I'm too tired right now to do the thorough research this question deserves.
From what I've heard, the exhaust from jet fuel contains far more greenhouse gases and in greater relative proportions that the exhaust from motorises land vehicles.
When I'm feeling a bit more energised, I may try to find the peer reviewed science reports.
From what I understand of it, although an individual flight by a long-distance jet aircraft does produce a considerable quantity of carbon-dioxide, overall, the total from air travel is a fairly small fraction of the total for transport generally.
That doesn't suggest complacency though - one could fairly question why so many business people for example, fly where and when a train would be just as efficient and less polluting, and not travelling at all would be even more efficient and even less polluting.
(The train may travel at 100mph, the plane at 400mph... but depending of course on distance, that can be offset by such factors as distances from town to airport, and the enforced long waits in airports. New York to Los Angeles by air is one thing... but London to Manchester or even Edinburgh?)
