Interpreting what someone says by dropping 4/5 of their argument isn't really grappling with it in good faith. Try, "I disagree that anything short of military force can accomplish meaningful international action on emission reductions."

In which case, assuming you actually believe this, you would be contradicting yourself and agreeing with DeLong that it isn't totally futile to expect concerted action to achieve significant emissions reductions.

All that being said, there are many cases of international efforts reducing the emissions of environmentally damaging substances (i.e. acid rain, ozone depletion) without resort to armed conflict. So I don't think your position holds up to the scrutiny of the real world. It feels more like an ideological reaction to the existence of global warming (which no-one seriously disputes these days) than an attempt to think through the underlying issues.