatively low pressure.

Emissions in 2040. The United States Department of Energy’s Annual Energy Outlook 2013 projects that light-duty vehicle stocks will increase by 26 percent in 2040 relative to 2010, and the number of miles traveled annually by each vehicle will be 11 percent greater in 2040 than in 2010. However, light-duty vehicles are projected to consume less fuel in 2040 than in 2010 because the average vehicle efficiency (expressed as miles per gallon) will increase by 72 percent between 2010 and 2040. The net result is that overall emissions from light-duty vehicles are expected to decrease in the coming decades, and by 2040 to be more 24 percent lower than 2010 emissions. With a phase-in of ammonia-fueled vehicles, however, far greater emissions reductions could be achieved during the same time frame.

Using a software tool called LEAP (the Long-range Energy Alternatives Planning system), developed by the Stockholm Environment Institute’s US center, I estimated the potential emissions reductions in the US transportation sector, through 2040, for two mitigation scenarios: In the first scenario, ammonia-fueled vehicles account for 10 percent of the light-duty fleet in 2020, 30 percent in 2030, and 50 percent in 2040. In the second scenario, ammonia-fueled vehicles account for 10 percent of the fleet in 2020, 50 percent in 2030, and 100 percent in 2040.

My simulations showed that the first scenario could reduce cumulative light-duty-vehicle carbon dioxide emissions from 2010 through 2040 by nearly 20 percent more than the Energy Department’s projections, while the second scenario could reduce emissions by 31 percent more than projected.

Costs and benefits. Besides emitting zero greenhouse gases from vehicles, ammonia fuel could reduce the well-to-tank carbon emissions associated with the production and delivery of conventional fuels. Assuming around 21 grams of carbon dioxide are emitted per megajoule of gasoline-produced energy, the estimated well-to-tank emissions for a gasoline-fueled vehicle in America are approximately 1.5 metric tons of carbon dioxide per year, based on an average fuel economy of about 21 miles per gallon and an average annual distance traveled of 12,000 miles.

Current industrial ammonia production plants, which run on fossil fuels, emit between 1.2 and 1.8 metric tons of carbon dioxide per ton of ammonia produced. Assuming that ammonia-fueled vehicles have a fuel-energy-input-per-mile equivalent to that of gasoline vehicles, a vehicle fueled with conventionally produced ammonia will cause ammonia-producing factories to emit somewhere between 4.2 and 6.1 metric tons of carbon dioxide per year, which is 7 to 36 percent less than the carbon dioxide emitted by a similar gasoline vehicle. However, if advanced ammonia production methods (for example, solid-state ammonia synthesis) can be commercialized, and non-fossil sources of electricity can be used to power ammonia production plants, no carbon dioxide will be emitted during the ammonia production process.

The average retail price of gasoline on February 13 was $3.33 per gallon, according to the US Energy Information Administration, so people who use conventional cars pay an average of about $1,900 per year for gasoline. The average bulk ammonia price in 2012 was estimated at about $634 per metric ton. Assuming the same retail-to-wholesale price ratio for ammonia as for gasoline, ammonia would retail for $2.07 per gallon. It takes about twice as much ammonia as gasoline to drive any given distance, which means it would cost around $2,700 per year to fuel an ammonia-powered vehicle—42 percent more expensive than using a conventional gasoline powered car. However, advanced ammonia production methods could decrease ammonia production costs, and thus prices.

Challenges. If a solid-state ammonia synthesis method can be commercialized, it would take an estimated 7,000 kilowatt-hours of electricity to produce one metric ton of ammonia. Implementing the more ambitious of the two mitigation scenarios described above would require increasing electricity generation, relative to the Annual Energy Outlook 2013 projections, by 11 percent in 2020, 42 percent in 2030, and 74 percent in 2040. Installing this additional generation capacity would be a major challenge in aggressive deployment of ammonia-fueled vehicles. But if ammonia could be produced with nuclear and renewable energy, the emissions associated with ammonia-fueled vehicles would be near zero.