Recent conversations in the general melting thread have brought up the subject of contrails. And in particular what effect would reduced air travel have on global and/or Arctic temperatures.
I couldnt find a more recent thread specifically on contrails and didnt want to mix it up with the aerosol thread which is a bigger topic.
Most of the literature I see out there say that aviation-induced radiative forcings (CO2, nitrogen oxides (NOx) and contrail/cirrus cloud formation) have a net warming effect. . NOx creates ozone and destroys methane and is a net warmer. As are the contrails and resultant cloud formation which trap radiation escaping from the Earth.
It's not all one way traffic though and this study in Nature found that contrail cirrus cause a significant decrease in natural cloudiness, which partly offsets their warming effect.
https://www.nature.com/articles/nclimate1068
So in reply to F.Tnioli post in the melting thread, would the reduced aircraft over the Arctic have a slight cooling effect overall ?
The way i see it, whenever insolation is close or at 24/7, incoming radiation is much higher than what Earth surface is able to "bleed back" to near-Earth space. Thus sunny places like equator are hot, ain't it. So during polar day, clearer sky - in particular, less contrails, - is definitely major warming of Arctic surface layers. Much of the summer, as we all know, insolation in Arctic is higher than even at the equator. High albedo slows the melt, of course, and it's a kind of balance which can and will be affected.
To me it's in general that simple: contrails are a sort of insulation, and as such it works both ways. I.e., slows down both cooling and warming: whichever would naturally happen given no contrails are present - would still happen, just slower: the more contrails, the slower temperature change will happen on average, if we speak the surface of Earth, that is.
As since for most of this winter we had "normal" amount of contrails over Arctic (let's not forget they don't entirely dissipate same day flights stop happening, far from), there was "slower/less cooling than it'd be without contrails" for freezing season, thus resulting in less ice thickness, weaker ice, warmer ice temperature on average, etc; but now with contrails gone, this "weak freezing season" - weak as is in compare to the old days, pre-1980s, - will be followed by "stronger (than recent years - with plenty contrails)" melt season.
So, before someone drops a vial with some coronavirus 2.0 - please tell 'em to wait about doing it all the way till NH summer, right? This way next "no contrails" season will happen in NH winter, clearer skies = more freezing. Then at least it'd be somewhat good for the ice outta next outbreak of the kind, eh...
