Why is international cooperation necessary for the protection of the natural environment?

Dealing with ecology can only be done by cooperating with each other. Our planet is surrounded by a single atmosphere, therefore, if one country deals with, for example, the problem of clean air, while others do not, then the problem will be impossible to solve. Air layers move constantly, and do not stand still. If each country had its own atmosphere, a kind of dome, it could purify the air for itself, but in our situation this is not possible. If all countries unite and start joint activities, then the problem can be solved.