Best solution I can think of.
They want to trade emissions credits, what about trading licenses to have children. A limited number of licenses would be issued each year worldwide. People would have to apply, pass certain DNA tests, and medical tests. There would have to be a system to enforce it: punishment for unlicensed pregnancies. I doubt sterilization would be necessary. But imprisonment might be.
Oh but then people will be screaming about human rights and the oppression of the impoverished masses who rely on their children to support them in their old age. And what about countries who don't ratify the treaty? Sanctions? Blockades?
Fuck. No.... don't.