The Kyoto framework, the UK’s Climate Change Act and the UK’s 2050 aviation target all exclude aviation non-CO₂ effects, since these do not derive directly from emissions of any of the six greenhouse gases covered by the Kyoto Protocol.

In considering how the UK can meet its 2050 aviation target, the Committee has therefore predominantly considered CO₂ emissions. However, there are potentially significant additional non-CO₂ effects from aviation which lead to both additional warming and cooling effects on the climate.

Non-CO₂ effects arise from the emission of gases and particles from aviation fuel, and also from induced cloudiness that is caused by aviation:

• Emission of gases - Aside from CO₂, combustion of aviation fuel results in emission of water vapour, nitrogen oxides (NOx) and aerosols. NOx are indirect GHGs and do not give rise to a warming effect (radiative effect) on their own, but they influence the concentration of other direct GHGs by enhancing ozone (which leads to warming), and by suppressing methane (cooling).
• Induced cloudiness – the flight of aircraft can also cause the formation of linear ice clouds (contrails) and can lead to increased cloudiness which in turn causes additional warming.

It is therefore highly likely that the net impact of non-CO₂ effects – particularly contrails and other induced cloud formation – is to increase the global warming impact of aviation beyond that suggested by CO₂ emission alone.

Aviation radiative forcing components in 2005

Source: Reproduced from Lee et al. (2009)

The precise scale of the additional impact of non-CO₂ effects on global warming is unclear and there are considerable scientific uncertainties still to be resolved. However, it is highly likely that these effects are significant and that they should therefore be accounted for in future international policy frameworks and in the overall UK policy framework for emissions reduction.

The implications for the UK framework are unclear. The options would be to increase required emissions reduction effort in other sectors, or increase required emissions reduction effort in aviation, or a combination of these two. It is likely that more emissions reduction effort would be required from UK aviation to allow for non-CO₂ effects, although further work would be required to determine the appropriate balance of effort between aviation and other sectors.