What could be the common ideal that keeps people together? Or could it be a global threat like global warming?
From past experiences, alliances between different groupings with conflicting interests only tend to be able to find common ideals where there is mutual benefit or a mutual threat.
Global warming is a problem that will affect the whole world, but unfortunately I doubt it will be able to unite the world's peoples. What people need is an enemy to work against - nothing brings people together like a common enemy unfortunately - but it is not possible to unite everyone against someone because there is no one left to fight. Hopefully some other common cause will emerge in the future to unite the world, space exploration for example.