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Flight traffic emissions
Traffic at such lofty heights produces a range of climate-impacting substances:
Carbon dioxide (CO2)
The aircraft emission with the largest impact on the climate is CO2, something which is created during every combustion process. In contrast to the other emissions, neither height above the earth nor geographic location matter when it comes to CO2. The IPCC regards our knowledge about the effects of CO2 as "good".
Water vapour (H2O)
Water vapour emissions near the ground (up to about 900m) don't affect the climate. At high altitudes, water vapour is responsible for the formation of vapour trails and the cirrus clouds they form. About 30% of the earth is naturally covered by cirrus clouds at any given time. High altitude air traffic can, under certain circumstances, increase the area covered by cirrus clouds. Vapour trails and cirrus clouds have a net positive greenhouse effect (warming). Sausen et al. 2005 (link to PDF) rate our knowledge about the climate effect of vapour trails as "fair", while they rate our knowledge of the formation and effect of cirrus clouds as "poor".
Nitrous oxides (NOx)
At ground level, NOx emissions lead to summer smog and raised ozone concentrations. The same substance, emitted at aircraft cruising altitude, has two effects that counteract each other. On the one hand, it leads to the formation of ozone which leads to warming, whilst on the other hand, it concurrently reduces the concentration of methane, another greenhouse gas, which results in cooling. According to the IPCC, the warming effect is predominant. Knowledge of NOx is judged as "fair" (ozone formation) and "poor" (methane depletion).
Aerosols (SO2 and soot)
Quality improvements in fuel and the combustion processes in modern turbines have led to the reduction of SO2 and soot. On the other hand, efficiency gains in jet turbines have increased the thrust per unit of kerosene whilst simultaneously increasing NOx emissions.
Soot and SO2 caused by air travel have a low impact on the climate.
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