'Have to', or what? What leverage does he or his organization have over these sovereign nuclear armed nations? If he said 'should' I would not have batted an eye, but talking as though you're in a position of power when you really aren't just makes you seem weak and out of touch.
I think this matters, because we need to be realistic about the limitations of a diplomatic/political approach to solving this problem. I am becoming increasingly convinced that a technological solution is more viable than a diplomatic one. A solar sunshade would cost hundreds of billions of dollars, which is not an insurmountable amount of money given the magnitude of the problem. It would not solve the acidification of our oceans, but it would address rising sea levels and temperature-driven severe weather. It's not a great solution, but it might be the best solution that doesn't hinge on diplomatic impossibilities.