Greenhouse emissions
Just another add-on to the Dupre/Notso debate;
If all us humung beans did was belch out CO2 it wouldn't be so bad, unfortunately we seem to have been clever enough to develop molecules which are thousands of times more potent than CO2 in terms of their greenhouse capability (and jolly stable too, so they hang around for ages'n'ages).
Furthermore, just to make everyone feel even worse, the exhaust from planes is injected into the upper atmos where things work better as greenhouse gases and are no so easily mixed up with the stuff that plants breathe down here at ground level. Ho hum, still I'm more part of the problem than part of the solution, so I don't wish to appear to be holier than anyone else....
I seem to remember reading something shortly after Sep 11th about the impact (or lack of) on the climate attributed to the empty skies over USA, anyone else remember that.... something about the insuylating properties of contrails if my booze addled memory serves...