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Aviation's role in climate change

Aviation is responsible for 2% of manmade CO2 emissions worldwide

This is according to The United Nations Intergovernmental Panel on Climate Change (IPCC).  The October 2006 report by Sir Nicholas Stern states that the largest contributor to human-induced CO2 is power generation (24%), mostly produced in coal and gas fired stations. Next is land use change at 18%, then agriculture, industry and transport at 14% each (aviation is part of transport). Buildings (8%), other energy related activities (5%) and waste (3%) make up the rest.

CO2 is not the only emission from aviation, however. The exhaust from aircraft engines is made up of: 7% to 8% CO2 and water vapour; 0.5% nitrogen oxides, unburned hydrocarbons, carbon monoxide and sulphur oxides; traces of hydroxyl family and nitrogen compounds and small amounts of soot particles (although the industry has managed to more or less eliminate soot emissions over the past few decades). Between 91.5% and 92.5% of aircraft engine exhaust is normal atmospheric oxygen and nitrogen.

The water vapour trails (contrails) created by aircraft may also have an impact, but research is inconclusive about whether these have a net warming or cooling effect on the earth. Under some meteorological conditions they can remain in the atmosphere and form ‘cirrus’ clouds, which may have an effect on climate change. For example, some research suggests these clouds may have different cooling and warming effects, depending on whether flights are during the day or night. This type of research can identify if there are any potential benefits in altering operational behaviour. More work is being done in this area and the aviation industry is assisting with research into the effects of contrails on climate change, including putting high-altitude atmospheric testing equipment on some passenger aircraft.

Cirrus clouds and vapour trails - inconclusive impactsCirrus clouds and vapour trails; evidence of their impact on climate change is inconclusive.  The aviation industry is assisting in further research to fully understand the effects.

There are many references to aviation having a greater effect than other industries because of the height at which the emissions are released. The most significant greenhouse gas, CO2, spreads in the atmosphere so quickly that it doesn’t matter where or at what altitude it is emitted, the impact is the same. However, other emissions such as NOx and water vapour can have more of an effect at higher altitudes. This greater effect is expressed by scientists as a multiplier.

Recent research suggests that aviation CO2 emissions should be multiplied by 1.9 times to take account of the added impact of these other gasses at altitude. However, it is important to realise that most other emitters also release non-CO2 gasses and require a multiplier to determine their overall climate change impact (also known as radiative forcing). The background rate for road transport, for example, is 1.5 times its CO2 emissions.

When these non-CO2 emissions and the multiplier are taken into account, the IPCC estimates aviation accounts for about 3% of total manmade climate impact.

Looking into the future, the industry is taking many measures to mitigate its climate change impact and the IPCC estimates that aviation’s total contribution, including CO2 and other effects,would likely rise to 5% (with a worst-case scenario of 15% of human emissions) by 2050. The proportional impact of aviation will also depend on the success of other sectors to regulate their emissions.

Find out more about aviation's local effects on air quality »