During jet kerosene combustion airplanes also emit nitrogen oxides (NOx). NOx affects the atmospheric concentrations of two GHGs:
- Methane (CH4) is a GHG that remains in the atmosphere for at least 10 years. Over a 100-year timeframe, methane is about 34 times as powerful as CO2 (IPCC, 2013).
- Ozone (O3) is a GHG that remains in the atmosphere for 2-8 weeks. The formation of O3 by aircraft is similar to the formation of smog by road traffic. But due to increased UV radiation at high altitudes, O3 is formed more effectively than on the ground.
The atmospheric chemistry involved is complex. To put it simply: NOx emissions from air travel lead to an initial increase in O3 (warming) that persists over a couple of months. It is followed by a longer-term (decadal timescale) decrease in CH4 (cooling) and O3 (cooling). The later decrease in CH4 and O3 does not outweigh the initial increase in O3. Therefore, NOx emissions produce a net warming effect (Lee, 2018).
NOx causes climate effects that are relatively short-lived. Short-term effects like these are relevant when the activities that cause them to persist or increase. Although there is no accumulation, like for longer-lived GHGs.