Why does the emission of aircrafts at higher altitudes have a greater effect on the climate?
Initial theories of the atmosphere assumed that it was "well mixed". You will see that term mentioned in many articles, and also the fairly made point that it is an assumption. Historical measurements of CO2 concentration had been pretty sporadic with the exception of Mauna Loa. It's instructive to look at the raw data from that observatory and the adjustments made to produce a smooth curve to view changes year by year. Note also the daily variations due to plants. A NASA satellite was launched in 2014 to measure CO2 high in the atmosphere so there is now evidence CO2 is not well mixed but we do not have enough data to look at long term trends as it has not given us even 3 years worth of data yet.
It seems to be claimed that about 40% of the CO2 produced from burning fossil fuels comes to show itself as CO2 concentration increases. The rest must go somewhere else (or we got the calculation wrong !). It is often assumed 30-40% goes into the oceans. Although I have seen claims that the earth has greened by about 18% in the past 40 years (increased intensive agriculture) I have not seen the increased CO2 from this source taken into account yet in calculating which proportion of the CO2 increase in the atmosphere is due to that. At a first examination this would seem to be quite important. If the 18% greening is responsible for a significant percentage of CO2 increase then clearly fossil fuel CO2 is less important. This looks like an important area that needs to be looked at. Maybe careful study of satellite data over green areas compared to deserts/ice will reveal something.