What effect do flight emissions have?

In addition to direct greenhouse gas emissions, aviation has other harmful effects on the climate through cloud formation and other chemical processes (non-CO₂ effects) – these effects are not taken into account when analyses consider only the CO₂ emissions resulting from the combustion of kerosene. A study published in 2020 estimates that, assuming no change in the growth of aviation, the total climate-damaging effect of aviation is globally on average about three times greater than the effect of CO₂ emissions alone.

How does air transport affect the climate?

Aviation has a stronger negative impact on the climate than just the effect of CO₂ emissions. Nitrous gases (NOx), water vapour, soot, aerosol and sulphate aerosol particles emitted by aircraft also affect the climate. In addition, flying creates contrails and contrail cirrus, i.e. clouds of ice crystals that can be generated by aircraft engines at high altitudes. This can have both warming and cooling effects.

Nitrogen oxides, for example, can lead to ozone formation, which warms the atmosphere. They can also contribute to the breakdown of methane in the atmosphere, which in turn has a cooling effect. Water vapour and soot particles, which absorb sunlight, have a direct warming effect. Sulphate particles can have a cooling effect by blocking sunlight from the atmosphere. In addition, aerosol particles can lead to the formation of contrails and cirrostratus clouds composed of ice crystals. The contrails trap infrared radiation in the atmosphere and thus have a warming effect. Although they reflect solar radiation back into space and can thus have a cooling effect, the warming effect is stronger. Cirrostratus or cirrus clouds composed of ice crystals absorb solar radiation and cause the atmosphere to become warmer.

Climate impact of air travel not limited to CO₂
Source: atmosfair (2020): Climate impact of air travel

Overall, aviation emissions clearly have a predominantly warming effect.

Cloud formation and the CO₂ emitted have the strongest climate impact. The direct impact of emitted particles is well documented by physical science. The exact effect of cloud formation has not yet been conclusively researched. The various impacts differ in terms of duration. While CO₂ remains in the atmosphere for centuries and warms the climate, cloud formation, soot and aerosols only have an effect over short periods of days to decades:

Climate impacts from aviation
Climate Factors CO₂ NOx → O3 Increase NOx → CH4 Decrease NOx → O3 Decrease Sulfate Aerosols Soot Contrails & Cirrus clouds
Climate Impact Warming Warming Cooling Cooling Cooling Warming Warming
Duration Centuries Weeks to months Decades Decades Days to weeks Days to weeks Contrails: hours
Cirrus clouds: hours to days
Spatial Distribution Global Continental to global Continental to global Continental to global Continental to global Local to global Local to global
Scientific understanding Good Fair Fair Fair Direct effects: good
Indirect effects: poor		 Direct effects: good
Indirect effects: poor		 Poor
Notes: NOₓ = nitrous oxide; O₃ = ozone; CH₄ = methane
Quelle: Carbon Offset Guide (2020): Total climate impacts from aviation.

How can the total climate impact be measured?

In some studies, the impact of aviation on the earth is estimated using radiative forcing (RF). Radiative forcing is a measure of how the energy balance of the Earth and atmosphere are influenced. A radiative forcing index (RFI), which considers the radiative forcing of all aviation effects relative to that of CO₂ emissions, is used in the calculations. The RFI estimates that the total climate-damaging effect of aviation is 1.9 to 4.7 times greater than the effect of CO₂ emissions alone.

In a study published in 2020, the climate impact of aviation is measured by what is known as effective radiative forcing (ERF), which is the increase or decrease in the balance between energy coming from the sun and energy emitted by the earth since the pre-industrial era. Based on this metric, the total climate impact of aviation is estimated to be about three times that of CO₂ emissions alone. The ERF is a better indicator of the warming effect of aviation because it takes greater account of rapid responses of the earth system.

For an individual flight, the specific climate impact depends on other parameters due to the above factors. These other parameters include the altitude of the flight, the prevailing temperature at that altitude, and the humidity in the atmosphere. For the calculation of the climate impact of a single flight it is important to take these conditions into account. An incentive can be created for reducing the climate-damaging effects of flying by, for example, flying at a lower altitude.

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