Why Not to Allocate Global Resources on a Per Capita Basis
Eric Posner has a good post on this subject, specifically with respect to put carbon into the atmosphere.
As I describe in a comment there is another good reason not to adopt such an allocation method: there are opportunity costs associated with setting up a regime that we have no good reason to believe would achieve any goal other one specific conception of equity. Generally put, there this would not necessarily improve the lot of humanity. More specifically, there is no reason to believe it would increase per capita utility, personal autonomy or improve the chance of survival of the species in comparison with other potential systems for allocating rights. As his piece hints, there is a reason other resources are not allocated this way: there are better ways to do so.