Climate impact calculation
The climate impact of emissions is a complex subject. Here are some of the main factors involved.
- Different gases have different "radiative forcing" factors (i.e. the extent to which a given amount of the gas affects atmospheric temperature). For example, methane is deemed to have about 21 times the radiative forcing of carbon dioxide. Radiative forcing factors can also be affected by altitude.
- Some gases have a direct warming effect, such as carbon dioxide, while others have an indirect effect due to their impact on atmospheric chemistry.
- Different gases have different lifetimes in the atmostphere. Moreover, the lifetime of a given gas varies with altitude. For example, stratospheric carbon dioxide lasts for a few years, while methane lasts for 10-15 years.
For air travel, there are a range of gases emitted that are relevant to climate change, where the most significant are carbon dioxide, nitrogen oxides and water vapour. At ground-level, water vapour has no climate effect, but at aircraft cruising altitudes it does, due to the creation of contrails (vapour trails). While nitrogen oxides have no direct warming effect, at aircraft crusing altitude they enhance troposheric ozone formation, which leads to a warming effect. Therefore, to produce a figure for climate impact expressed in terms of "carbon dioxide equivalents", we need to apply a conversion factor to the actual carbon emissions.
This is a complex topic, and keeps a bunch of doctorates busy at research institutes around the world. It's complicated by the fact that the different emissions have different lifetimes, and also by the fact that aviation also leads to increased cloudiness, although this aspect is poorly understood.
Scientific reports from various respected bodies have produced values for this conversion factor in the range 1.6 to 4.2. The Fourth Assessment Report from the Intergovernmental Panel on Climate Change (IPCC) in April 2007 discussed this topic at length (in Chapter 2), with the conclusion that the components excluding "aviation induced cloudiness" correspond to a factor 1.9, while aviation induced cloudiness could account for an increase in this factor by a further 1.2 but that this figure is highly uncertain (see section 2.6.3). We have taken a conservative "two-thirds" approach to cloudiness, adding in two-thirds of this uncertain figure, to arrive at a final figure for Radiative Forcing Index of 2.7.
All we can say for sure is that the conversion factor we have chosen is unlikely to be exactly correct. One concrete way in which it is wrong is that we apply it linearly to all carbon dioxide emissions associated with the flight, whereas in reality the (significant) emissions at takeoff happen at a lower altitude than cruising emissions. A more sophisticated model would apply separate conversion factors to the takeoff, cruising and landing components of the total emissions.
We monitor research in this area on an annual basis and update our model accordingly.http://ipcc-wg1.ucar.edu/wg1/Report/AR4WG1_Print_Ch02.pdf