Just imagine how this will get magnified and drive the world nuts when half the things we try to do to tackle climate change end up being plausibly explainable by natural variation. It's already a hard enough problem to solve without natural variance being thrown into the mix.

But that's not conclusive either. There is this:

The research by Hong and his colleagues, published in Geophysical Research Letters, follows other studies arguing that the September 2001 temperature variations can be explained by the clear, dry weather on the crucial days4, and that climate-modelling results do not support the contrail effects claimed by Travis's group5.

I contend the clear dry weather was a consequence of the lack of planes. It sounds the one of the original authors does too.

All I know is the weather here was beautiful. And very uncharacteristically so. Unfortunately nobody is going to repeat the experiment by grounding the entire fleet for 3 days just to test it.