Just throwing out what I think I read, about 80% of greenhouse gas is from coal and oil electrical generation, about 20% from transportation. Close to what XP said, I heard 4% is from aviation, leaving 16% for other transportations. That's not only oil though, add coal to the electrical generation.
But robert, think of this. Doesn't liquid hydrogen have more energy per pound than avgas? Makes it ideal for airplanes that must generate the lift to carry the weight of their fuel, unlike trucks, cars