They can be relatively localized. Consider contrails, which with the grounding of air travel in the US after 9/11 had climate impacts that could be studied.

https://globalnews.ca/news/2934513/empty-skies-after-911-set...

http://www.bbc.co.uk/sn/tvradio/programmes/horizon/dimming_t...

One of the interesting aspects of that is that it returned to normal (well, contrails being normal) rapidly.