What Is Your Sin?

Over at Green Car Reports, the “guide to cleaner, greener driving,” electric car advocate David Noland asks, “Which sins worse: cars or planes?” The “sin,” of course, is carbon emissions, and his answer, while interesting, is flawed in many respects.

“The passenger jet blows away the automobile in terms of efficiency and CO2 emissions per mile,” he says, a result he apparently considers surprising. But it’s not surprising at all to anyone familiar with the Department of Energy’s Transportation Energy Data Book. According to tables in the book, airlines emitted about 2,568 grams of carbon per passenger mile in 2013, while the average car emitted 3,144 grams (or 3,564 if SUVs and other light trucks are included).

But it’s not enough to show that both cars and airlines have been rapidly improving their energy efficiency. Noland wants to really blow cars out of contention, so he biases his analysis in several ways.

First, his original question was, “How much carbon will be added to the atmosphere if I take this flight, compared to staying home?” But, as many comments on his article noted, that isn’t the question he answered. Instead, the question he answered was, “How much carbon will be added to the atmosphere if I take this flight, compared to driving the same distance in a car?” Simply staying at home is likely to use far less energy than either travel by plane or car.

His problems go deeper than that. He compares the average carbon cost of driving with the marginal cost of flying. That is, instead of asking, “How much carbon do the planes we fly emit?” he prefers to focus on “How much more carbon will be emitted by the plane if I am a passenger vs. an empty seat?” It other words, as one comment notes, he makes the “outrageous assumption that planes fly whether or not we do.” In fact, if people decided to stop flying due to the environmental impact, the planes wouldn’t go.

Beyond this, his numbers are suspect. He claims that “the average load factor per [automobile] trip had fallen to about 1.4.” That’s simply not true, and it is certainly not true for intercity travel that would compete with flying. The National Household Travel Survey found that average auto occupancies in 2009 were 1.67 people per car. That improves auto energy efficiencies and carbon emissions per passenger mile by about 20 percent.
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Several years ago, the Antiplanner interviewed one of the authors of this survey who said that its occupancy numbers applied primarily to urban travel. For intercity travel, several surveys have found that average occupancies are 2.2 or greater. That improves the results another 32 percent, and reduces the energy used by cars and light trucks in intercity travel to 2,700 BTUs per passenger mile, only a little more than planes.

Poland’s article might also encourage policy makers to favor airlines over driving in funding or regulation due to the former’s supposed greenhouse gas advantage. But this assumes that mode selection is the best way to save energy and reduce carbon emissions. In fact, as Noland is well aware, there are many other ways including alternative fuels and increased fuel economy.

While Noland may favor electric cars, they have fewer greenhouse gas emissions only for people living in regions that get most of their electricity from renewable sources–in other words, California, Oregon, and Washington. Almost everywhere else in the United States gets most of its electricity burning fossil fuels. On the other hand, biofuels, which are (or soon will be) available both for highway vehicles and aircraft, can have truly zero net carbon emissions.

We can also make all kinds of vehicles more fuel efficient. Since 1970, the average fuel economies of cars have been cut almost in half, while the fuel efficiency per passenger mile for planes has been cut nearly 75 percent. Some of the latter reduction is due to increased occupancy rates, but much of it is due to increased fuel economy.

Volvo, Audi, and other European manufacturers are selling plug-in Diesel hybrid cars that can get the equivalent of more than 150 mpg. People willing to use biodiesel fuel and who live in areas that get most of their electricity from non-carbon-emitting sources would produce almost no greenhouse gas emissions.

Unfortunately, manufacturers aren’t selling Diesel hybrid plug-ins in the U.S. due partly to regulatory issues and partly to higher taxes on Diesel fuel (plus Volkswagen’s Diesel scandal hasn’t helped). Americans who want to reduce their personal carbon emissions should ask the manufacturer of their choice to start selling them in the United States.