If the US, brimming with wealth, technologically advanced and educated can't commit to fighting climate change, what will the poorest developing countries decide? They would need to take the burden themselves and falling behind economically (something China in particular would strongly resist). A few defectors can ruin the entire plan because of economic pressure. I think we should be nearing an accord with greater control anyway -- defectors will need to suffer harsh economic sanctions greater than what they'd spend on the accord anyway. I expect this to happen within a few years.