Generally I feel pretty good about my carbon footprint. I live in a tiny apartment that doesn't use much energy to cool in the summer or heat in the winter. I gave up my car and walk or take public transportation to get most places. I try to be conscious of how my choices affect the world around me.
Except when it comes to flying. In the last two years, I've been to Chicago several times, Seattle, London and Sydney.
But a new study, published last week in Environmental Science & Technology, is making me feel a little less guilty, at least about the trips I've taken to U.S. locales (and theoretically had a choice between car and plane). Scientists in Austria and Norway calculated and compared the climate impact of several modes of travel---plane, car, two-wheeled vehicles, bus and rail. (They also looked at freight transport, but that's a subject for another day.) They went beyond simple carbon emissions, though, and included other factors that can influence the Earth's climate, such as aerosol emissions, ozone production, methane oxidation and cloud formation. And because the impact of these and other factors on climate changes over time (some last longer than others, for instance), the scientists also looked at how the climate impact of each mode of travel and transport varied over time. One trip can have a very different effect on climate five years into the future versus 50 years.
The calculations were done in terms of global temperature change per passenger-kilometer, that is, how much of a temperature increase (or decrease) would result from one person traveling one kilometer via a specific mode of transport. In the short- and long-term, rail is clearly the winner, with bus travel catching up 50 years after the trip. Motorcycling falls into third place. But the shocker is that though air travel is the worst on a short time scale, it falls to fourth, ahead of the car, by the 50 year mark. So that trip to Seattle I took earlier by plane earlier this year will have less of an impact on the planet's climate than if I had taken a road trip. At least according to these calculations.
We've all heard that air travel is the worst offender in terms of climate change. So how can this be?
“As planes fly at high altitudes, their impact on ozone and clouds is disproportionately high, though short lived. Although the exact magnitude is uncertain, the net effect is a strong, short-term, temperature increase,” lead author of the study, Dr. Jens Borken-Kleefeld, said. “Car travel emits more carbon dioxide than air travel per passenger mile. As carbon dioxide remains in the atmosphere longer than the other gases, cars have a more harmful impact on climate change in the long term.”
While this study may lessen a bit of my guilt for traveling by plane, it truly reinforces my preference for the train. Not only does it have the least climate impact of all five modes of travel in the study, it's also the one I enjoy the most.