"So what will Paris actually do in practical terms?" How about, at the very least it is symbolic of the fact that the US will fully take part in one of the mos

"So what will Paris actually do in practical terms?" How about, at the very least it is symbolic of the fact that the US will fully take part in one of the most important global programmes there is. That it will set targets and be transparent, reporting on its efforts to reach those targets. It will share know-how and expertise with countries that are trying to solve a problem that is an existential threat to civilisation. And if, as you suggest, the US is actually beating the progress it needs to make to keep global temperature rise to within 1.5C, then that's great - it can get the global plaudits it deserves, with no further action needed.