When will it be time for a world government?
A wise man once said that if the governments of the world really DID know that the Earth was being visited by aliens using technologies beyond our wildest dreams, they would all suddenly become the best of friends. The point being that groups of people can, and have in the past, set aside more petty and parochial differences in order to deal with more serious threats collectively.
The trouble is getting people to agree that a threat exists, and that banding together is worth the trouble of dealing with the threat. In the case of Nazi Germany, the threat was not agreed upon in time and the alliance that formed after that mistake became undeniable broke apart after the problem was "solved." On the other hand, the problem of British tyranny over the American colonies resulted in a more long-term union of varying states with disparate needs: the federation that resulted was able to create unity where needed and allow states to address more local needs as they saw fit.
Now the world faces some rather interesting problems. Some are time-worn, such as terrorism. Others are relatively new, such as global climate change. And our current favorite thing to discuss, the economic troubles.
Now it seems to me to be just common sense that all of these problems could far better be dealt with if humanity had a more central government. But a world government won't happen until humanity as a whole recognizes a threat so large and so general that there is a push for unification.
Now if the ice that currently covers Greenland were to slide into the ocean, turning Miami into an island... that MIGHT just drive some folks to agree that a serious change of standard procedures is needed. Or will we end up acting as we did in the case of Nazi Germany... waiting until millions are getting killed before saying "Hm... we done screwed up. Better start to fix things now!"?

