Plant-based fuels are awaiting final clearance for take-off after Lufthansa and others have put them to the test in commercial flight.
Biofuels taxi up to the runway as airlines go for cleaner alternatives
The airplane of the future will not emit any carbon dioxide.
It will be powered by electric engines or hydrogen and may consist of just one gigantic, supremely aerodynamic wing, a see-through floor and cavernous space for up to 1,000 passengers.
But that day is still decades off and in the meantime, a race is on to limit the considerable environmental damage caused by conventional planes.
The world's current fleet of about 15,000 passenger jets, although increasingly fuel-efficient, accounts for at least 2 per cent of all greenhouse-gas emissions.
Global aviation is among the fastest-growing polluters. Carbon dioxide emitted by aircraft high in the sky remains in the atmosphere and the climate impact is far worse than emissions on the ground.
For example contrails, or vapour trails, from the engines cause cirrus clouds that compound global warming. Some scientists estimate the warming effect to be as big as 10 per cent.
On top of that, pressure to make flying greener is mounting because aviation is growing at an annual rate of about 5 per cent. The International Air Transport Association (Iata) wants that growth to be carbon dioxide-neutral by 2020 and to cut emissions by 50 per cent by 2050, compared with 2005 levels.
That's a highly ambitious goal, and at present there is a limited range of options for achieving those targets.
Electrically-powered aircraft are still a long way off because it is a huge challenge to develop batteries that are sufficiently light and yet powerful enough to give engines the necessary thrust.
Any revolution in aircraft design won't go into effect before 2040 at the earliest because of the time it takes to develop, test and build new models, and the long operational lifetimes - about 30 years - of jets now coming on the market.
One solution is biofuel, which many scientists say offers the best and quickest way to curb carbon dioxide emissions from aviation because engines do not have to be redesigned to use it.
"Biofuels will be used for the transition," says Mirko Hornung, a professor of aviation systems at the University of Munich. "For example, synthetic kerosene that can be made from rapeseed oil or the jatropha plant. All tests so far have shown that it can be used like conventional kerosene."
Despite supply problems and its relatively high cost at present, the benefit of biofuel is that burning it only releases carbon dioxide that was previously absorbed by the plants from which it was made.
The thought of being on a plane fuelled by algae, animal fats or reprocessed cooking oil may not inspire much confidence when 15,000 metres in the air, but tests have shown biokerosene performs better than conventional jet fuel.
"It has a higher energy density. This leads to lower fuel consumption, meaning that you can fly further with the same amount," says Lukas Rohleder, a spokesman for the Aviation Initiative for Renewable Energy in Germany (Aireg).
A number of airlines around the world, including Lufthansa and Air France-KLM, have been testing a kerosene mix partly consisting of biofuel on commercial flights in recent years - including trans-Atlantic flights - and have reported that they are satisfied with the engine performance.
Lufthansa last year completed a long-term test of biokerosene on 1,187 scheduled flights, involving one of the engines of an Airbus A321 being powered by a fuel mixture containing 50 per cent biosynthetic kerosene. A 50 per cent fuel mix is the maximum allowed under current international rules for commercial flights. That is because the effect on engines of using higher percentages of biofuel has yet to be fully researched, says Aireg.
Lufthansa was the first to conduct long-term tests of biokerosene on scheduled flights.
"Engineering inspections all gave favourable results and we were able to verify that biokerosene does not emit higher pollutant levels than normal kerosene," says Joachim Buse, the head of aviation biofuel at Lufthansa.
Iata wants 6 per cent of kerosene used in planes to be biofuel by 2020. The European Union wants to create production capacity for two million tonnes of biokerosene by 2020 - about 4 per cent of the region's total kerosene requirement.
At the end of October, a twin-engined, 10-seater Falcon jet made history in the sky above Ottawa by becoming the world's first plane to fly with 100 per cent biofuel.
Boeing also believes biofuels hold the key to radically cutting carbon dioxide emissions from aviation.
"We are convinced that sustainable biofuels can provide a way to reduce the CO2 by between 60 and 80 per cent on an airplane," John Tracy, Boeing's chief technology officer, told the BBC last month.
"We really do believe that by 2030, 30 per cent of all airline fuel could be provided by sustainable biofuels."
But there are some major hurdles, and doubts about the benefits of biokerosene.
For one, the potential of biofuel to cut emissions depends on the type of plant it is derived from and where it is grown. This is because, in some cases, the cultivation of suitable plants can have an even bigger carbon footprint than fossil fuel.
If the land used for growing the plants was previously a woodland or tropical swamp, which absorb more carbon dioxide than monoculture plantations, then producing the fuel does more environmental harm than good. And if the land is taken away from growing food crops, the process can be even less justifiable.
The trick is to derive the fuel from plants grown on land that would not otherwise be used for farming, or that can be planted in rotation with food crops, a process that benefits the soil.
Algae, for example, can be grown on wasteland, in vertical vats or in ponds. The jatropha plant, also known as the Barbados nut, thrives in tropical and subtropical regions and can be cultivated in areas where hardly any other plant would grow. The camelina plant, another source of biokerosene, can be grown in rotation with cereals.
But at present, there is not nearly enough biokerosene available.
"Research is needed on whether the material can be produced in the necessary quantities and whether production at acceptable prices is possible," says Dietmar Schrick, the managing director of the German Aerospace Industries Association.
Such crops are not being grown on anything near the scale needed to feed the aviation industry and there is a dire shortage of refineries. That contributes to making biokerosene far too expensive to be commercially viable now.
It costs at least twice as much as fossil-based kerosene, and as fuel makes up about 30 per cent of an airline's costs, it is unsurprising the demand for green fuel will not be going through the roof anytime soon.
This is where companies such as Jatro come in.
Based in Frankfurt, it is in the process of accumulating €200 million (Dh979.7m) from investors to develop the large-scale cultivation of the hardy jatropha plant in Thailand, Indonesia, Laos and Cambodia.
"Crude jatropha oil offers a sustainable and scalable source of energy at a commercially viable price," the company says.
"Engine tests and test flights have demonstrated that the use of biofuels from jatropha as 'drop-in' fuels is technically sound and works on all commonly used engine types properly and efficiently."
Despite the cost, the incentive for airlines to use biofuels is set to grow in coming years because the EU has included all airlines landing at its airports in its carbon emissions trading scheme. The move, which took effect on January 1, requires airlines to pay for their greenhouse-gas emissions by obtaining special permits.
For now, most of the permits are being given away free. But assuming the move survives legal challenges from countries including the United States and China, it could end up forcing carriers to spend about US$15 billion (Dh55.09bn) between now and 2020 to comply with the trading scheme.
The expansion of production, and the prospect of further increases in the price of fossil fuels, will also help to make the price of biokerosene less of a hindrance.
But companies and governments need to step up their research and investment to make that happen.
"To build up the necessary capacities and to secure the competitive advantage, investments must be done now," says Mr Rohleder. "The amount of biokerosene available in the long term depends on which raw materials are available and what results the research in bio-refinery technologies produce."