Over the past few days, I’ve been trying to pull together some data on how airplane travel affects global warming, as part of a broader project on transportation and climate change.
My stunningly obvious conclusion: it’s complicated. Worse, different calculation methods yield wildly different results.
Take, for instance, this brilliant chart (below) from the Stockholm Environment Institute, comparing many of the major online emissions calculators. Emissions are represented by the light blue lines. As you can see, the online calculators find that a Boston-D.C. round trip has the impact of somewhere between 0.19 tons and 0.48 tons of CO2 emissions, depending on which calculator you use. (By the way — as I discuss below, it looks like the Atmosfair calculator is probably the most accurate and comprehensive.)
The high-end estimate is over two and a half times higher than the low-end estimate. What’s up with that?
To be brief, the complications in calculating air travel emissions include:
- Distance of flight. Generally speaking, short flights use more fuel per mile traveled. It takes a lot of fuel to lift a plane, and shorter flights have more lift per mile traveled. Taxiing also burns through fuel, and the amount of taxiing doesn’t vary that much by the length of the flight. So you can think of short flights as “city” driving for a car — with lots of idling and acceleration — and long flights as “highway” driving. That said, fuel efficiency drops for very long flights, since the plane has to carry extra fuel for a long flight — and it takes fuel to carry fuel.
- Altitude. Although long flights tend to use less fuel per mile, they also tend to fly higher, and stay at high altitudes for longer. And high-altitude flying has some troubling side effects. On net, airplane contrails, and the cirrus clouds they form at high altitudes, are believed to trap heat in the atmosphere, especially at night.
- Timing and location. Flying at night, and at northern latitudes, may increase the formation of heat-trapping clouds. During the day, though, clouds may reflect sunlight back into outer space.
The problem is, there’s tons of uncertainty about all of these effects, particularly for high-altitude emissions. (See this IPCC report, especially this page, for more on this.) But on average, the IPCC recommends multiplying the CO2 emissions from fossil fuels by 2.7 to get the total global-warming impacts from a given flight. That is, the comprehensive global warming impacts (counting contrails, clouds, and trace gases) of flying are about 2.7 times as high as the CO2 alone. Note, however, that according to this pdf, a more recent study pegs that figure at 1.9, rather than 2.7.
Confused? I certainly am.
As should be obvious, these complications can make it really difficult to estimate the climate impacts of any given flight. And to add to the confusion, if you really want to be accurate you’ll need to account for the type of plane you’re flying, how old it is, whether you’re flying first-class or economy (first class seats take more room on the plane), and how full the plane is. Sheesh! It makes calculating the climate impacts of your car a piece of cake.
There are two ways around these complications. First, you can make some simplifying assumptions — some rules of thumb that work pretty well, all things considered. And second, you can rely on the best, most reliable online calculator you can find.
For the second tack, the Stockholm Environment Institute recommends Atmosfair, since it has the broadest and most comprehensive assessment of the climate impacts of flying — covering not just CO2 emissions, but also other gases, contrails, and the like. As a consequence, Atmosfair tends to give a higher estimate of climate impacts than the other flight calculators — depressing, but probably pretty accurate.
For the first tack — well, any rule of thumb is going to be overly simplistic. But for my purposes, it may be best to stick with the CO2 releases from airplane fuel only, divided by short, medium, and long-distance flights. The World Resources Institute (PDF) issued some widely used estimates, based on a 2005 report by a British government agency (PDF), that have been used at this reputable GHG protocol site. However, The British agency updated its figures in a 2007 report, here (PDF), allegedly based on this EU report (PDF) — though I can’t figure out how. So I’m going to use the most recent British figures (see table 6 in the pdf) as the best available estimate of CO2 releases per passenger-mile. Converting from metric, they are:
- 0.56 pounds per mile for short flights,
- 0.46 pounds per mile for medium flights, and
- 0.37 pounds per mile for long flights.
Of course, that doesn’t match up at all with Atmosfair’s results. But it’ll have to do for now. I’m done with complexity.