I'd say Xi Jinping. US presidents don't have the same level of influence within their own nation as Xi does in China, and are limited to (at most) 8 years in charge. Xi's term doesn't end until he does.
We need to get more carbon into the atmosphere to help plants grow stronger when they breathe it in.
Don't be silly. We've discussed this before. That's a ludicrous oversimplification of how the science works.