The paper, ‘21st century civil aviation: Is it on course, or is it over confident and complacent? – thoughts on the conundrum of aviation and the environment’ published in the Aeronautical Journal, Vol 121, Issue 1236, pp 115-140, 2017 by D. I. A. Poll, offers insight into this complex dilemma.

Aviation delivers great commercial and social benefits, and as the global economy expands, demand for air transport will increase. The current industry expectation is that the global capacity growth rate will be running at approximately 5% per annum for the foreseeable future. Growth in the global economy is clearly a good thing. However, aviation also contributes to climate change at a time when there is increasing international pressure to limit mean global temperature rise. Therefore, the future success of aviation is likely to depend upon the industry’s ability to hold its environmental impact within politically acceptable limits.

The environmental impact comes from a number of sources, such as the carbon dioxide that is released as fuel is burned. The impact of carbon dioxide is well understood. However, less well understood are the impacts of the other emissions, namely – the other principal product of the combustion process – water vapour. Water vapour emissions play a central role in the creation of contrails. Of particular importance are the “persistent” contrails that are a common sight in the northern latitudes, which have the potential to influence the amount and duration of cloud cover – a significant contributor to global mean temperature. Persistent contrails occur when an aircraft flies through a region of air that contains a high concentration of water vapour at a very low temperature. Under the right conditions, the engine’s water and particulate emissions can initiate a massive precipitation of atmospheric water in the form of ice crystals. Initially, these crystals are trapped in the aircraft’s flow field, producing the familiar two parallel white line structure. Some estimates put the amount of ice formed to be up to 10 million times that attributable to the engine’s water production. Therefore, contrail formation is a major, although relatively short lived, atmospheric event with the potential to influence global mean temperature. In fact, current estimates suggest that the impact may be comparable to that of aviation’s carbon dioxide.

At present, the International Council for Civil Aviation is focussing on reducing carbon dioxide emissions and it is recognised that, with current market growth rates, new technology alone cannot deliver an acceptable global, carbon emission rate in the necessary time scales. It is proposed that market based offset schemes be used to bridge the gap, but, for a variety of reasons, these are not an ideal solution. However, the available data suggest that the fuel currently burned per annum is almost double that needed if all aircraft were operating at their maximum economic efficiency. This suggests that all options have not yet been considered, or even identified, and that much may be gained by a careful assessment of airline operations and air traffic management practices and procedures.

Carbon dioxide may be only half the problem and greater consideration needs to be given to the mitigation of both the carbon dioxide and the non-carbon dioxide effects in an optimum, balanced way. Unfortunately, time is not on our side.