Many of the threads in airliners.net have looked at this and no one has really given an answer. Always wondered why!
The answer may be in this article which appeared in the British press recently.
The full story is in this forum in the editorial articles section "Is it OK to fly?"
"... An aviation fuel expert at Shell dispelled any real hope that a paradigm shift is within reach whereby planes will no longer be powered by highly polluting kerosene but instead by a much cleaner alternative. Liquid hydrogen is pure fantasy, he said, for at least another 50 years. And forget any hope that biofuels, as is the hope with cars, could come to the rescue. Ethanol, he said, is a poor performer as an aviation fuel.
Currently, the only real alternative is "synthetic" kerosene made not from oil but natural gas, biogas or coal. It has the major advantage of working within current aircraft and therefore does not need new costly infrastructure. In fact, there are already planes flying in South Africa fuelled on this technology - but made from coal and therefore not offering any significant emissions advantage over kerosene.
The bottom line, he said, is that kerosene will be the preferred fuel for the next 30 years. And hearing Boeing assert that new planes being bought today will have a service life of up to 60 years, it appears that we are now "locked-in" to this technology - and its resultant emissions - for the long-term. (Not exactly good news for Tony Blair who recently said he favoured waiting for a technological solution to reduce aviation's impact.)
Couple that depressing news with the fast-developing taste for flying around the world, particularly in India and China, and it becomes clear that aviation-related emissions are guaranteed to soar in the next few decades. Tweaks here and there in efficiency savings are going to be rendered virtually insignificant by increased demand. In Europe, demand is growing 5% a year on average, whereas in China it stands this year at 14% and India at 15%, albeit from a smaller base. But India, fast attempting to play catch-up with the West, is currently spending $12bn on airport building. Its airlines have 330 new aircraft on order, largely driven by its booming new low-cost sector - its current civil fleet numbers just 200. And by 2020 India's minister for aviation is predicting that up to 2,000 planes could be operating. China, meanwhile, says it plans to buy 100 new planes every year over the next five years to increase its current fleet of 863 aircraft. Since 2000, passenger numbers have doubled in China. To put all this growth in perspective, there are about 12,000 civil aircraft presently flying in the world ...."