Yeah. I mean, there's no definitive proof to back that such a thing as "global warning" even exists, but if Al said it on TV, it must be true, right?
Finally, someone that knows their stuff.
If by definitive proof, then you mean "a scientific experiment whereby global increases in temperature can be shown as being the direct result of carbon dioxide emissions" then, you're right. There is no definitive proof. That whole ethical dilemma of increasing global warming significantly enough that you can look directly at it and say it was because of your contribution of CO2 is preventing this experiment from occuring. Besides, where would we get the control? Do you have a spare Earth lying around?
However, there is a metric %^&ton of correlational data showing a positive relationship between CO2 levels in the atmosphere and increasing temperature.