What effect do emissions from aircraft have?

The majority of emissions from air traffic are emitted at cruising altitude, i.e. at an altitude of around 10–12 kilometres. Carbon dioxide has the same heating effect on the atmosphere irrespective of the altitude of the emissions. The other exhaust fumes and particles from the engines react in the atmosphere in a complicated way, partly increasing and partly decreasing the warming effect. Not all the reactions and effects are completely understood.

The warming effect of air traffic

Carbon dioxide emissions (CO2) from air traffic account for around 2 per cent and nitrogen oxide emissions for around 3 per cent of the total emissions produced by human activities. Nitrogen oxide emissions have both an accelerating and retarding effect on the increase in the greenhouse effect. At a cruising altitude of 8-12 kilometres, NOx emissions produce ozone, which warms the atmosphere. However, NOx emissions simultaneously reduce methane in the atmosphere, which is a greenhouse gas.

Air traffic produces small amounts of emissions of oxides of sulphur and particles compared with other sources of emissions. At cruising altitudes, i.e. the troposphere, particles increase the formation of clouds and the warming of the climate, but at the same time sulphate particles formed from sulphur dioxide (SO2) have a cooling effect on the atmosphere. The water vapour (H2O) created in the fuel combustion reaction disappears from the troposphere in 1–2 weeks.

The formation in suitable conditions of the condensation trails of water vapour visible behind aircraft flying at cruising altitudes and their effect on the development of cirrus clouds is part of the warming effect of emissions. Currently, little is known about how this process works.

According to estimates, the share of emissions and fuel consumption of air traffic of all human activity on the warming of the atmosphere is in the region of 3.5–4 per cent. The effect is approximately double in relation to the proportion of carbon dioxide emissions from air traffic. This estimate does not include the effect of the formation of cirrus clouds, which is not yet sufficiently well known.

Emissions have no effect on the ozone layer

The ozone layer in the upper atmosphere, i.e. the stratosphere, protects the Earth from ultraviolet, i.e. UV radiation. Emissions of nitrogen oxides from air traffic are released in the troposphere and lower layers of the stratosphere, where they react with oxygen and produce ozone. Civil aircraft do not, therefore, reduce the ozone layer protecting the Earth. Of civil aircraft, only the supersonic Concorde flew in its time in the stratosphere where the emissions may have reduced the amount of protective ozone.