Most of the countries already collaborated and came to an agreement for environmental solutions. There are international agencies that provide guidelines and urged all the nations to sign on international environmental agreements, each with critical issues to address.

Important two you should know are Montreal Protocol to address CFC effects on ozone layer and Kyoto Protocol to address issue of greenhouse gases and climate change.

Currently, we are said to be living with the climate change consequences rather than trying to prevent it years ago. Extreme weather events, rising sea levels, and changes to weather patterns. Last year, for e.g pakistan floods, big river dried up in China and France. Also at US river? With the dino footprints. We dont need these hypothetical scenarios when climate change threaten a country food security and costed billions in damages.

Its important to note that some country are more prone to the effects of climate change. So, when disaster struck, unprecedanted as it'll be, it will not change environmental stance of other country.

Edit: I failed to imply that we are already working together on this critical issue (except north korea) regardless of hypothetical scenarios op mentioned.