There are many websites that calculate the CO₂ emissions of flying. Many of them then allow you to offset the emissions by buying an equivalent amount of activity that will replenish the CO₂ you have generated, such as planting trees. Figures vary for the same trip, and most of them ignore the additional greenhouse impacts of other pollutants such as nitrogen oxides and contrails (vapour trails) in the upper atmosphere. Our calculations triple the basic CO₂ emission values (which are in themselves figures averaged over more than one source) to account for this. But estimates of the real impact vary betwen 7 and 11 times the basic CO₂ figure.

Contrails (condensation trails) are streams of water vapour left behind by aircraft flying at high altitudes. Scientific debate continues, but these are likely to contribute to the greenhouse effect, and also to the formation of high level cirrus clouds, which contribute to raising night-time temperature by acting as a “blanket” to the atmosphere.

Figures given by the airline industry will almost never include these additional impacts because, they would argue, the scientific debate is not resolved to the extent that the figures can be considered accurate. The 3x multiplier we have used may yet prove to be an underestimate.