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.
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.
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 .
No! There was never a claim that temperatures rose after 9/11. The suggestion was only about an increased temperature spread, which is definitely true. Not that it could be told from post 9/11 observation for sure, but there is more research on the subject which shows contrails reduce the daily temperature spread. As to be expected anyway.
However the overall effect of contrails, according to the IPCC, is positive! So contrails are warming Earth. Yet it must be said, that the IPCC rates this effect as minimal (+0.01W/m2 in 2004 for linear contrails). If that is true (and I doubt it is), a reduction in contrails would cause non-detectable cooling.