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.
I don't think Elon Musk is very important. He's certainly wealthy and successful, but he wouldn't be anywhere near the top of the list.
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.
Musk is rich and business savvy.