Aviation’s climate impact

Aviation is often seen as a major source of greenhouse gas emissions. And it is true that as a private individual, flying is the single most climate-impacting activity you can undertake (if we disregard space travel). Having said that, we don’t fly all that often, and most people in the world never fly at all. From the global perspective, aviation accounts for only about 2% of total carbon dioxide emissions.

Last week an estimate was made of how much carbon dioxide the family saved by not flying to Switzerland for a skiing holiday. However, the real difference is actually less than the figures given in last week’s presentation. That’s because it is rather difficult to calculate exactly how much climate impact a flight causes. The amount of energy needed to fly one person over a distance of one kilometre depends to a considerable extent on the total length of the flight. Flying from Göteborg to Stockholm requires almost twice as much fuel per kilometre than flying to Beijing, for instance. And of course another vital parameter is how full the aircraft is.

The calculation only takes into account the CO2 emitted during the flight itself, but producing the fuel also requires energy. To this should be added the fact that airports and their various peripheral activities also cause emissions, but these emissions are seldom taken into account when calculating aviation’s total climate impact.

What is most complicated with aviation, however, is the warming caused by aviation apart from the carbon dioxide effect. For one thing, aircraft produce nitric oxide emissions. Nitric oxides have both a warming and a cooling effect on the climate. Warming comes from the fact that they help create ozone. When we talk about ozone we usually refer to the ozone layer in the stratosphere that prevents the sun’s ultraviolet (UV) light from penetrating through to the earth’s surface. However, ozone that is formed at lower altitudes in the atmosphere functions as a greenhouse gas. Furthermore, nitric oxides are also part of a process that breaks down methane, which is a greenhouse gas. If we take all these effects into account over a hundred-year perspective, nitric oxides from aviation will have a slight overall warming effect.

Aviation also causes contrails (also known as vapour trails, seen as white streaks in the sky). The density of these contrailsvaries with factors such as the aircraft’s altitude and ambient temperature. Just how much warming effect this has depends on whether you are flying at night or during the day, and whether the ground below is dark or light.

If we add together the effects of nitrogen oxides and vapour trails, we find that aviation causes an overall warming effect that is about 30% higher than that caused solely by carbon dioxide over a hundred-year period. This time perspective is important: carbon dioxide remains in the atmosphere for a very long time. Ozone remains for weeks, while contrails disappear after a few hours. However, while they are present, they have a significant warming effect. If we were to examine this comparison on a shorter time frame, we would have to increase aviation’s emissions by more than 30%.

To this should be added that contrails can later turn into more long-lasting cirrus clouds high up in the atmosphere. These clouds cause a rise in temperature down on the surface of the earth, although there is some uncertainty as to the extent to which these clouds are caused by aviation. If we also take these clouds into account, the warming effect of aviation is about 70% higher than that caused solely by carbon dioxide. But there are large uncertainties present.

If we now try to calculate the family’s trip to Geneva, the journey is 1680 km long, and flights of that length require about 0.40 kWh fuel per person-km. If we examine aviation fuel from a lifecycle perspective but disregard emissions from airport operations, we end up with 410 kg CO2/person for a return trip. If we now add in the effect of nitric oxides and contrails, the final figure is between 530 kg CO2 eq/person and 700 kg CO2 eq/person.

Emissions per km for the flight are about 158 g CO2 eq/km, which is comparable with doing the trip alone in a large car.

Fredrik Hedenus, Chalmers