The Friends of the Earth claim that air travel is "the world's fastest growing source of greenhouse gases" in a 6 November 2007 CNN article. The airline industry estimates air travel releases about 3% of overall greenhouse gases. The discrepancy between estimated impacts is due to the controversial idea of "radiative forcing", which is a belief that the high-altitude airline emissions cause disproportionate damage.
Environmental groups who argue this point were elated when the United Kingdom's aviation minister this year agreed that air travel was responsible for about 13% of greenhouse gas damage.
I'm not really certain what qualifies an aviation minister to assess environmental impact, but take it for what it's worth.