And, I’d add, a global problem.
We do not have a Chinese or an American atmosphere. We have a global atmosphere. We cannot run independent experiments upon it. We have instead been running a joint experiment. This was not a conscious decision: it happened as a result of the industrial revolution. But we are consciously deciding not to stop.
While it is rational to ask if the benefits of mitigation outweigh the costs, it is irrational to deny the plausibility of man-made climate change.