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I recall that after 9/11, when all US flights were canceled, there was a statistically detectable bump in average temperature. Something about jet contrails disappearing for 4 days. Has anything like that happened during the recent COVID-19 drop in US air traffic? Sorry I don't have the reference to the 9/11 data/story.

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  • $\begingroup$ If you are referring to CO2 ; refer to the Mauna Loa (?) CO2 data. $\endgroup$ – blacksmith37 May 6 at 20:24
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    $\begingroup$ int-res.com/articles/cr2004/26/c026p001.pdf A report stating that the warming was caused by slightly abnormal weather patterns, rather than 'global cooling' to which you refer. $\endgroup$ – Software Engineer May 6 at 20:42
  • $\begingroup$ I lot of the literature suggests that the diurnal temperature range may be a better quantity to look at than the temperature itself. The contrails have a cooling effect during the day (enhanced albedo) and a warming effect at night (enhanced insulation), so an analysis of diurnal temperature ranges would be interesting. $\endgroup$ – M Juckes May 7 at 22:20
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It's not clear yet; this is an area of active research which it itself hampered by COVID-19 social distancing requirements, according to this report on Climate Change News:

Scientists with Nasa and European research groups hope to use clear skies to narrow down massive uncertainties about the warming effect of condensation trails – the wispy white lines that criss-cross the skies in the wake of jets engines.

“The air traffic system has not been diminished to the current levels since the days following 9/11,” said Patrick Minnis of Nasa Langley Research Center, who is joining a research effort to study high-altitude clouds.

“Flight groundings at the scales initiated in response to the coronavirus pandemic are a significant opportunity to better quantify the impact of air traffic on cloud cover via contrail formation.”

As to the post-9/11 effect:

The new research builds on studies after the 9/11 suicide hijackings in the US grounded flights for a few days. One study, for instance, found that the plane-free skies had an impact on temperature variations in the United States, but some researchers say the findings might have been caused by natural variations.

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I had to look at Mauna Loa data myself for the heck of it. No change to the roughly 2 ppm increase each year, presently at 416.6 ppm. I am somewhat surprised , not because of the reduced air traffic, but the reduced auto traffic which is the significantly greater amount of CO 2 .

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  • $\begingroup$ I don't think this answers the question, which directly mentions the loss of the dimming effect of contrails, which is noted in news.psu.edu/story/361041/2015/06/18/research/… $\endgroup$ – jeffronicus May 6 at 22:38
  • $\begingroup$ That is "old school" pre 1980 , when environmentalists were convinced there was global cooling. The theory was tiny ice crystals released at high altitude by jets, reflected enough solar energy to cool the earth. $\endgroup$ – blacksmith37 May 7 at 18:45
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    $\begingroup$ The effect might not be as much as the 1980's environmentalist thought, but it is still a topic of active research. It is not the ice emitted by the jets that matters, but the clouds seeded by them .. which can cover quite an area and have a measurable local effect. I I believe question is about this local effect. $\endgroup$ – M Juckes May 7 at 22:16
  • $\begingroup$ The pre '80 theory was global , not local. $\endgroup$ – blacksmith37 May 8 at 19:39

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