I should also answer skorpio's earlier question about ATR's diagram: the diagram is no miracle, although it does present the worst-case scenario for the other modes of transport versus the best-case scenario for aviation. This is normal in such documents. Additionally, because gas-turbines are highly efficient, they do reduce nitrous oxide and carbon monoxide emissions to levels far below those of inefficient small piston engines. The critical problem with the diagram is that it doesn't measure the right variable. One could chart ice-creams consumed per passenger km, but it wouldn't be very on-topic.
The most important gas affecting global warming - by a country mile - is carbon dioxide. The amount of CO2 produced is directly proportional to the weight of fuel burned, and aircraft burn vastly more fuel than other modes of transport. There is no way around this: an aircraft must pull a huge wing through the air merely to keep itself aloft, with all the pressure, skin, and induced drag associated with that. Additionally, it does that at high speed, where efficiencies are inherently lower.
This high-speed element is of course why aviation is so well-suited to long distances and crossing water. My only point in all this is that aviation is inherently ill-suited to short distance over land, and is only able to compete with rail and road over short distances because of the market distortion caused by zero tax on aviation fuel.