It's hard to remember a time when our skies weren't filled with aeroplanes. Wherever you are in the world, the noise of jet engines and the sight of vapour trails are part of our daily lives - lives that have changed dramatically as a result. Thanks to revolutions in cheap air travel, the number of people taking international flights is exploding - over 1.8 billion passengers travelled by air in 2004, a figure set to grow by 6% a year between now and the end of the decade.

This new freedom comes at a price. One side-effect of burgeoning air travel is a corresponding rise in the amount of carbon dioxide emitted by aircraft -the so-called 'greenhouse gas' that many scientists believe is at least in part responsible for global warming. CO2 emissions from aircraft have doubled across Europe since 1990 thanks to the boom in low-cost air travel, a trend that looks likely to continue unchecked well into the future.

There's a cost for aircraft manufacturers too, whose profit margins have been squeezed ever further as competition across the airline industry increases. And in a world still nervous about flying after the 9/11 attacks, security and safety of aircraft have never been higher on passengers' agendas. One way or another, something dramatic has to change if air travel is to remain economically and environmentally feasible in the 21st century.