So far, the argument seems to be primarily that we're pumping SO much carbon into the environment that some, if not all, of the warming is caused by us. We know how much carbon we're pumping out but I don't believe we know how much warming it is actually causing (versus natural effects).

That would be important to ensure our response is at the appropriate scale. Unfortunately, without that quantification, we're stuck with either doing "EVERYTHING" or "NOTHING".