What mode of transportation is most environmentally friendly? A paper released online by the journal Environmental Science and Technology attempts to answer this question. Using a powerful, predictive chemical climate model that takes into account the effect of a wide range of pollutants, the authors examine the overall effect on temperature and radiative forcing from a given transport work unit and extend the analysis many years into the future. Turns out, air travel will result in a lower temperature rise than the equivalent car travel, but only after a long period of time.

The paper uses as inputs one year's worth of emissions from various transportation methods, both freight and personal. The authors computed the rise (or fall) in temperature over 5-, 20-, and 50-year horizons, as well as the radiative forcing impact over 20-, 100-, and 500-year periods.

To account for obvious differences in capacity and capability, effects were measured against comparable units. Freight transport's effect is measured per ton-km or per vol-km; likewise, passenger transport is measured per passenger-km, or per passenger-hour.

The freight categories were aviation, light-duty trucking, heavy-duty trucking, rail, and ship. For passenger transport, the authors looked at aviation, car, motorcycle/moped/scooter, bus, and rail.

Ship and rail were the respective winners in freight and passenger transport; both resulted in lower average global temperatures over the 5-year horizon windows. These drops in mean temperature are the result of a stronger cooling effect from sulfate aerosols and methane destruction (via ozone creation), which more than offset the rise in temperature due to the added CO 2 .

The biggest, non-obvious result is the finding that, over the 20- and 50-year horizons, air travel had a lower impact on the global temperature per passenger-kilometer than driving. Air travel has a significant immediate impact on clouds and ozone, and this is reflected in its short-term effects; however, the CO 2 released by driving spends a long time in the atmosphere, leading to long-term impacts on the Earth's climate.

For those who dread car travel (like myself), there's now another reason for picking flying over a road trip: it's better for the environment.

Environmental Science & Technology, 2010. DOI: 10.1021/es9039693