We’ve gotten a few questions already about how we came up with our charts on the climate-warming impacts of travel choices. The charts compare the global warming impacts of cars, SUVs, vanpools, planes, buses, and trains. And since I couldn’t find a single, unified data source for all of that—at least, not one that I couldn’t poke some holes in—I compiled our figure from scratch, using a bunch of different sources.
So in case you’re the sort of person who gets excited about numbers and footnotes, there’s plenty—plenty, I say—of data wonkery to follow…
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First off: the data for these charts represent mid-point estimates for both direct and indirect climate-warming emissions from travel. They do include an estimate for carbon dioxide emissions from extracting, transporting, and refining crude oil, as well as emissions of trace climate-warming gases in vehicle exhaust. The air travel emissions also include the effects of contrails, cirrus cloud seeding, and other wacky airplane stuff. But the estimates don’t include emissions from vehicle construction and maintenance, or any of the life-cycle emissions related to building roads and parking lots, insurance company operations, etc. Including these other life-cycle emissions would push up the totals a bit, but probably not enormously.
Second, the geekery:
- Petroleum fuel cycle emissions represent a mid-point estimate from fuel cycle models, including the GREET model, which is maintained by the Argonne National Lab. See table 14-1 of this pdf for upstream emissions estimates. In addition, our emissions estimates include trace gases such as NOx from tailpipes, which add roughly 5 percent of CO2 equivalents to total tailpipe emissions, per p. 4 of this EPA report. By our final count, burning a gallon of gasoline releases 25.8 pounds of CO2 equivalents into the atmosphere, including about 19.5 pounds of CO2 from the tailpipe itself. For diesel, it’s 27.8 pounds of CO2 per gallon total, including 22.3 pounds of CO2 directly from the tailpipe.
- Real-world passenger vehicle mpg is taken from a variety of sources. My Prius figure (46.4 mpg), is the average of values from here and here; the second link has real-world data for other efficient vehicles, too. If you care, Hummer H2 fuel efficiency (10.2 mpg), along with real-world mpg ratings for other efficiency duds, can be found here.
- More generally, passenger vehicle mpg for cars and light trucks is from this spreadsheet from the US Bureau of Transportation Statistics, which pegs light trucks at 16.2 mpg and passenger cars at 22.9 mpg, as of 2005.
- Aircraft emissions are tough to calculate in the abstract. Not only do estimates vary, there are also a lot of variables in play—how many seats are filled, how far and how high a plane travels, its route, the time of year and time of day, flight delays and reroutings, etc. This awesome analysis of online air travel emissions calculators suggests that Atmosfair.com is the best of the lot, so I averaged Atmosfair’s values for a short, medium, and long plane flight, coach class from Northwest cities. Emissions work out to just under a pound of CO2 equivalents per mile of point-to-point travel. But note that direct CO2-only emissions from air travel are actually quite a bit lower than the figure suggests—and usually are somewhat better than the per-passenger emissions from car travel. Much of the net impact of air travel aren’t from CO2, but from other gases and contrails. Herearesomegood sources for CO2-only emissions from air travel; but note that they disagree. This spreadsheet from GHGProtocol.org also lets you calculate your direct emissions from travel, including air travel.
- For rail and bus transit, as well as vanpools, fuel consumption and ridership figures are derived from tables 17 and 19 of the National Transit Database, a program of the US Department of Transportation. I’ve used King County, WA figures for buses, and national averages for rail transit (combining light, heavy, and commuter rail). On its face, bus transit doesn’t do all that well, since many buses around the US run almost empty. Late-night buses carrying only a few passengers are pretty much efficiency duds. But a full bus—like many of the rush hour buses in Cascadia—is quite efficient. So when it comes to the climate, filling seats is just as important as choosing the right kind of vehicle. And just for the curious—transit buses average 10.7 riders per vehicle mile traveled in King County, and trains average 25.9 riders per train car. Trains genuinely do attract more riders than buses, but much of their advantage stems from the fact that trains tend to be built in dense, transit-friendly neighborhoods, while buses often service more sparsely populated suburbs.
- Train travel poses a puzzle, since trains—particularly transit trains—often run on electricity. And electricity is slippery—when you turn on the lights, there’s no telling where the juice really came from. Which means that, in effect, the climate impacts of train travel depend on where you think the electricity comes from. Using national averages for the generation mix gives lots of credit for hydropower and other low-carbon generation. On the margins, though, much of the nation’s electricity comes from coal, which is much worse for the climate than “average” power. On the other hand, “peak” electricity—the marginal power produced at times of day when demand spikes—typically comes from natural gas, which is generally less carbon-intensive than the US average. I’ve used US averages here; but other, arguably equally valid assumptions would yield different estimates. Also, there’s one potential source of error: for lack of easily accessible data, I assume that the upstream and trace emissions from electricity—the energy used to extract and transport coal and gas, for example, or trace emissions from coal combustion—accrue at the same rate as the upstream and trace emissions for diesel fuel. It’s a wee bit sketchy, really, but it solved the problem of accounting for Amtrak, which runs on both diesel and electricity.
- Amtrak figures come from the “CO2 emissions from transport or mobile sources” spreadsheet at GHGProtocol.org. Note that these agree pretty closely with the figures used by the Transportation Energy Data Book, with intercity train travel assumed to be predominantly diesel.
- I chose a nominal value for walking and biking, as well as additional passengers. However, the calories from walking and biking have to come from somewhere — and the food system, from farms to stores to refrigerators and stoves, consumes lots of energy. So some people have suggested that, mile for mile, walking and biking aren’t quite as benign for the climate as is commonly thought. I’m not sure I agree, but I’ll leave this analytical dilemma as an exercise for the reader.
I think that’s all for now. Also, many, many thanks are due to former research intern Justin Brant, who did most of the early research for this project!!