moving on
What Extinction Level Event do you feel would bring the peoples of the world together in unity?
I don't think there is such an event - by it's definition there wouldn't be anyone left to unite.
Overpopulation - will become 'self regulating' in that if there is not enough food to go around the population will decrease - it won't just disappear in a puff of smoke - alien invasions excepted.
Change of Habitat - in earlier ice ages the population (inc animals) moved to habitats they were comfortable in or adapted to the new conditions (or became extinct if they did neither). If that means the available 'good' habitats are smaller in size or non existent see Overpopulation
Meteor Strike - see Change of Habitat
Global Warming - see Overpopulation
Reduction in Earths Magnetic Field - see Meteor Strike
Nuclear War - See Change of Habitat
Life is about survival, nothing more, nothing less. Humans are way past basic survival and into comforts, power, control etc. There are too many 'factions' at odds with each other and there will always be those who want to manipulate a situation to their own ends or be happy to sit back and let others do all the work.
To paraphrase a well known saying:
"You can bring some people to work together all of the time or all people to work together some of the time but not all people all of the time"