From this article by George Monbiot:
"the total warming effect of aircraft emissions is 2.7 times as great as the effect of the carbon dioxide alone. The water vapour they produce forms ice crystals in the upper troposphere (vapour trails and cirrus clouds) which trap the earth’s heat. According to calculations by the Tyndall Centre for Climate Change Research, if you added the two effects together (it urges some caution as they are not directly comparable), aviation’s emissions alone would exceed the government’s target for the country’s entire output of greenhouse gases in 2050 by around 134%."
http://www.monbiot.com/archives/2006/12/19/preparing-for-take-off/
http://www.guardian.co.uk/environment/2006/oct/06/travelsenvironmentalimpact.travelnews
http://www.sightline.org/research/energy/res_pubs/rel_air_travel_aug04 (It has a great list of sources at the end of the article.)
http://www.direct.gov.uk/en/Environmentandgreenerliving/Greenertravel/DG_064429
Honestly, it's even mentioned in the sports pages!