The thinking just doesn't follow...
"Global warming may be a bad thing and would require us to take action."
"Global warming isn't happening because we can't absolutely prove it's totally the fault of human activity."
The results of global warming and the cause of global warming are two different arguments, but it's actually evidence that deniers already know global warming is a real threat by trying to suspend the one argument with another. The difference isn't belief that global warming is happening or not; nobody wants it to happen (assuming it will harm us and we're not including misanthropists), some people are just afraid to admit to themselves that the threat is real.
People are pendulums, you push them away and they'll come right back. You can't just show someone evidence and expect them to rationally use it, you have to let them talk long enough for them to hear themselves be wrong.