Now I am not an expert, but do have an opinion.
While I agree with much of what has been said above, I believe that the main reason that commercial aviation has been under pressure is that the environmental impact per unit weight of fuel used is greater.
This is because much of the aircraft exhaust gases are placed directly into stratosphere. This means that it stays there for a long time (measured in 10s of years I believe; it could be as long as 100 years) as there is little mixing of air layers. The CO2 and Nox are also placed closer to the Ozone layer. The high altitude cirrus and contrails formed by water vapour in the exhaust are another concern for the 'greenhouse effect'.
I'm interested to hear further views on this and learn some more