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I am trying to understand the footprint of a flight. First Wikipedia says (without quoting a source):

The level and effects of CO2 emissions are currently believed to be broadly the same regardless of altitude.

But then there are numerous references to radiative forcing, which is supposedly almost doubling the effects of the emissions (at this point only co2, or also NOx and others?). These are never explained (it says "see below" once, but there follows no explanation below.

The article on radiative forcing says that

Typically, radiative forcing is quantified at the tropopause in units of watts per square meter of the Earth's surface.

However this does not seem to explain why quantifying it "at the tropopause" makes sense, or what that means, since the measure given is measured on the surface. Does this imply that emissions below / above the tropopause are not considered contributing to radiative forcing? (I presume they are.)

Also, how come that emissions at greater heights force more? After all, at lower heights the same amount can cover a greater surface area at the same thickness / density. Maybe, the change in wavelength of incoming radiation is greater since the traveled distance is greater, and thus more is absorbed?

Lastly, does CO2 not ascend after being emitted at surface level, such that it will enter the stratosphere just as well as if it is emitted there? Or are there further factors partially mitigating this?

You can see I have too many questions, and find the Wikipedia articles very unrevealing / lacking.

Thank you!

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2 Answers 2

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Check out https://en.wikipedia.org/wiki/Environmental_impact_of_aviation . NOx from airplanes near/in the stratosphere increases ozone formation plus aerosols help form contrails... both of which increases warming. If the airplane emissions aren't in the stratosphere, then the chemistry and lifetime of those non-CO2 emissions are not nearly as potent.

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  • $\begingroup$ Thank you. I had read that, hence my mention of NOx. But it also states that NOx emissions are believed to be at least partially mitigated by a number of factors, of which some have not yet been quantified. That sounds like we don't really have any idea how much impact NOx up there has, if at all, or am I interpreting something wrong? $\endgroup$
    – foaly
    Mar 3, 2017 at 7:46
  • $\begingroup$ further, NOx impacts are said to be local, or at most regional. Thus, depending on the route I suppose "global" warming effects are not necessarily a direct result (although flight routes pretty much cover the globe today I suppose). $\endgroup$
    – foaly
    Mar 4, 2017 at 6:33
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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.

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    $\begingroup$ Overall, good answer but needs references $\endgroup$
    – L.B.
    Apr 5, 2017 at 13:16
  • $\begingroup$ Increasing bio-mass (greening) won't add CO2 to the atmosphere - it will be taking CO2 out. We do know where the CO2 is coming from. $\endgroup$
    – Ken Fabian
    May 26 at 23:54
  • $\begingroup$ Estimate of about 1.3% increase in biomass between 2000 and 2019 here - science.org/doi/10.1126/sciadv.abe9829 - but I think it unwise to presume a dependable ongoing trend; vegetation will be increasingly affected by climate change in a variety of ways including changed drought and fire intensity that may prevent vegetation retaining carbon. $\endgroup$
    – Ken Fabian
    May 27 at 0:48

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