To return to topic, one of the most serious greenhouse gas emitters is not cars, or aeroplanes, but agriculture. Vast tracts of land are set aside for grazing instead of being forest, and the animals on that land being fed high protien diets so they get fat then emit great quantities of methane. One estimate put the total contribution to the greenhouse gasses from agriculture at 80%. This includes the animals emissions themselves, and all the farm machinery needed to tend and process such herds.
http://www.indsp.org/IAGW.php
We are only just coming to understand this component but I expect it to be talked about a lot more over the coming months and years. If it's true (and I see no reason to think it isn't), then one of the major things we'll need to do to combat global warming is change our diet. Vast herds of animals so we can eat meat with each and every single meal is not sustainable. Oh, and just in case anyone is confused here, no, I am not a vegetarian.