We need global democratic government. It's the only solution to everything from climate change to tax evasion and worldwide inequality. Is it possible? Yes. Is it probable? Less and less likely every day. We need to have a forceful push for this, or else we're going to be in a very bad state in the next 50 years.
The biggest obstacle to global governance is communication. How can the will of the people and that of a governing body be made intelligible to the masses if a country is large in size? Until recently, it wouldn't have been possible, but with the advent of the internet, it seems more and more feasible that we might one day see a centralized governing body for the entire world. A mediating force would be necessary to account for the differing social norms across the globe, but it seems logical that technology will eventually mitigate language and communication difficulties, and that the world will be so well-connected (commercially and culturally) that national divisions will begin to seem arbitrary.