a proof (scientific one)
Greehouse gasses create a nice rock in the solar system that is capable of supporting life.
previous shifts in climate have corresponded to higher or lower concentrations of Greenhous Gasses in the atmosphere
If humans, since the industrial revolution have been placing more GHG into the atmosphere, while at the same time, removing natural carbon sinks (rainforest) ,
reasonably, doesn't that at least put the possiblity of human exaggeration of the natural warming and cooling cycles?
Reasonably, any possibility, no matter how improbable, is to be considered if other explanations fail.
The relation of CO2 to Temperature, except for 3 examples across 4 and one half billion years, has always been that the rise of temperature causes the rise of CO2. By citing this link and deducing that a rise in CO2 will produce a rise in temperature is demanding that the future causes tha past.
Those 3 examples are all from many, many years ago. Antarctica has melted twice in the mean time.