Air Trails.

Richard Branson (Virgin Airlines et al) has pledged $3 billion to be invested in technologies intended to alleviate the impact of climate change. Rumour has it that it is his own companies that will be the main beneficiaries of this, but in any event, it looks like a step in the right direction. He has named biofuels as a particular area of focus. I have done the sums in previous postings and from those numbers it may be deduced that to run the U.K.'s current fleet of aircraft would require (at a biodiesel yield of 2 tonnes/hectare) 6 million hectares = 60,000 km*2 square kilometers of arable land, which is about the total area of arable land we have in the U.K. So, if we grew no food at all we could just about meet demand. Biodiesel has various disadvantages for aviation, namely that at the tropospheric temperatures encountered by commercial aircraft particularly at higher altitude, as is used for transatlantic flights (e.g. 'Virgin Atlantic'), the fuel becomes very viscous and unfit for purpose. If we were to run them on bioethanol (which stays fluid down to much lower temperatures), we would still need to turn over half our available arable land to bioethanol production - ignoring all other forms of transportation, which need another three times as much again (i.e. a total of twice that arable land area). Agreed, planes could fly at lower altitudes, where it is warmer, but there they would encounter greater air-resistance and need to carry and to use more fuel for the trip.
Ethanol only packs about 70% of the energy punch of oil-based aviation fuel ('spirit' it is sometimes called, in view of its volatile nature and flammability) and so about half as much again would need to be taken on board for the flight. Hydrogen powered planes are another alternative, but even if the gas is stored in zeolites there are many problems attendent to the extreme cooling required (which would probably need liquid nitrogen), and all other kinds of hazard associated with this material. Also the plane would need to be a lot bigger, since hydrogen weight for weight is a poorer fuel than normal hydrocarbon based plane fuels.
There is no way around the problem of planes pumping out CO2 and water, which is itself a more potent greenhouse gas than CO2 when emitted into the atmosphere at altitude (even hydrogen produces water when it burns, about 2.6 times as much as burning a fuel equivalent quantity of hydrocarbons), unless we reduce the number of flights in the first place. In short, neither in terms of fuel provision nor cutting greenhouse emissions from fuel exhaust gases, can technology provide a neat solution. Biodiesel can be mixed (up to about 10%) with other fuels like 'aviation spirit' and not be subjected detrimentally upon by low temperatures, but there is no reduction overall in the amount of CO2 each flight emits. There are calls to reduce our CO2 emissions by 87% by 2030 to prevent the globe warming by the magic 2 degrees C (which computer models tell us will be very bad news, and irrevocable). This is in every sector, including aviation, and yet the number of flights offered and taken increases year on year.
The fact is that cheap flights are on the way out, and so we can kiss goodbye to being whisked away on cheap foreign holidays, or overseas business conferences. Such activities will be done in virtual space using internet video-conferencing, and other personal journeys will be more planned-for events, making the most of the whole experience, of seeing distant family and friends, or exploring foreign parts, with the journey itself providing a large part of the experience. There is the saying that 'it is better to travel than to arrive'; a celebration of the mind-broadening adventure that travel can become, when the voyager has to make various encounters on his progress that are avoided by air-travel. If that seems like an inconvenience or even an intrusion into personal liberty, then we should consider ourselves fortunate to be so disadvantaged, since the majority of the world's population have more pressing annoyances to deal with.