There were some post 9/11/2001 studies on this.
Having almost all air traffic grounded for a few days allowed a comparison to be made of the effect of aircraft on clouds, etc.
E.g. [link|http://facstaff.uww.edu/travisd/pdf/jetcontrailsrecentresearch.pdf|Contrails reduce daily temperature range] - a 1 page article from Nature. Basically, the contrail clouds reflect light during the day and act as a blanket at night to reduce the maximum daily temperature range. The temperature range was about 1 degree C larger when the planes were grounded than the 3 days before; 3 days later (when the planes were flying again) the temperature range was about 1 C less than the 3 days before 9/11.
The sky was interesting those 3 days. We may be the last people to see such a sky over cities in the US.
Cheers,
Scott.
