What is really known about aircraft exhaust and the effect on the atmosphere?  Although jets create far less greenhouse gas than power plants or automobiles, they have an outsized impact because of where they deposit it - the delicate upper troposphere and lower stratosphere, 7 to seven 11km up from Earth's surface.

The result: rising and alarming scientific concern that jets may be turning the skies into a polluted, heat-trapping place.

Airliners are unique because even though their total emissions are relatively small, compared to other sources, they're putting their emissions directly into the upper troposphere.  The long term issues of which are still relitively unknown.

When injected and mixed together into the icy atmosphere, the mix of exhaust gases - including water vapour, unburned hydrocarbons, particulates, sulfates, nitrogen oxides (NOX), and carbon dioxide - produces clouds and has two to three times the warming effect of carbon dioxide alone, Massachusetts Institute of Technology researchers reported last year.

The question for the future are:

1. What are the effects of the emissions with respect to changes in the concentrations of ozone?

2. What are the effects of the emissions with respect to greenhouse gases? 

3. What are the effects of emissions of future air traffic?

It will be interesting to see how evironmental policy makers and even the airline industry address this issue in the foreseeable future.