Why does it matter whether humans caused climate change? The important question surely is whether there is anything humans can or should do to reduce the future damage of climate change.
According to some religious traditions, humans are the trustees of the planet. But you don't have to be religious to believe we should care for the planet. If someone is drowning, we don't stand on the bank arguing who pushed him in, or whether he slipped. Our responsibility to help people is not limited to those we have already harmed.
I am not a climate change expert, but I think that the evidence for humans having contributed to climate change looks pretty convincing. (Good visual summary of the evidence on both sides here.) But even if it wasn't, I don't think it follows that we should just let the planet destroy itself without trying to do something.
But many people (on both sides of the climate change debate) apparently believe that climate change denial leads inevitably to a policy of non-intervention. As if to say - if God wants the planet to get warmer, who are we to stand in His way?
Clearly the factual debate has emotive consequences for international policy and collaboration. But the link is based on rhetoric rather than rational logic.