The treaty’s advocates, hoping to forestall a Trump exit, are trying to save the accord by arguing that it is largely meaningless. In this spirit, a piece in the liberal website Vox explained, the Paris accord “asks participants only to state what they are willing to do and to account for what they’ve done. It is, in a word, voluntary.” In other words, “Nothing to see here, just us climate-change alarmists playing pretend.”

And there is indeed much to be said for the worthlessness of Paris. Beijing pledges that China’s emissions will “peak around 2030.” By one estimate, this is when its emissions would peak regardless. So the world’s largest emitter is using the accord as a platform for climate virtue-signaling.

According to Benjamin Zycher of the American Enterprise Institute, even if Paris is fully implemented and you accept the Environmental Protection Agency’s model for how emissions affect warming, it will produce a rounding error’s worth of decline in the global temperature by 2100 — .17 of a degree Celsius.

If Paris is such a nullity, why shouldn’t we simply pull out? This is where its supporters reverse field and contend that it will be a global disaster if the U.S. leaves. Supposedly the moral suasion involved in countries coming up with voluntary targets and having to defend their performance meeting them will drive an ever-escalating commitment to fight global warming.