When studies look at the amount of greenhouse gases emitted by transportation, the focus is nearly always on the emissions created in fuel combustion – gasoline and diesel for cars and trucks, bunker fuels for maritime vessels, jet fuel for aircraft, and so on. One excellent example of this kind of study is the Getting There Greener study by the Union of Concerned Scientists (UCS). The UCS study shows that travel by bus emits the least carbon at all distances traveled and for one, two, or four travelers. Similarly, the study found that flying first class was almost always the worst option, with driving a typical SUV any appreciable distance coming in a close second.

But what most studies lack is a detailed analysis of the overall cradle-to-grave lifecycle of the transportation modes being compared. A new study by two University of California-Berkeley researchers has attempted to analyze the bulk of the lifecycle of multiple types of passenger vehicles, including fuel production, manufacturing and maintenance of the vehicles themselves, infrastructure construction and repair costs, all in addition to the basic fuel consumption. And the study also looks at three commonly regulated pollutants in addition to energy consumption and greenhouse gas (mostly carbon dioxide) emissions.

And the results are quite a bit different from purely fuel consumption-based analyses.

Transportation in the United States accounts for about 33% of all carbon dioxide (CO 2 ) emissions, about 75% of carbon monoxide, about 5% of sulfur dioxide (SO 2 ), and around 57% of nitrogen oxides (NO x ). In addition, transportation accounts for about 28% of all energy consumed in the United States. Therefore, understanding how best to improve the fuel efficiency and lower the emissions created by the transportation sectors is a natural approach to improving overall air quality and slowing climate disruption.

But fuel consumption is only one part of the equation, and not necessarily the most important part. Not all pollutants are emitted in large amounts by vehicles. SO 2 , the compound responsible for acid rain, is largely produced by coal fired power plants. But what happens if, in the future, most vehicles are plug-in hybrid and electric vehicles? Fuel consumption will go down, but SO 2 emissions will likely rise dramatically if coal plants remain the dominant source of electricity, and those increased emissions could reasonably be applied to the transportation sector.

In an attempt to estimate the real greenhouse gas (GHG) and pollutant emissions, the Berkeley study chose three “typical” automobiles, four “typical” rail lines, urban buses, and three “typical” aircraft. For each of those typical vehicles, the researchers collected data on the mining and processing of raw materials, fabrication of components, the shipping of components to the assembly factory, electricity used in the assembly process, how much energy was consumed in maintaining the vehicles, how much fuel was burned in shipping replacement parts from factories to the maintenance facilities, the pollution created in making infrastructure like roads, train tracks, or airports, the lifespan of the infrastructure, transportation to and from central depots like airports, the GHGs emitted in building parking lots, how often roads needed to be repaired, the amount of time spent idling vs traveling, and so on. And, of course, the direct consumption of fuel in order to move the vehicle from one place to another. Only the decomissioning of the vehicle itself was left out of the analysis due to complexities created by the many varied ways that vehicles can be scrapped – turned to scrap metal, picked over for individual parts, left to rust in the Mojave desert, etc.

In all cases, though, the emissions of GHGs and pollutants due to the vehicle’s lifecycle significantly boosted the overall emissions, in one case by a factor of 8 (800%). And as a result, the usual “airplanes terrible, cars bad, trains OK, buses best” analysis result was turned around some.

First off, when it comes to overall energy consumption, the lowest energy consuming transportation method is urban diesel bus with peak ridership while the highest consumption is the very same bus with off-peak ridership. And as you might expect, the sedan, SUV, and pickup are all worse than all the aircraft and mass transit option. But there’s already an interesting data point – on a pure energy consumption basis, all three commercial aircraft (small, midsize, and large) consume more energy than all the train options. But when the energy consumed during the rest of the lifecycle is included, large aircraft are actually more efficient on a passenger-kilometer traveled (PKT) basis than one of the two light rail systems profiled.

Looking at greenhouse gases reveals another change due to lifecycle GHG emissions. The highly electrified light rail systems in the Northeast not only emit more greenhouse gases than diesel-powered commuter rail and west coast light rail, it also emits more GHGs than midsize and large aircraft even though the Northeast light rail consumes the least overall energy. According to the paper, this is because “The San Francisco Bay Area’s electricity is 49% fossil fuel-based and Massachussetts’ is 82%.” This results in a reversal of one of the UCS study’s conclusions about rail travel – “Ride the rails in the Northeast to cut carbon and congestion.” (Chapter 4, page 20).

The problems with light rail powered by fossil fuel power plants become even more clear when you look beyond just GHGs into standard pollutants. SO 2 emissions were the worst for light rail due to coal’s high sulfur emissions. Urban diesel buses at peak ridership were still the best performers, but all three aircraft sizes were better than all of the rail options, and the lowest SO 2 rail option was actually the diesel commuter rail. And the three auto options were in the middle – worse than aircraft on a PKT basis, but better than all but one of the rail options.

57% of nitrogen oxides come from transportation, and the worst offender was again the off-peak urban bus. Commuter rail and pickups were tied for next worst, with electric rail having the lowest NO x emissions. Aircraft were in the middle of the pack.

Finally, carbon monoxide pollution is mostly a result of autos, and nothing in the overall lifecycle changed that – the typical sedan (the best performer of the three autos) was still between five and six times worse than its nearest mass transit competitor, the off-peak urban bus. And while electric rail lines had an extremely low fuel emission profile for carbon monoxide, the construction of the rail lines themselves resulted in enough carbon monoxide emissions to make them worse overall than an urban bus at peak ridership – even though the bus was powered from a comparably highly polluting diesel engine.

As illuminating as the results of the lifecycle emissions estimates are, however, they aren’t the complete story.

The paper’s graphs illustrate that even if you were able to reduce the energy consumption, carbon emissions, and pollution to zero, the results are still counter-intuitive. As the table below shows, urban peak buses remain the overall best transportation, but large and medium size aircraft continue to perform extremely well. In fact, in energy consumption, GHG emissions, SO 2 , and NO x , air transportation performs as well as or better than all of the rail options. This is again because of the relative lack of infrastructure required for air vs. ground transportation.

Of course, we can’t reduce the effects of active vehicle operations (mostly fuel consumption) to zero, and aircraft are likely to have a greater problem with this than most other forms of transportation. But let’s assume that new research into low carbon aircraft fuels produce the 60-80% cuts in GHG emissions mentioned in this article, and let’s further assume that we can reduce GHG emissions for all the other forms of transportation by 90%. Again, GHG emissions of air travel continue to look good compared to rail travel due to the infrastructure costs of the train.

These thought experiments could go on forever, and I’m certain that there will be others who look at the paper’s results in far greater detail than I can by “eyeballing” the graphs. The overall point is this – the complete lifecycle of a form of transportation must be considered before we can reasonably judge whether one form of transportation is “better” than another. Replacing the pollution from personal cars and trucks with pollution from a coal plant may not, in fact, be a good tradeoff even if overall GHG emissions and energy consumption fall as a result. Similarly, ridership should also be considered before determining what the “best” solution for a given region will be. And finally, the mix of energy and infrastructure required for the “optimal” transportation scheme will be dramatically different from region to region, from urban to suburban to rural, and from short to long distance transportation.

As with renewable energy, one size will most definitely not fit all, and research and development in all of the main modes of transportation is well warranted.

Image Credits:

Vincent Laforet, NYTimes

Environmental Research Letters

S&R