[s1artibartfast]

>It wouldn't make a difference if the absolute volume of emissions into the atmosphere

First off, it literally would make a difference in the the absolute volume. 7 tons/person is objectively less than 15tons/person.

IF your point is simply that the US can't stop climate change all by itself, then of course you are correct.

Climate change is not a binary situation. it can be more or less extreme.

Do you throw your trash on the street just because someone else did? Less trash is obviously better than more trash.