A number of studies have been done over the years which compare the emissions of greenhouse gases and/or noxious gases for different types of transport.
The answers can vary a good deal depending on which examples of trips or vehicles are picked and according to assumptions made on matters such as vehicle fill. Nonetheless, there is general agreement on ranking orders.
Most studies show that passenger air travel produces more far more emissions than train, coach or ship. Cars are generally shown to be higher than train, coach and ship, but usually still less than flying.
One example of such a study by CE Delft ‘To shift or not to shift, that’s the question: The environmental performance of the principal modes of freight and passenger transport in the policy-making context’ (2003).
The top chart on page 38 shows the emissions of carbon dioxide per passenger kilometre. As can be seen, CE Delft concludes that CO2 emissions by aircraft are anything from 3 to 10 times higher than rail for medium distance journeys of 500km. For longer journeys of 200km, air is somewhat better in terms of emission per passenger km, but still 1 ½ to 6 times worse than rail.
Aircraft’s emissions of other gases – nitrogen oxides (NOx) and small particulates (PM10) – are not always as much as other modes. However, it should be noted that NOx and PM10 emissions are not usually a problem except in urban areas.