Temperature change per transport work by mode for various years after the emissions: per passenger kilometer for passenger travel (left) and per ton-kilometer for freight transport (right). Global average values for the year 2000. Bars represent 1 SD. Credit: ACS, Borken-Kleefeld et al. Click to enlarge.

A new study comparing both the absolute and normalized climate impacts of current passenger and freight transportation finds that driving a car increases global temperatures in the long run more than making the same long-distance journey by air. However, in the short run travelling by air has a larger adverse climate impact because airplanes strongly affect short-lived warming processes at high altitudes.

The researchers used a suite of climate chemistry models to consider the climate effects of all long- and short-lived gases, aerosols and cloud effects, not just carbon dioxide, resulting from transport worldwide.

Jens Borken-Kleefeld and colleagues performed calculations for the integrated radiative forcing and mean temperature change, for different time horizons and various measures of transport work. The study was published the in ACS journal Environmental Science & Technology.

An unambiguous ranking of the specific climate impact can be established for freight transportation, with shipping and rail having lowest and light trucks and air transport having highest specific impact for all cases calculated. Passenger travel with rail, coach or two- and three-wheelers has on average the lowest specific climate impact also on short time horizons. Air travel has the highest specific impact on short-term warming, while on long-term warming car travel has an equal or higher impact per passenger-kilometer.

—Borken-Kleefeld et al.

They concluded that in the long run the global temperature increase from a car trip will be on average higher than from a plane journey of the same distance. However, in the first years after the journey, air travel increases global temperatures four times more than car travel. Passenger trains and buses cause four to five times less impact than automobile travel for every mile a passenger travels.

Findings for freight transport (year 2000) include:

  • The specific climate impact of air transport is 3 to 42 times higher, for a light truck it is 2 to 8 times higher than average truck transport.
  • Rail transport of heavy goods has a 4 to 10 times lower specific climate impact than trucking, while it varies from negligible to half to a similar impact for volume products.
  • Ship transport has by far the lowest climate impact: It exerts 5 to 10 to 30 times less warming per transport work than trucking and is even cooling on shorter time scales.

Findings for passenger transport (year 2000) include:

  • Rail travel has at least a factor 4 lower specific impact and is cooling on shorter times, bus and coach travel has 2 to 5 times lower specific impact, while travel with two- or three-wheelers has up to a factor 2 lower specific climate impact than car travel.
  • Air travel results in a lower temperature change per passenger-kilometer than car travel on the long run; the integrated radiative forcing of air travel is on short- to medium time horizons much higher than for car travel
  • Per passenger-hour traveled however, aviations climate impact is a factor 6 to 47 higher than the impact from car travel.

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. 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.

—Dr. Jens Borken-Kleefeld


  • Jens Borken-Kleefeld, Terje Berntsen and Jan Fuglestvedt (2010) Specific Climate Impact of Passenger and Freight Transport. Environ. Sci. Technol., 44 (15), pp 5700–5706 doi: 10.1021/es9039693