The Montreal Protocol constitutes a significant success of international cooperation: since the 1980s, the ozone layer seems to slowly recover. By contrast, the UNFCCC regime, despite being almost the same age, has not achieved significant successes yet.
→ what, in your view, explains this difference?
I think the difference in success between the Montreal Protocol and the UNFCCC is mostly due to the fact that the goals mentioned in the Montreal Protocol are more precise.
The Montreal Protocol focuses on the ozone layer. Therefore some relevant chemicals needed to be phased out. Internationally there is more consensus on the urgency of this problem. Phasing out these chemicals has not so much impact on the national economies.
The UNFCCC is just a framework and does not set a precise and binding limitation on emissions. I think that internationally there is less consensus about the urgency of the problem and how to deal with it. The problem is more complex since climate change has different causes and a lot of different consequences. People are afraid that the limitation on CO2 emissions will have a big impact on their economies and therefore a lot of countries are unwilling to make ambitious commitments to reduce CO2 emissions. Further, developing countries mostly have the priority of development, of getting their people out of poverty. Binding limitations on CO2 emissions could frustrate the development of these countries.