How about this? Tax goods from (or with large inputs from) China AND the US. Heavily. Harshly. Punitively.

Between them, those 2 are most culpable in terms of greenhouse gas emissions.

If you are truly motivated by a genuine desire to address what IS the single most grevious threat, then you ought to agree.

Wholeheartedly.

If you're just going to harp on about China, and ignore the US as being equally the problem (and if we take history into account, the US is the BIGGEST problem), then there's another word for that motivation.

And that word is not "environmentalism"

By volume don't really make sense, the only way is to count per person. Counting by volume depend on how you decide to put groups together for counting. You would always be able to say that "the others" are larger than the one you are and say that what you do don't matter. But the easiest to change is what you do, not what everyone else do