Presumably she's referring to her first proposal -- a carbon tax. It's my understanding that there's nearly consensus among economists that a carbon tax is the most efficient solution to global warming, but political consensus that it would never happen when framed as a tax.
If you don't know what a carbon tax is, or why it's orders of magnitude more efficient a solution than "eat less meat" I'd encourage you to look into it a bit. Essentially if you create a system where what is best for the planet is also the cheapest course of action (for individuals and businesses), you no longer have to rely on convincing every single individual to change their morality/beliefs.
No sovereign is going to agree to subordinate itself without all others doing the same
Classic prisoners dilemma
The international political system has a long track record of cajoling nations into joining treaties that theoretically are like this.
It's not prisoner's dilemma, because the main property of the PD problem is that parties are prevented from communicating and cooperating with each other. When there's negotiations, transparencency/monitoring, etc, parties can cooperate. If you think international treaty development is like PD, you are due for a for a big optimism update!