The climate impact of air travel is often divided into CO2 emissions and "high altitude effects" like that of water vapour and cloud formation. The effect of the emitted CO2 is cummulative, as CO2 has a high retention time in the atmosphere. On the other hand, as I have understood it, the high altitude effects are more short lived. Their impact on temperature are determed by current levels of air travel, and not past.

How can the size of these two effects be compared to each other or to other sectors such as travel by car? Is there any videly accepted method for this? It seems a bit like apples and oranges to me.

I know about GWP, that uses a somewhat arbitrary timeframe of 100 years. Is and can that be used here? I have also heard about RFI, but I am unsure how that works or is applied to this.

  • $\begingroup$ My understanding of this issue is incomplete, so please do correct any misunderstandings in the question. $\endgroup$
    – Anders
    Jun 26 '19 at 14:32
  • $\begingroup$ It has been agreed; the CO 2 heating is worse. Until about 1980 half the environmentalists said humans were causing global cooling ( because of high altitude water vapor reflecting solar heat.) The other half said global heating caused by CO2. after 1980 it has been all warming. $\endgroup$ Jul 5 '19 at 15:27
  • $\begingroup$ @blacksmith37 I am not talking about cooling, or what is worse. I am talking how you compare two different heating effects. $\endgroup$
    – Anders
    Jul 6 '19 at 8:47

The CO2 is a greenhouse gas which causes a net heating effect as it retains additional solar radiation, the exact magnitude of this effect is still highly debated. The high altitude effects you've mentioned are often referred to as "jet exhaust shading" and it's estimated, based on some temperature data from the week after the 9/11 attacks in the US, that it causes roughly 4°C of cooling, this is because the jet exhaust contains water which condenses out as high albedo clouds, and sulfur compounds that absorb solar radiation as well as the CO2.

  • $\begingroup$ Thanks for the answer! I thought the high altitude effects contributet to warming, but maybe I have misunderstood that. Do you have any source for the 4 degree number? I don¨t think you really touch on the heart of the question, though - how do you compare the different effects? $\endgroup$
    – Anders
    Jul 10 '19 at 21:04
  • $\begingroup$ @Anders I think I first heard the 4° figure on An Inconvenient Truth but Guy McPherson quoted a similar number, from a NOAA study, in a talk that, among other things, discussed what would happen to climate if we stopped using jets, I can't actually find that study though so I'm not sure what he was looking at. Unfortunately without some verified data on the warming effect we can't really compare or contrast the effects accurately, this study NOAA did on contrails seems to conclude that the warming effect is the bigger concern. $\endgroup$
    – Ash
    Jul 12 '19 at 11:05
  • $\begingroup$ AIUI there's also an increase in the potency of the CO2 released by releasing it at altitude instead of ground level - but I don't know if this is correct, and if so I don't know the mechanism. $\endgroup$ Jul 12 '19 at 17:15
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    $\begingroup$ @SemidiurnalSimon I know there's a lot of unknowns about the exact effects of high altitude clouds as it appears that they can both heat and cool the atmosphere simultaneously but I've never heard anything about altitude effecting the magnitude of CO2's heating effect but high altitude may contribute to it persisting longer in the atmosphere which would increase its net potency. $\endgroup$
    – Ash
    Jul 12 '19 at 17:28
  • $\begingroup$ @Ash ah, that may be it! Definitely not my area :) $\endgroup$ Jul 12 '19 at 20:21

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