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 .