Planes, trains, and automobiles: comparing the [CO.sub.2] impacts.

• Author: Kolwey, Neil

More and more people are asking what they can do to reduce their personal impacts on climate change. These include greenhouse gas (GHG) emissions (mainly carbon dioxide, or [CO.sub.2]) from our household fuel and electricity consumption and from our vehicle fuel consumption. They also include a host of other indirect emissions to which we contribute through our commercial travel choices, eating habits, and consumption of other products and services. This article discusses and compares emissions from personal transportation, including air travel, buses, trains, and the cars we drive. If we understand these emissions better, perhaps it will help us to make smarter and greener choices.

Comparing Transportation Emissions

Many online calculators are available to help individuals calculate their household or individual [CO.sub.2] emissions. These calculators tend to focus on four types of emissions: household fuel use, household electricity use, vehicle fuel use, and air transport emissions. When I calculated my own family's [CO.sub.2] emissions, I was surprised to find that our emissions from air travel (not including those for business purposes) were greater than the fuel- and electricity-related emissions from our house, and greater than the emissions from our two cars. This is probably true for families that take more than 10 plane trips per year (combined). But the calculators do not reveal how emissions from cars (per kilometer) compare to those of commercial jets, or how these two modes compare to trains and buses.

Good, clear comparisons of emissions from transportation are hard to find. Figures 1 and 2 are the result of my attempts to sift through published data from various sources to assemble a reasonable comparison. Figure 1 compares the emissions for longer trips, for which people often choose to fly, and Figure 2 compares the options for shorter trips, such as commuting to work. One key question in calculating emissions for buses, trains, and planes is whether to use the published data on "average occupancy" (numbers of passengers per trip) or to assume the buses, trains, and planes are full. In these figures, I provide a range of emission factors, with these two different assumptions forming the basis for the endpoints of the ranges shown. For example, if one assumes the trains are fully occupied, this emission factor (in kilograms of [CO.sub.2] per passenger-kilometer) is at the minimum point of the range. If, in practice, the trains are only approximately 50-percent occupied, then the emissions per passenger-kilometer are twice...