Aeroplanes and greenhouse

Forest and Bird, Issue 266, 1 November 1992, Page 7

Aeroplanes and greenhouse

AIRCRAFT contribute eight times as much per passenger kilometre to the greenhouse effect as cars, and 22 times as much as electric inter-city

trains according to a British study. The government’s Energy Technical Support Unit has made the calculations based on the effect of various greenhouse gases over a potential 50-year period. The surprising conclusion is that the main greenhouse problem in relation to aircraft is not carbon dioxide but nitrogen oxides. During the 1970s and 80s aircraft designers concentrated on improving fuel efficiency by increasing engine temperature and pressure. This had the environmentally beneficial result of reducing emissions of carbon dioxide to only one third of an aircraft’s total emissions, but only at the expense of increasing the amount of nitrogen oxides which now contribute the other two thirds. Nitrogen dioxide breaks down in a photochemical reaction to form ozone, a greenhouse gas. At ground level nitrogen dioxide has three times the greenhouse potency of COs, but at the cruising altitude of most planes in the cooler air 10-12 kilometres above ground level, chemical reactions are slowed down and the greenhouse effect of relatively short-lived gases such as ozone is enhanced. Thus the heat-trapping potential of nitrogen dioxide emissions increases more than a hundredfold at these higher altitudes. So even though aircraft produce only about three percent of the world’s nitrogen oxide emissions, these emissions probably contribute as much to global warming as all the remaining output of nitrogen oxides put together. Aircraft designers are now looking at ways of reducing engine temperature and thus the level of nitrogen oxides. The problem is that a "lean burn" engine with reduced emissions of all greenhouse gases, while safer for the environment, might not be as safe for passengers. Such an engine would be more difficult to restart in the cold air of the upper atmosphere. Source: New Scientist