I,m by no means any sort of expert on this subject, but for my two(sense) worth it seems to me that most are almost completley missing the issue, (as in being in a forest but can,t find a stick) as already correctly stated jet engine efficency has vastly improved over the last 50 years espessially in power output and emissions control, but does anyone consider why this is the case overall, in both nature and engineering you still don,t get something for nothing, the amount of oxygen required to burn that fuel and produce that power is exponentially a much more serious problem that any form of polution generated by it. As a complete layperson In the eighties, as now, it was once simlpy explained to me by a very savvy pilot, that if we could view oxygen usage by high altitude jet engines in the same way we can view the contrails they make, the (skytubes) for each would be hundreds of times bigger for each engine than the contrails themselves. But then again I,m just a igorant storch pilot pickin on me tablet. Cheers hargraves