As some posters have already indicated, we're not likely anytime soon to have conclusive proof either that global warming is in large part due to human activity--- or that it is not.

If it is not primarily due to human activity and we take steps to try to curtail human contributions to global warming anyway, we have lost little.

If global warming is in fact due primarily to human activity and we take no steps to curtail human contributions to it, we have condemned the world to disaster--which we might have mitigated or even avoided.

Therefore it is clear that the prudent--i.e. conservative--thing to do is to take steps to curtail human contributions to global warming.