Greenhouse gases from aircraft
During flight, aircraft engines emit carbon dioxide, oxides of nitrogen, oxides of sulphur, water vapour, hydrocarbons and particles. All of these gases alter the chemical composition of the atmosphere in a variety of ways, both directly and indirectly. Most of these emissions are at high altitude, since commercial aircraft generally cruise between 9 and 14 km. Because high concentrations of greenhouse gases are emitted at such high altitudes, their emissions are especially damaging.
Carbon dioxide is widely seen as the main greenhouse gas, simply because it is emitted in the highest quantities (although other greenhouse gases, such as methane, are much more powerful). However, because of the effect of the extremely hot gases from the aircraft mixing with extremely cold air at high altitude, the other gases appear to have just as much of an effect. The overall impact is estimated by the Intergovernmental Panel on Climate Change to be about 2.7 times the effect of CO2 alone – this is called radiative forcing.
Water vapour, a harmless emission at ground level, can have also have a significant effect at high altitude. The vapour trails you can see from planes high in the sky are the result of water condensing from the emissions of planes flying close to the imaginary line between the upper troposphere and the stratosphere. These appear to cause the formation of cirrus clouds, high wispy formations of ice crystals. These reflect some of the sun's heat back to space, but also trap some heat in the atmosphere, and this appears to be the stronger of the two effects. The effects of supersonic aircraft are even worse, as they fly higher into the stratosphere and release much more water vapour.
Between 1990 and 2004, UK carbon dioxide emissions from transport rose by 50%, whilst emissions from every other sector started to fall. Aviation is responsible for most of this increase: its annual emissions almost doubled from 20.1 to 39.5 million tonnes. This is 5.5% of the country's CO2 emissions, but if you include the radiative forcing, it is widely accepted that it produces 13% of the country's greenhouse gases.
In 2005 Friends of the Earth commissioned the Tyndall Centre for Climate Research to determine what effect the Government's plans for airport expansion would have on our targets for reducing carbon dioxide emissions. It found that if we were to stabilise emissions at 450 parts per million by 2050, aviation would on its own emit more carbon dioxide than we were allowed.
A Technological solution?
There is no question that technological developments have massively improved the fuel efficiency of aircraft. The problem is that the limits have moreorless been reached, and only small improvements are now possible.
The Government has now committed to an 80% reduction in CO2 emissions by 2050. This needs to be across all sectors, for reasons which will be explained in the section on the Emissions Trading Scheme. Therefore, if aircraft numbers were to be maintained at current levels, every aircraft wold need to reduce its emissions by 80%. If aircraft numbers are to double, as envisaged by the Government’s Air Transport White Paper (see above), every aircraft would need to reduce its emissions by at least 90%. No one would argue that this is remotely possible.
The most efficient aircraft are the turboprop planes, and these appear to make up the bulk of aircraft flying out of Southampton. They use just 59% of the fuel of turbofan jet aircraft and are generally quieter, but are only used on short-haul flights, presumably because they cannot reach the same altitude as jets. Short-haul flights should be discouraged in any case, as greener alternatives such as coach and rail should be developed instead.
Some aviation industry bodies and companies have announced voluntary targets for further improvements in efficiency and environmental performance. In Europe, these are co-ordinated by the Advisory Council for Aeronautical Research in Europe (ACARE). ACARE has set targets for improving the environmental performance of the whole industry, covering engines, airframes and operations, but these are purely voluntary.
One design for more efficient aircraft uses ‘open rotor’ engines. Its proponents claim it could reduce CO2 emissions by 50%. The only problem is that it is much noisier than current designs. Another design is a ‘blended wing’ design, in which the wings become hollow and carry passengers. This is still just a concept, its stability and safety far from proven.
Some airlines are proposing that biofuels, or as campaigners call them, agrofuels are the solution. Such biofuels include ethanol from sugar and corn, biodiesel from jatropha, coconut oil, and fuel derived from algae. Virgin and New Zealand Airlines have already ran experimental flights partially fuelled by biofuels. Unfortunately, running planes off biofuels would use more agricultural land than the planet has, even if everyone gave up eating. Air New Zealand’s flight from Sydney to Auckland had one of its four jet engines powered by a 50/50 mix of Jet A1 and coconut oil – in other words, it was 12.5% powered by biofuel. Even this used up 150,000 coconuts. A single transatlantic flight would need 45 million coconuts. Already, biofuels grown for road transport have caused deforestation, displacement of indigenous people, and rising food prices in the developing world.
Finally, there is the age-old proposal for hydrogen fuelled planes. There are plenty of technological problems to resolve, for example the small energy-volume ratio, but if it worked, these planes would produce no CO2. Instead they would produce a tenfold increase in water vapour, which at that altitude would be even more damaging.
There are some other possible solutions, such as improving air traffic control to reduce the amount of time aircraft are held in a stack waiting to land, keeping flights on direct routes, even possibly flying at lower altitudes (which would reduce the contrail effect of water vapour but increase fuel burn due to increased drag in higher pressure air), but none of these can deliver reductions in CO2 of more than about 10%.
The Emissions Trading Scheme
The principle is a sound one – if you want to emit lots of carbon dioxide, you will have to buy permits to do so, theoretically from someone who does not emit lots of carbon dioxide. The aviation industry publicly support the inclusion of aviation in the Emissions Trading Scheme, not just so that they can keep flying, but also to stimulate the development of low carbon technology in both aviation and other industries. In other words, the money paid by the aviation industry will stimulate the development of clean technology in other industries.
In practice, however, it is simply not going to work. In fact it is very likely to push up the price of carbon for other sectors, such as heating, electricity, and surface transport. Given that only 38-50% of Britons fly in any given year (depending on your sources), whilst virtually 100% of people use heating, electricity and surface transport, this would be socially unjust.