Cooperation is often difficult to achieve because it can be difficult to ensure that individual interests and collective interests align. What will do the most good for the group may require individuals to make sacrifices, and not everyone is sufficiently altruistic to make those sacrifices.
This same problem affects efforts to address climate change: what will be best for the entire globe is not the same as what is best for an individual nation, or an individual person.
But the problem is even worse in the case of climate change because it requires cooperation across generations. This makes it even harder to achieve the sort of reciprocation that is usually a part of successful cooperative schemes.