This paper discusses the feasibility and performances of simple mechanisms to implement international environmental agreements in the multilateral externalities context of global warming. Asymmetric information and voluntary participation by sovereign and heterogenous countries are key constraints on the design of those agreements. Mechanisms must prevent two sorts of free-riding problems - free riding in effort provision and free riding in participation. As markets might fail to solve simultaneously those two problems, we construct instead a simple menu of options that trades off the provision of incentives for participating countries and the provision of incentives to participate.With such mechanism, all countries voluntary cont...