What observation would disprove man-made global warming?

Just as with all other science we improve our skills all the time. Two years ago we didn't think anything would happen on 2012 that would be related to natural causes, at least nothing that could be predicted. Now we know that we will be passing the center point in our rotation around our galaxy and that our magnetic poles will change when this happens, as it has before. This could or could not cause major problems in tidal currents and tectonic stability.

Global warming may or may not be related to man. It is more likely then not that we are the cause, but we can not prove it one way or the other yet.

I would consider failure of the models to predict with any accuracy, without having to be adjusted to account for reality as it plays out, to be sufficient cause to cast doubt in the theory, yes.

The models are the primary, if not really the only, "proof" of man's role in climate change at the present and during the recent past. Failure of the models to reasonably reflect reality casts doubt on the validity of the models, and thus the conclusion that derives from the application of those models.

AGW proponents seem to wish to fly by the seat of their pants, so to speak, by taking any change as proof of their contentions. Hard to argue against a theory that allows pretty well any result as a confirmation of the theory.

This, of course, is part of the problem for us doubters as well, as we claim that too little is understood about the climate and its natural behavior to allow prediction, and thus any result is also compatible with the concept of nature being the primary if not exclusive source of climate variation.

What doubters have in wealth, however, is a vast amount of information showing that climatic variations are normal, not abnormal, and thus a change, in and of itself, is no cause to presume an unnatural origin for any given climatic change. AGW proponents really need to demonstrate, unequivocally, that a given climatic change is not a result of natural processes.

I do not think they have achieved the burden of proof on that question as yet.

Until any one of the so-called models accurately predicts something correctly, the proof of AGW has not occurred. So far, they have not accurately matched the past by even 50%.