This is a hugh issue - to add with the other two hugh issues ( 1) noise 2) emissions ) facing new aircraft designs - particularly HIGH ALTITUDE aircrafts, MULTI-ENGINE aircrafts - and MOST IMPORTANTLY - HYDROGEN POWERED AIRCRAFTS. ..I think Boeing has done a study on this - the problem with HYDROGEN & LOTS of HIGH ALTITUDE AIRCRAFTS is that the former GENERATES WATER VAPOR the persists at altitude for a very long time - now, if you load up the skys with these things - they could effectively start to impact the solar radiation comming through .. Now, if you are business paranoid Boeing - this is something else to mitigate a 10-30 billion new airplane strategy - There is still cold war nuclear bomb fallout persisting at very high SR71 altitudes - so everything that goes up - takes awhile to come down.. I knew a British air force engine mechanic who had significantly gotten exposed to distilled radiation - due to cleaning engine exhaust nozzles on high altitude British bombers or reconnassiance aircrafts during the 70s80s. Another big risk factor for the A380 is noise - risky to but 4 very large engines on an airplane in view of future unknown noise & emissions standards..