The recent upsurge of concern over global warming and the financial crisis has reinvigorated advocates of world government, who claim that it is the only way to solve global problems that cross state boundaries. Left to themselves, individual states might "free ride" on the efforts of others, and the issue in question might remain unaddressed. This recent piece by Financial Times columnist Gideon Rachman is one example of the argument. For a more detailed and more academic statement, see here.
This case for world government is superficially appealing, but seriously flawed. Even if world government advocates are right to assume that some global problems are too big for any one nation to solve, it doesn't follow that world government is needed to address them. The problems in question can be addressed equally effectively through cooperation between a few major powers. For example, the United States, the European Union, India, Japan, and China produce the lion's share of the world's greenhouse emissions. An agreement between these major powers could therefore drive emissions way down, even if other states sought to free ride. Similarly, these major powers have the vast majority of the world's banks and other financial institutions, and could therefore cooperate with each other to address the financial crisis (assuming, for the sake of argument, that such international regulation is necessary).
Both economic collective action theory and basic common sense suggest that cooperation between a small number of like-minded actors isn't difficult to achieve and is not likely to be plagued by free-riding. Free-riding would be inhibited by the fact that each of players knows that the whole arrangement is likely to fall apart if they don't do their share (i.e. each is big enough for it's failure to contribute to have a decisive impact). In other words, efforts at free-riding would be prevented by the knowledge that if they are attempted, there will be nothing left to free-ride on. For a fuller statement of these points and cites to relevant literature, see pp. 1241-43 of this article that I coauthored with John McGinnis.
Obviously, cooperation might be prevented not by free-riding but by honest disagreement over the nature of the problem, the kind of action needed to address it, and whether or not the costs of action exceed the benefits. However, such disagreement can also arise even within the confines of a single worldwide government. Unless that government takes the form of a dictatorship or very narrow oligarchy, it too will sometimes be prevented from acting by internal disagreement. And we can't assume that the advocates of stronger action are necessarily right. For example, the the US, China, and India may be correct in their belief that the costs of radically reducing fossil fuel emissions in the near future outweigh the benefits. In cases where action is likely to cause more harm than good, the possibility that disagreement will block it is actually a good thing. In sum, there is no reason to believe that a world government can act to solve global problems more effectively than a consortium of the world's major powers. To the extent that honest disagreement might inhibit the actions of a concert of great powers more than those of a world government, that is as likely to be beneficial as harmful.
The argument sketched out here merely suggests that world government is unnecessary. In later posts, I will explain why its establishment would pose severe dangers of its own.
UPDATE: I think many commenters are conflating free riding (a situation where actors agree on the problem and on the need to act, but try to get others to bear all the costs) with genuine disagreement over the existence of a problem, the action needed to solve it, or the relative costs and benefits of that action. My contention is that the failure to act on global warming is caused by the latter: key players such as China, India, and the US believe that the costs aren't worth the benefits. If so, world government could not solve the problem, because presumably these parts of the world would have enough clout in that government to prevent it from adopting major cuts in fossil fuel initiatives. At least that would be the case if the world government were at all democratic. Between them, these three nations have nearly half the population of the world.