Flying beyond our means – air travel and the environment

Box 1 | What aviation emissions do

Most subsonic planes fly between 9 and 13 kilometres above the Earth, in the upper troposphere and lower stratosphere (largely depending upon latitude). Development of new commercial supersonic planes that will fly between 17 and 20 kilometres above the Earth are also predicted to take to the skies in the not too distant future. Flying in the stratosphere with a supersonic aircraft will have different (not better) impacts on climate change and ozone depletion.

Although aviation only contributes around 2.5 per cent of annual global carbon dioxide emissions, emissions at altitudes of 9 kilometres or more have a much greater effect than if they were at ground level because of the non-carbon dioxide emissions. And at supersonic altitudes it's even worse, the radiative forcing – greenhouse effect – is five times greater than for planes travelling at the lower subsonic levels because of the water vapour emitted into the naturally dry stratosphere.

Modern planes are responsible for a cocktail of emissions that can affect the environment at the regional level where they fly, as well as contribute to global warming. When jet engines burn fossil fuels – generally kerosene – the main discharges that affect climate include carbon dioxide, nitrogen oxides, soot, water vapour and sulfate particles.

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Carbon dioxide is a greenhouse gas that contributes to global warming. Of particular concern is that carbon dioxide can remain in the atmosphere for up to 200 years and longer, and spreads in a blanket around the Earth.

Nitrogen oxides emitted from jets have two main effects. Firstly, they break up methane gas present in the skies. Methane is a greenhouse gas that spreads out around the globe, so reducing it reduces global warming slightly. Secondly, nitrogen oxides also react in sunlight to create the greenhouse gas ozone, which has a localised warming effect. At tropical latitudes the production of ozone occurs five times faster than in temperate zones. Because the two reactions involving methane and ozone occur in different regions, they do not simply cancel each other out.

Water vapour and particles from planes in cold air trigger the formation of distinctive condensation trails (contrails) seen so often trailing behind planes in the sky (most of the water content of contrails is actually condensed from the background atmosphere). Contrails that persist may eventually form the wispy looking cirrus clouds that can last for weeks in the atmosphere. Contrails and cirrus clouds act to trap the Earth's heat on a regional scale. Cirrus clouds also reflect some of the heat from the sun away from the Earth, in what's called the albedo effect. However, the warming effect of these clouds seems to be greater than the cooling effect. In fact this warming effect may be as large as that from carbon dioxide emissions from planes. Researchers are trying to clarify the effects of contrails and cirrus clouds created by planes.

Soot and sulfate particles have little direct effect in comparison to other emissions. Soot has a small warming effect and sulfate particles have a minor cooling effect on climate. Both can trigger cloud formation though, so indirectly cause warming.

Boxes
Box 2. Flying smarter
Box 3. Saving our skies – what can we do?

Related site
Does flying cost the earth? Animation (Science Museum, UK)

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Posted February 2009.