Frustrating article. During his time at CATO the author had all the necessary mental models, data sets, and journal articles required to come to this conclusion.
So what changed his mind about managing the risks of global warming? His non-answer is, "I changed my mind about that, however, because (among other things) I changed my mind about risk management."
I suspect that the real reason is that he has left CATO and is no longer paid to look at the world in a certain way. His revelation that global warming is too risky to play games is mostly a sad confirmation of Sinclair. "It is difficult to get a man to understand something when his salary depends upon his not understanding it."
If I was going to find a silver lining it would be that people such as him are finding it somehow worthwhile to believe in the science that they've spent decades trying to refute. Until change is profitable, nothing will change.