The Paris Agreement cannot be legally binding, and they will not even call it that.
The Kyoto Protocol protocol was called “legally binding.” But Canada was “legally bound” and simply announced in the middle of the Copenhagen conference (2009) that it was leaving. Not one government said a word about it. Why? Because there is no international government, and there were no sensible penalties in the treaty. There was nothing behind the words. They were simply a fraud.
If there is no enforcement (nothing at all), then what is there to bind anyone to the agreement?
Suppose the Paris agreement was written as a Treaty, and the U.S. Senate passed it (totally impossible, but say it did). Now say the US decides it does not want to be in the treaty. Well, the US Senate can just un-ratify the treaty. In fact George Bush decided on his own to ignore quite a few treaties. Nothing happened!!
There is only one way to make an agreement bind, and it has little to do with “legally.” Countries can decide that those who join the “coalition of the willing” will voluntarily penalize those who should be in but don’t cooperate. Realistically this punishment would be mild, for example taxing all goods imported from a non-participant at some rate related to carbon content.
This is weak, but even this tiny step would make the agreement bind a little. Any country that left could not escape this small penalty. So the agreement “binds” a little when you think of leaving.
Of course it would be best to combine this with Green Climate Fund payments for poor countries that joined the coalition.
But could we stop talking about “legally binding”?! It’s just a meaningless distraction designed to make people feel good so they don’t think about taking meaningful action.