Once upon a time, doubting the link between carbon and a warming planet was a perfectly reasonable position to have. That was before Svante Arrhenius came along and demonstrated the fundamental link in 1896. That's eighteen-96.
Later, it was somewhat reasonable to doubt the human component to it all. That stopped making sense long ago and definitely by 2007 when "unequivocal" entered the dictionary.
There's still plenty of room for budding science Ph.D.s to make their mark, but by now, denying the basic chemistry and physics is just plain ignorant.
What to do if someone does doubt? Smile politely, pull out a recorder or your phone, and ask to have their statement on the record. Their children may want to know.