You are missing the point. Imposing tariffs is a way to force other countries to follow some rules. But if your own country doesn't follow those rules, what sense does it make? And that's the situation here, because the 3 largest CO2 producers are US, Russia and China, naturally, because they are biggest. But China actually invested heavily into solar and lowering their CO2, such as electric buses, while US left Paris Accord, rejected Green New Deal, started more oil drills and did a bunch of other things that actually increased their CO2 production. So if anyone should impose carbon duties, it's everyone else around US to force US to start following the rules.