Whether God or global warning exists is difficult to answer conclusively and in a meaningful way. (Eg., Everybody on this site likely accepts that the earth has always gone through cycles of cooling and heating.)
The burden of proof (onus probandi) is frequently misunderstood. And whether proof is provided doesn't affect the truth of the statement---(1 + 1= 2, but probably most people on here would not be able provide it.)
But it would seem appropriate for anyone to ask anyone to at least "back up" what they say---if not actually prove what they've said. Otherwise it's just their opinion---which may be right or wrong, even with so-called "proof."
The period is appropriate---it separates an "if-then" clause. The presence or absence of a comma after "But" is at the discretion of the writer---and depends on where he wants you to pause when you read that sentence.
Salt and Red Pepper 