I do not remember the particulars, but after 9/11 and there was not any air traffic, some lab did a test. If I remember correctly, they found the earth was warmer since there were no contrails.
The upshot of this theory is that pollution, and especially jet contrails are blocking some of the light of the sun from hitting the Earth. 9/11 gave us some real data. Unfortunately I can’t remember what the name is but there was an impact on US temperatures when we were grounding planes that was not felt in Mexico and Canada where air traffic was normal.
Global Dimming does not change the theory of Global Warming. Climate change is occuring and there is a balance between the effects of carbon dioxide and pollution. The fear is what will happen when the scales tip.
I was just looking at the PBS site I listed above and saw that there was an article posted about the effects of contrails on the climate: http://www.pbs.org/wgbh/nova/sun/contrail.html.
This quote from the article refers exactly to what you were asking
Interesting, but what makes it a “conspiracy”?
The conspiracy is about chemtrails.