This paper studies dynamic mechanisms for a global commons. A leading example is carbon consumption. We posit a model in which each country benefits both from the use and the aggregate conservation of an open access resource at each date. Conservation is beneficial because it reduces a country's environmental costs of resource use. A country's relative value of consumption-to-conservation is summarized by a privately observed parameter -its resource type -which evolves stochastically each period. An optimal quota system is an international agreement over resource consumption that maximizes world welfare subject to being implementable by Perfect Bayesian equilibria in compliance and disclosure strategies. With complete information, we show that the optimal quota is first best; it allocates more of the resource each period to countries with high value of consumption (and low value for conservation). However, under incomplete information, we show that the optimal quota is fully compressed -it is invariant to the country's resource type at every point in time. In the case of CO 2 , this means that all ex ante identical countries receive the same emissions restrictions despite having differently evolved environmental costs and resource needs. We show that full compression holds under fairly general distributions on private shocks. JEL Codes: C73, D82, F53, Q54, Q58