Basically the reduction of the amount of light that get's through the atmosphere. It's been proved, simply because the only way it could happen is the increasing amount of greenhouse gasses in the atmosphere.
Yes, global dimming is a very real phenomenon that has been measured (though it is notoriously difficult to get really good measurments because the aeresols that provide for it are so short-lived in the atmosphere). Dimming has nothing to do with greenhouse gases though--they don't interact with visible light. Global dimming is caused primarily by aeresols and dust particles put into the atmosphere. Sulphur aeresols have been especially important. cj
