Of course I would give these people a chance and listen to what they say. Still, if you tell me that there's no proof that a raise of CO2 in the atmosphere is our fault I just don't understand your doubts. You can calculate how much fuel gets burned and how much CO2 you get as an outcome of all these chemical processes. And then, if you measure the concentration of CO2 and compare what has been added to it by what we have burned, and these numbers match exactly, following every little deviation over the years - I don't see that there is need for another proof.

And then CO2 is known to be a greenhouse gas and its effects are understood quite well. There is a direct correlation between the global temperature and CO2 concentration.