Also, a large country is likely to have regions that will lose big in case of global warming, while other regions are indifferent or even stand to gain. The large country's single government has reason to oppose global warming in that case, which means that it can put all of its diplomatic weight on the side of global warming countermeasures. And once countermeasures were agreed to, they can be enforced across the entire country. If it were split up into smaller countries, the ones that will be less affected by global warming have an incentive to avoid countermeasures so they can exploit their competitive advantage there.