Klima 1-2008

The price of air travel

When it comes to air travel, it’s not only the CO2 emissions that have a negative effect on the climate. What a flight should actually cost depends on which of these effects we want to pay for.

The EU emissions trading system will include CO2 emissions from air travel from 2012. It was originally proposed to multiply the CO2 emissions by a factor of 2 to take into account the entire climate effect of air travel. Some net-based calculators for carbon offsets include only the emission of CO2, while others add more. What are these other climate effects, and should they be included in the price of offsets?
CO2 emitted at high altitudes has exactly the same effect on climate as CO2 emitted from other sources closer to ground level. But the altitude makes a difference for many of the other components emitted from air travel. The most important additional climate effects come from contrails, cirrus clouds, and ozone (see fact box).

Short lifetimes

Air travel thus has additional effects on climate that are not regulated through the Kyoto Protocol. But comparing the contribution from these additional effects with the climate effect from CO2 is no straightforward task. A contrail or cirrus cloud has a lifetime less than one day, while a large amount of the CO2 we emit today will still be in the atmosphere in about 100 years. Many claim, with reference to the IPCC special report on aviation from 1999, that the overall additional effect is two to four times as great as that of CO2. This factor, or index for radiative forcing, is based on historical emissions from aviation. It does not tell us what emissions today mean for future climate change. The Kyoto Protocol uses GWPs (global warming potentials) for a time horizon of 100 years in the future to compare the climate effect of various gasses. The additional factor calculated in terms of GWPs is 1.2 if we include contrails, and 1.8 if we take into account the uncertain contribution of cirrus clouds. The net contribution of NOx via ozone and methane is negligible. For a shorter time horizon, the weighting will increase. A weighting factor of 2, as was originally suggested in the EU emissions trading system and in many climate calculators, thus means either that the precautionary principle is being adopted and allowance is made for the great uncertainty that lies in a GWP for air travel, or that for this sector a shorter time horizon is being used than what is used in the Kyoto Protocol. That is, greater emphasis is given to the warming that takes place over the next few years compared to that which occurs in 100 years.

Should the additional effects be included?

The next question is whether it is reasonable to use this kind of weighting factor in emissions trading systems, such as that used by the EU. The answer to this question depends on what we want to achieve by using this kind of additional factor. By attempting to take into account all climate effects, we buy more offsets and thus the flight becomes more expensive. That is, we pay for the damage the flight causes the climate. A higher price can mean that we travel less.
But there are also counterarguments. In other sectors, the emissions trading system is limited only to CO2. The emissions trading system will probably be expanded to include other Kyoto gases, but it is unlikely that the first expansion will cover gases and particles not included in a climate treaty. The additional effects beyond CO2 are also relevant for other sectors. Estimates for road traffic suggest a factor of up to 1.25 due to ozone formation and soot particles, while for some other sectors the factor will be lower than 1. There are few arguments for treating one sector – in this case, the air travel sector – differently than other sectors with respect to which emissions and effects will be included.

Alternative air routes

A final consideration is whether including the entire climate effect of air travel in the emissions trading system through weighting will trigger measures that can actually reduce the climate effect from this sector. The climate effect from emissions of gases and particles with short atmospheric lifetimes will depend on where and at what time of day the emission takes place. That is, an airline can reduce the climate effect from the formation of contrails by choosing an alternative flight path – which might, on the other hand, increase emissions of CO2. The airline could also choose an engine that reduces CO2 emissions, but releases more NOx. Connecting various effects of air travel together in an emissions trading system by using a fixed weighting factor will thus not necessarily result in measures aimed at reducing each of these effects. In the medium term, an option could be that the additional effect is calculated on the basis of the plane’s actual flight path and the meteorological conditions along this path. Then the airlines could optimize their flight plans on the basis of prognoses from a weather forecast model. Other instruments should be used to reduce the additional effects. These could include taxes and development of technical standards for NOx emissions, or guidelines for which route and altitude the planes should choose.


  • Lian, Jon Inge et al. (2007) Bærekraftig og samfunnsnyttig luftfart. [Sustainable and socially beneficial air traffic.] TØI rapport 921/2007
  • Forster, P. M. D., et al. (2006), It is premature to include non-CO2 effects of aviation in emission trading schemes, Atmospheric Environment, 40, 1117-1121.
  • IPCC, 1999, Aviation and the global atmosphere. A special report of IPCC working groups I and III. J.E. Penner, D.H. Lister, D.J. Griggs, D.J. Dokken and M. McFarland (eds).

Sist oppdatert: 29.07.2008

THE PRICE VARIES. The climate effect from air traffic is complicated, and this means that the price of offsets varies considerably. According to CICERO, the CO<sub>2</sub> emissions should be multiplied by a factor between 1 and 2 if the real climate impact is to be taken into account. Photo: Stockxpert THE PRICE VARIES. The climate effect from air traffic is complicated, and this means that the price of offsets varies considerably. According to CICERO, the CO2 emissions should be multiplied by a factor between 1 and 2 if the real climate impact is to be taken into account. Photo: Stockxpert

Additional effects from air traffic

Contrails are linear clouds of ice formed by emissions of water vapor from aircraft. Contrails reflect incoming solar radiation, but also absorb long-wave heat radiation from the ground. The cooling effect of contrails reflecting sunlight occurs only when the sun is shining, while the warming effect is independent of whether it is day or night. Globally, it is the warming effect that dominates for contrails.

Cirrus clouds can be developed from a contrail under certain conditions. Like contrails, a cirrus cloud can have a warming effect. It is difficult to distinguish the natural cirrus clouds from those formed by air traffic. The climate effect of increased numbers of cirrus clouds from air traffic can be considerable, but there is substantial uncertainty connected with these estimates.

Nitrogen oxide (NOx), carbon monoxide (CO), and volatile organic compounds (VOCs) are precursors of ozone (O3), which is a greenhouse gas. In a relatively clean background atmosphere, as in the upper troposphere (air traffic height), emissions of NOx cause the production of ozone to increase. Ozone found high in the troposphere (in the airplanes’ cruising altitude) is more effective as a greenhouse gas than ozone at lower levels. But emissions of NOx also contribute to a reduced lifetime for the greenhouse gas methane (CH4) in the atmosphere. Thus the emissions of NOx also have a cooling effect on the climate that counteracts its warming effect via ozone.

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