Hugh Somerville on strategies for sustainable aviation

Q: The world increasingly relies on mobility, yet we must find ways to reduce its side effects. What specific initiatives can meet the need of the modern world: increased mobility with decreased harm to our surroundings? In short, what is the path to sustainable mobility in the aviation sector?

A: Aviation is a vital component of the world's communication and transportation system. It has grown five-fold over the last 30 years and is set to continue growing at around 4-5% per annum, with some regional variation. Such growth is not possible without impact on the environment. Over the last 10 years it has become clear that this impact on the global climate is the most serious long-term issue facing the aviation industry.

There are, of course, other environmental issues. All airports have to deal with the effect of the noise generated by aircraft movements. A target has been set by the Advisory Council for Aeronautical Research in Europe (ACARE) of a 50% reduction in perceived noise from individual aircraft by 2020 (relative to levels in 2000). The overall noise impact will be managed within the Balanced Approach instigated by ICAO (the International Civil Aviation Organization), using a mix of new technology to reduce noise at source, operational improvements, better land use and, where necessary, restrictions on the operation of some noisy aircraft.

Another issue is the quality of air around some airports. Ground transport close to the airport, some of it associated with airport activity, is the major contributor, and technological improvements will reduce input from aircraft (ACARE target: 80% reduction in NOx emissions by 2020). Substitution of low-emitting vehicles, on- and off- airports will contribute to the achievement of acceptable air quality levels.

The major challenge is to address the direct and indirect global-warming (greenhouse) effects of engine exhaust gases. Carbon dioxide is emitted as a direct product of the combustion of kerosene in the same way as it is produced by cars from gasoline. It lasts in the atmosphere for around 100 years and the impact is the same whether it is emitted from sources on the ground or at the high altitudes where aircraft cruise.

Emissions of nitrogen oxide (NOx) are associated with local air quality, but also have indirect impacts at cruise altitudes. They generate ozone in the troposphere which contributes to warming. While the lifetime of ozone is weeks, the effect is longer and the effect is regional rather than global. The effect depends on altitude, location and atmospheric conditions. NOx emissions also lead to a cooling effect through the removal of some of the methane (a major greenhouse gas) arising from other, ground-based, sources. An understanding of these indirect NOx impacts is essential.

Condensation trails only form in a very cold, very humid atmosphere. The lifetime of a contrail may vary from seconds to hours, and contrails may in turn lead to the formation of cirrus clouds. The effects are highly dependent on altitude, location and atmospheric conditions. The role of water vapor and particles derived from kerosene combustion in both contrail formation and the enhancement of cirrus cloud is not clear. There is, however, some evidence of a correlation between cirrus cover and air traffic.

Overall carbon dioxide, from all sources, accounts for about two thirds of the global-warming effect of man's activities. Aviation accounts for around 2.5% of present CO2 emissions while transport as a whole constitutes about 20%, mostly from road transport.

Clearly aviation is not the only transport mode requiring attention. A reduction of 60-70% in emissions may be required to achieve stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous interference with the climate system. Some estimates suggest that aviation could, as a result of growth and disproportionate contribution of its exhaust emissions, contribute all of these acceptable emissions by 2050.

Clearly doing nothing is not an option. Meeting this challenge will require action at a global level and strategy could include the following elements:

• Better understanding of the climate impacts of aviation related to cirrus, contrails and NOx.
• A drive for continued improvement by reducing emissions at source through technology. Fuel efficiency of aircraft in terms of fuel per tonne kilometer has improved by some 70% over the past 40 years. Improvement is likely at around 1% per year for some time, but this will only go a part of the way to compensate for 4-5% growth.
• Operational improvements may include using aircraft best suited for individual routes, substitution by rail where it makes sense, and even options such as in-flight refueling. Improvements in ATC/ATM systems could on their own contribute some 5-10%.
• Emissions trading in open systems (ETS) where, within an overall cap, aviation compensates for growth by buying reductions in emissions from other sectors or by investing in certified emission reduction projects such as those described under the Kyoto Protocol is another possibility. Some industry commentators prefer this option to taxes, pointing out that while taxes have a known financial penalty they do not define any environmental benefit and have already proved ineffective, for example in private road transport. In trading, the environmental benefit is defined, but the cost of the "negative emissions" is not clear, at least initially. The European Commission is committed to looking at incorporation of aviation in the European ETS by 2008; however, not all parts of the industry agree that this is the best way forward.
• Other possible measures include a charge on actual emissions where the revenue is used to promote reductions in emissions, for example through research and development of new technology.
• One other possibility, which is beginning to be used by companies such as British Airways, is to offer individuals or organizations the option to offset emissions arising from air travel by paying for reductions achieved through a credible third party. Such offsets are generally not certified with the same rigor as Kyoto linked reductions.
• There has been much speculation about alternative fuels. Liquid hydrogen is feasible but would require about three times the volume of kerosene; methane would require around two times the volume; and, of course, these beg questions as to the production and distribution of such fuels and the environmental effects of burning them at high altitudes.
• A more likely option in the foreseeable future is the introduction of fuels derived from biomass as kerosene extenders. Such renewable kerosene could play an important part in managing the future climate impact of aviation.
• As more becomes known of the overall climate impacts, it may be that systems evolve which allow operational avoidance of contrail and cirrus formation, for example by flying lower. Work is already underway on such possibilities, as it is on aircraft that might fly lower, and somewhat slower, but have less climate impact.

There are other relevant initiatives already. Some of the U.K.'s airlines, airports, manufacturers and the main air navigation service provider have cooperated to develop "Sustainable Aviation," a first collective strategy addressing climate and other environmental issues. "Greener by Design" a broad group of aviation experts has examined some of the possible options. With support from some sectors of the industry, The European Commission is looking at the possible incorporation, in 2008, of aircraft emissions in the European Emissions Trading System, which already covers some 40% of Europe's CO2 emissions. British Airways and some other travel and tourism organizations offer customers the means to offset their individual flights at a cost of U.S.$20-U.S.$50 for all the CO2 generated in a return flight form London to Miami.

Aviation is a global industry and action is needed at the global level, to avoid distorting the market. Sadly, while some aviation companies are recognized as leaders in management of corporate responsibility, many individuals seem to cast off their environmental credentials when they board an aircraft. The time has come for this to change and for government, the wider industry and the traveling public to take action.