interesting stuff i've just read about contrails contributing more to global warming than the actual burning of fuel. sadly, i've lent the book to my neighbour, so haven't got the references to hand, but basically, it's about contrails seeding cirrus clouds, so they thicken, causing more cloud cover, which "hot" wavelengths from the sun can penetrate, but which prevent heat energy from the earth radiating out again. i can't remember the exact figures, but apparently, we could cut something like 85% of the effect by flying 500 feet lower, or something.
gobsmacking, i thought.