What sort of evidence are we talking about? We don't have a another exact copy of Earth so we can't perform a controlled experiment by somehow inducing low clouds to form in one and not in the other. But if you need this sort of proof then how can you turn around and justify the belief that CO2 is definitely causing climate change?
Even NASA would disagree with you (https://visibleearth.nasa.gov/view.php?id=54219):
Clouds play a complex role in the Earth's radiation budget. Low Clouds reflect much of the sunlight that falls on them, but have little Effect on the emitted energy. Thus, low clouds act to cool the Current climate.
(Note the paper is specifically referring to low clouds)
> The problem with this line of thinking is that, when you do something to a system, usually lots of properties of the system change. So you can always say "Look, properties A and B changed at the same time right after we did X! So maybe A caused B, not X."
Sure, but also, when you make changes X1, X2, X3, ... Xn, you can't just arbitrarily say that X1 must be the cause and X2 .. Xn are not.